result -> results (#27)
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Jacek Sieka
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stew/result.nim
577
stew/result.nim
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@ -1,575 +1,4 @@
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# nim-result is also available stand-alone from https://github.com/arnetheduck/nim-result/
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import results
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export results
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# Copyright (c) 2019 Jacek Sieka
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# Licensed and distributed under either of
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# * MIT license (license terms in the root directory or at http://opensource.org/licenses/MIT).
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# * Apache v2 license (license terms in the root directory or at http://www.apache.org/licenses/LICENSE-2.0).
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# at your option. This file may not be copied, modified, or distributed except according to those terms.
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type
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ResultError*[E] = object of ValueError
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## Error raised when using `tryGet` value of result when error is set
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## See also Exception bridge mode
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error*: E
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ResultDefect* = object of Defect
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## Defect raised when accessing value when error is set and vice versa
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## See also Exception bridge mode
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Result*[T, E] = object
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## Result type that can hold either a value or an error, but not both
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##
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## # Example
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##
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## ```
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## # It's convenient to create an alias - most likely, you'll do just fine
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## # with strings or cstrings as error
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##
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## type R = Result[int, string]
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##
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## # Once you have a type, use `ok` and `err`:
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##
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## func works(): R =
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## # ok says it went... ok!
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## R.ok 42
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## func fails(): R =
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## # or type it like this, to not repeat the type!
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## result.err "bad luck"
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##
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## if (let w = works(); w.isOk):
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## echo w[], " or use value: ", w.value
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##
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## # In case you think your callers want to differentiate between errors:
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## type
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## Error = enum
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## a, b, c
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## type RE[T] = Result[T, Error]
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##
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## # In the expriments corner, you'll find the following syntax for passing
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## # errors up the stack:
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## func f(): R =
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## let x = ?works() - ?fails()
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## assert false, "will never reach"
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##
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## # If you provide this exception converter, this exception will be raised
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## # on dereference
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## func toException(v: Error): ref CatchableError = (ref CatchableError)(msg: $v)
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## try:
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## RE[int].err(a)[]
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## except CatchableError:
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## echo "in here!"
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##
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## ```
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##
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## See the tests for more practical examples, specially when working with
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## back and forth with the exception world!
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##
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## # Potential benefits:
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##
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## * Handling errors becomes explicit and mandatory at the call site -
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## goodbye "out of sight, out of mind"
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## * Errors are a visible part of the API - when they change, so must the
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## calling code and compiler will point this out - nice!
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## * Errors are a visible part of the API - your fellow programmer is
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## reminded that things actually can go wrong
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## * Jives well with Nim `discard`
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## * Jives well with the new Defect exception hierarchy, where defects
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## are raised for unrecoverable errors and the rest of the API uses
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## results
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## * Error and value return have similar performance characteristics
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## * Caller can choose to turn them into exceptions at low cost - flexible
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## for libraries!
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## * Mostly relies on simple Nim features - though this library is no
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## exception in that compiler bugs were discovered writing it :)
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##
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## # Potential costs:
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##
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## * Handling errors becomes explicit and mandatory - if you'd rather ignore
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## them or just pass them to some catch-all, this is noise
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## * When composing operations, value must be lifted before processing,
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## adding potential verbosity / noise (fancy macro, anyone?)
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## * There's no call stack captured by default (see also `catch` and
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## `capture`)
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## * The extra branching may be more expensive for the non-error path
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## (though this can be minimized with PGO)
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##
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## The API visibility issue of exceptions can also be solved with
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## `{.raises.}` annotations - as of now, the compiler doesn't remind
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## you to do so, even though it knows what the right annotation should be.
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## `{.raises.}` does not participate in generic typing, making it just as
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## verbose but less flexible in some ways, if you want to type it out.
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##
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## Many system languages make a distinction between errors you want to
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## handle and those that are simply bugs or unrealistic to deal with..
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## handling the latter will often involve aborting or crashing the funcess -
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## reliable systems like Erlang will try to relaunch it.
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##
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## On the flip side we have dynamic languages like python where there's
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## nothing exceptional about exceptions (hello StopIterator). Python is
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## rarely used to build reliable systems - its strengths lie elsewhere.
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##
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## # Exception bridge mode
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##
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## When the error of a `Result` is an `Exception`, or a `toException` helper
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## is present for your error type, the "Exception bridge mode" is
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## enabled and instead of raising `Defect`, we will raise the given
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## `Exception` on access.
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##
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## This is an experimental feature that may be removed.
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##
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## # Other languages
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##
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## Result-style error handling seems pretty popular lately, specially with
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## statically typed languages:
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## Haskell: https://hackage.haskell.org/package/base-4.11.1.0/docs/Data-Either.html
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## Rust: https://doc.rust-lang.org/std/result/enum.Result.html
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## Modern C++: https://github.com/viboes/std-make/tree/master/doc/proposal/expected
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## More C++: https://github.com/ned14/outcome
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##
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## Swift is interesting in that it uses a non-exception implementation but
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## calls errors exceptions and has lots of syntactic sugar to make them feel
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## that way by implicitly passing them up the call chain - with a mandatory
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## annotation that function may throw:
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## https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/ErrorHandling.html
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##
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## # Considerations for the error type
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##
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## * Use a `string` or a `cstring` if you want to provide a diagnostic for
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## the caller without an expectation that they will differentiate between
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## different errors. Callers should never parse the given string!
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## * Use an `enum` to provide in-depth errors where the caller is expected
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## to have different logic for different errors
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## * Use a complex type to include error-specific meta-data - or make the
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## meta-data collection a visible part of your API in another way - this
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## way it remains discoverable by the caller!
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##
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## A natural "error API" progression is starting with `Option[T]`, then
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## `Result[T, cstring]`, `Result[T, enum]` and `Result[T, object]` in
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## escalating order of complexity.
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##
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## # Other implemenations in nim
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##
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## There are other implementations in nim that you might prefer:
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## * Either from nimfp: https://github.com/vegansk/nimfp/blob/master/src/fp/either.nim
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## * result_type: https://github.com/kapralos/result_type/
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##
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## # Implementation notes
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##
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## This implementation is mostly based on the one in rust. Compared to it,
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## there are a few differences - if know of creative ways to improve things,
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## I'm all ears.
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##
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## * Rust has the enum variants which lend themselves to nice construction
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## where the full Result type isn't needed: `Err("some error")` doesn't
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## need to know value type - maybe some creative converter or something
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## can deal with this?
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## * Nim templates allow us to fail fast without extra effort, meaning the
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## other side of `and`/`or` isn't evaluated unless necessary - nice!
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## * Rust uses From traits to deal with result translation as the result
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## travels up the call stack - needs more tinkering - some implicit
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## conversions would be nice here
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## * Pattern matching in rust allows convenient extraction of value or error
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## in one go.
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##
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## # Performance considerations
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##
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## When returning a Result instead of a simple value, there are a few things
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## to take into consideration - in general, we are returning more
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## information directly to the caller which has an associated cost.
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##
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## Result is a value type, thus its performance characteristics
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## generally follow the performance of copying the value or error that
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## it stores. `Result` would benefit greatly from "move" support in the
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## language.
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##
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## In many cases, these performance costs are negligeable, but nonetheless
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## they are important to be aware of, to structure your code in an efficient
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## manner:
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##
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## * Memory overhead
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## Result is stored in memory as a union with a `bool` discriminator -
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## alignment makes it somewhat tricky to give an exact size, but in
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## general, `Result[int, int]` will take up `2*sizeof(int)` bytes:
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## 1 `int` for the discriminator and padding, 1 `int` for either the value
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## or the error. The additional size means that returning may take up more
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## registers or spill onto the stack.
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## * Loss of RVO
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## Nim does return-value-optimization by rewriting `proc f(): X` into
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## `proc f(result: var X)` - in an expression like `let x = f()`, this
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## allows it to avoid a copy from the "temporary" return value to `x` -
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## when using Result, this copy currently happens always because you need
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## to fetch the value from the Result in a second step: `let x = f().value`
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## * Extra copies
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## To avoid spurious evaluation of expressions in templates, we use a
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## temporary variable sometimes - this means an unnecessary copy for some
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## types.
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## * Bad codegen
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## When doing RVO, Nim generates poor and slow code: it uses a construct
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## called `genericReset` that will zero-initialize a value using dynamic
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## RTTI - a process that the C compiler subsequently is unable to
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## optimize. This applies to all types, but is exacerbated with Result
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## because of its bigger footprint - this should be fixed in compiler.
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## * Double zero-initialization bug
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## Nim has an initialization bug that causes additional poor performance:
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## `var x = f()` will be expanded into `var x; zeroInit(x); f(x)` where
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## `f(x)` will call the slow `genericReset` and zero-init `x` again,
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## unnecessarily.
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##
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## Comparing `Result` performance to exceptions in Nim is difficult - the
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## specific performance will depend on the error type used, the frequency
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## at which exceptions happen, the amount of error handling code in the
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## application and the compiler and backend used.
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##
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## * the default C backend in nim uses `setjmp` for exception handling -
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## the relative performance of the happy path will depend on the structure
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## of the code: how many exception handlers there are, how much unwinding
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## happens. `setjmp` works by taking a snapshot of the full CPU state and
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## saving it in memory when enterting a try block (or an implict try
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## block, such as is introduced with `defer` and similar constructs) which
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## is an expensive operation.
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## * an efficient exception handling mechanism (like the C++ backend or
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## `nlvm`) will usually have a lower cost on the happy path because the
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## value can be returned more efficiently. However, there is still a code
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## and data size increase depending on the specific situation, as well as
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## loss of optimization opportunities to consider.
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## * raising an exception is usually (a lot) slower than returning an error
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## through a Result - at raise time, capturing a call stack and allocating
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## memory for the Exception is expensive, so the performance difference
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## comes down to the complexity of the error type used.
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## * checking for errors with Result is local branching operation that
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## happens on the happy path - even if all that is done is passing the
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## error to the next layer - when errors happen rarely, this may be a cost.
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##
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## An accurate summary might be that Exceptions are at its most efficient
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## when errors are not handled and don't happen.
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##
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## # Relevant nim bugs
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##
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## https://github.com/nim-lang/Nim/issues/13799 - type issues
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## https://github.com/nim-lang/Nim/issues/8745 - genericReset slow
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## https://github.com/nim-lang/Nim/issues/13879 - double-zero-init slow
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case o: bool
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of false:
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e: E
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of true:
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v: T
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func raiseResultError[T, E](self: Result[T, E]) {.noreturn, noinline.} =
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# noinline because raising should take as little space as possible at call
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# site
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mixin toException
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when E is ref Exception:
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if self.e.isNil: # for example Result.default()!
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raise (ref ResultError[void])(msg: "Trying to access value with err (nil)")
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raise self.e
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elif compiles(toException(self.e)):
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raise toException(self.e)
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elif compiles($self.e):
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raise (ref ResultError[E])(
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error: self.e, msg: "Trying to access value with err: " & $self.e)
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else:
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raise (res ResultError[E])(msg: "Trying to access value with err", error: self.e)
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func raiseResultDefect(m: string, v: auto) {.noreturn, noinline.} =
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if compiles($v): raise (ref ResultDefect)(msg: m & ": " & $v)
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else: raise (ref ResultDefect)(msg: m)
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template checkOk(self: Result) =
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# TODO This condition is a bit odd in that it raises different exceptions
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# depending on the type of E - this is done to support using Result as a
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# bridge type that can transport Exceptions
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mixin toException
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if not self.isOk:
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when E is ref Exception or compiles(toException(self.e)):
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raiseResultError(self)
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else:
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raiseResultDefect("Trying to acces value with err Result", self.e)
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template ok*[T, E](R: type Result[T, E], x: auto): R =
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## Initialize a result with a success and value
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## Example: `Result[int, string].ok(42)`
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R(o: true, v: x)
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template ok*[T, E](self: var Result[T, E], x: auto) =
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## Set the result to success and update value
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## Example: `result.ok(42)`
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self = ok(type self, x)
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template err*[T, E](R: type Result[T, E], x: auto): R =
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## Initialize the result to an error
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## Example: `Result[int, string].err("uh-oh")`
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R(o: false, e: x)
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template err*[T, E](self: var Result[T, E], x: auto) =
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## Set the result as an error
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## Example: `result.err("uh-oh")`
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self = err(type self, x)
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template ok*(v: auto): auto = ok(typeof(result), v)
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template err*(v: auto): auto = err(typeof(result), v)
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template isOk*(self: Result): bool = self.o
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template isErr*(self: Result): bool = not self.o
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func map*[T, E, A](
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self: Result[T, E], f: proc(x: T): A): Result[A, E] {.inline.} =
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## Transform value using f, or return error
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if self.isOk: result.ok(f(self.v))
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else: result.err(self.e)
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func flatMap*[T, E, A](
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self: Result[T, E], f: proc(x: T): Result[A, E]): Result[A, E] {.inline.} =
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if self.isOk: f(self.v)
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else: Result[A, E].err(self.e)
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func mapErr*[T: not void, E, A](
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self: Result[T, E], f: proc(x: E): A): Result[T, A] {.inline.} =
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## Transform error using f, or return value
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if self.isOk: result.ok(self.v)
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else: result.err(f(self.e))
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func mapConvert*[T0, E0](
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self: Result[T0, E0], T1: type): Result[T1, E0] {.inline.} =
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## Convert result value to A using an conversion
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# Would be nice if it was automatic...
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if self.isOk: result.ok(T1(self.v))
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else: result.err(self.e)
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func mapCast*[T0, E0](
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self: Result[T0, E0], T1: type): Result[T1, E0] {.inline.} =
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## Convert result value to A using a cast
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## Would be nice with nicer syntax...
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if self.isOk: result.ok(cast[T1](self.v))
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else: result.err(self.e)
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template `and`*[T0, E, T1](self: Result[T0, E], other: Result[T1, E]): Result[T1, E] =
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## Evaluate `other` iff self.isOk, else return error
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## fail-fast - will not evaluate other if a is an error
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let s = self
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if s.isOk:
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other
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else:
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type R = type(other)
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R.err(s.e)
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template `or`*[T, E](self, other: Result[T, E]): Result[T, E] =
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## Evaluate `other` iff not self.isOk, else return self
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## fail-fast - will not evaluate other if a is a value
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let s = self
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if s.isOk: s
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else: other
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template catch*(body: typed): Result[type(body), ref CatchableError] =
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## Catch exceptions for body and store them in the Result
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##
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## ```
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## let r = catch: someFuncThatMayRaise()
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## ```
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type R = Result[type(body), ref CatchableError]
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try:
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R.ok(body)
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except CatchableError as e:
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R.err(e)
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template capture*[E: Exception](T: type, someExceptionExpr: ref E): Result[T, ref E] =
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## Evaluate someExceptionExpr and put the exception into a result, making sure
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## to capture a call stack at the capture site:
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##
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## ```
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## let e: Result[void, ValueError] = void.capture((ref ValueError)(msg: "test"))
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## echo e.error().getStackTrace()
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## ```
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type R = Result[T, ref E]
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var ret: R
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try:
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# TODO is this needed? I think so, in order to grab a call stack, but
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# haven't actually tested...
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if true:
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# I'm sure there's a nicer way - this just works :)
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raise someExceptionExpr
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except E as caught:
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ret = R.err(caught)
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ret
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func `==`*[T0, E0, T1, E1](lhs: Result[T0, E0], rhs: Result[T1, E1]): bool {.inline.} =
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if lhs.isOk != rhs.isOk:
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false
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elif lhs.isOk:
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lhs.v == rhs.v
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else:
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lhs.e == rhs.e
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func get*[T: not void, E](self: Result[T, E]): T {.inline.} =
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## Fetch value of result if set, or raise Defect
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## Exception bridge mode: raise given Exception instead
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## See also: Option.get
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checkOk(self)
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self.v
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func tryGet*[T: not void, E](self: Result[T, E]): T {.inline.} =
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## Fetch value of result if set, or raise
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## When E is an Exception, raise that exception - otherwise, raise a ResultError[E]
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if not self.isOk: self.raiseResultError
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self.v
|
||||
|
||||
func get*[T, E](self: Result[T, E], otherwise: T): T {.inline.} =
|
||||
## Fetch value of result if set, or return the value `otherwise`
|
||||
if self.isErr: otherwise
|
||||
else: self.v
|
||||
|
||||
func get*[T, E](self: var Result[T, E]): var T {.inline.} =
|
||||
## Fetch value of result if set, or raise Defect
|
||||
## Exception bridge mode: raise given Exception instead
|
||||
## See also: Option.get
|
||||
checkOk(self)
|
||||
self.v
|
||||
|
||||
template `[]`*[T, E](self: Result[T, E]): T =
|
||||
## Fetch value of result if set, or raise Defect
|
||||
## Exception bridge mode: raise given Exception instead
|
||||
self.get()
|
||||
|
||||
template `[]`*[T, E](self: var Result[T, E]): var T =
|
||||
## Fetch value of result if set, or raise Defect
|
||||
## Exception bridge mode: raise given Exception instead
|
||||
self.get()
|
||||
|
||||
template unsafeGet*[T, E](self: Result[T, E]): T =
|
||||
## Fetch value of result if set, undefined behavior if unset
|
||||
## See also: Option.unsafeGet
|
||||
assert isOk(self)
|
||||
self.v
|
||||
|
||||
func expect*[T: not void, E](self: Result[T, E], m: string): T =
|
||||
## Return value of Result, or raise a `Defect` with the given message - use
|
||||
## this helper to extract the value when an error is not expected, for example
|
||||
## because the program logic dictates that the operation should never fail
|
||||
##
|
||||
## ```nim
|
||||
## let r = Result[int, int].ok(42)
|
||||
## # Put here a helpful comment why you think this won't fail
|
||||
## echo r.expect("r was just set to ok(42)")
|
||||
## ```
|
||||
if not self.isOk():
|
||||
raiseResultDefect(m, self.error)
|
||||
self.v
|
||||
|
||||
func expect*[T: not void, E](self: var Result[T, E], m: string): var T =
|
||||
if not self.isOk():
|
||||
raiseResultDefect(m, self.error)
|
||||
self.v
|
||||
|
||||
func `$`*(self: Result): string =
|
||||
## Returns string representation of `self`
|
||||
if self.isOk: "Ok(" & $self.v & ")"
|
||||
else: "Err(" & $self.e & ")"
|
||||
|
||||
func error*[T, E](self: Result[T, E]): E =
|
||||
## Fetch error of result if set, or raise Defect
|
||||
if not self.isErr:
|
||||
when T is not void:
|
||||
raiseResultDefect("Trying to access error when value is set", self.v)
|
||||
else:
|
||||
raise (ref ResultDefect)(msg: "Trying to access error when value is set")
|
||||
|
||||
self.e
|
||||
|
||||
template value*[T, E](self: Result[T, E]): T = self.get()
|
||||
template value*[T, E](self: var Result[T, E]): T = self.get()
|
||||
|
||||
template valueOr*[T, E](self: Result[T, E], def: T): T =
|
||||
## Fetch value of result if set, or supplied default
|
||||
## default will not be evaluated iff value is set
|
||||
self.get(def)
|
||||
|
||||
# void support
|
||||
|
||||
template ok*[E](R: type Result[void, E]): auto =
|
||||
## Initialize a result with a success and value
|
||||
## Example: `Result[int, string].ok(42)`
|
||||
R(o: true)
|
||||
|
||||
template ok*[E](self: var Result[void, E]) =
|
||||
## Set the result to success and update value
|
||||
## Example: `result.ok(42)`
|
||||
self = (type self).ok()
|
||||
|
||||
template ok*(): auto = ok(typeof(result))
|
||||
template err*(): auto = err(typeof(result))
|
||||
|
||||
# TODO:
|
||||
# Supporting `map` and `get` operations on a `void` result is quite
|
||||
# an unusual API. We should provide some motivating examples.
|
||||
|
||||
func map*[E, A](
|
||||
self: Result[void, E], f: proc(): A): Result[A, E] {.inline.} =
|
||||
## Transform value using f, or return error
|
||||
if self.isOk: result.ok(f())
|
||||
else: result.err(self.e)
|
||||
|
||||
func flatMap*[E, A](
|
||||
self: Result[void, E], f: proc(): Result[A, E]): Result[A, E] {.inline.} =
|
||||
if self.isOk: f(self.v)
|
||||
else: Result[A, E].err(self.e)
|
||||
|
||||
func mapErr*[E, A](
|
||||
self: Result[void, E], f: proc(x: E): A): Result[void, A] {.inline.} =
|
||||
## Transform error using f, or return value
|
||||
if self.isOk: result.ok()
|
||||
else: result.err(f(self.e))
|
||||
|
||||
func map*[T, E](
|
||||
self: Result[T, E], f: proc(x: T)): Result[void, E] {.inline.} =
|
||||
## Transform value using f, or return error
|
||||
if self.isOk: f(self.v); result.ok()
|
||||
else: result.err(self.e)
|
||||
|
||||
func get*[E](self: Result[void, E]) {.inline.} =
|
||||
## Fetch value of result if set, or raise
|
||||
## See also: Option.get
|
||||
checkOk(self)
|
||||
|
||||
func tryGet*[E](self: Result[void, E]) {.inline.} =
|
||||
## Fetch value of result if set, or raise a CatchableError
|
||||
if not self.isOk: self.raiseResultError
|
||||
|
||||
template `[]`*[E](self: Result[void, E]) =
|
||||
## Fetch value of result if set, or raise
|
||||
self.get()
|
||||
|
||||
template unsafeGet*[E](self: Result[void, E]) =
|
||||
## Fetch value of result if set, undefined behavior if unset
|
||||
## See also: Option.unsafeGet
|
||||
assert not self.isErr
|
||||
|
||||
func expect*[E](self: Result[void, E], msg: string) =
|
||||
if not self.isOk():
|
||||
raise (ref ResultDefect)(msg: msg)
|
||||
|
||||
func `$`*[E](self: Result[void, E]): string =
|
||||
## Returns string representation of `self`
|
||||
if self.isOk: "Ok()"
|
||||
else: "Err(" & $self.e & ")"
|
||||
|
||||
template value*[E](self: Result[void, E]) = self.get()
|
||||
template value*[E](self: var Result[void, E]) = self.get()
|
||||
|
||||
template `?`*[T, E](self: Result[T, E]): T =
|
||||
## Early return - if self is an error, we will return from the current
|
||||
## function, else we'll move on..
|
||||
##
|
||||
## ```
|
||||
## let v = ? funcWithResult()
|
||||
## echo v # prints value, not Result!
|
||||
## ```
|
||||
## Experimental
|
||||
# TODO the v copy is here to prevent multiple evaluations of self - could
|
||||
# probably avoid it with some fancy macro magic..
|
||||
let v = (self)
|
||||
if v.isErr: return err(typeof(result), v.error)
|
||||
|
||||
v.value
|
||||
{.deprecated: "import results".}
|
||||
|
|
|
|||
|
|
@ -0,0 +1,575 @@
|
|||
# nim-result is also available stand-alone from https://github.com/arnetheduck/nim-result/
|
||||
|
||||
# Copyright (c) 2019 Jacek Sieka
|
||||
# Licensed and distributed under either of
|
||||
# * MIT license (license terms in the root directory or at http://opensource.org/licenses/MIT).
|
||||
# * Apache v2 license (license terms in the root directory or at http://www.apache.org/licenses/LICENSE-2.0).
|
||||
# at your option. This file may not be copied, modified, or distributed except according to those terms.
|
||||
|
||||
type
|
||||
ResultError*[E] = object of ValueError
|
||||
## Error raised when using `tryGet` value of result when error is set
|
||||
## See also Exception bridge mode
|
||||
error*: E
|
||||
|
||||
ResultDefect* = object of Defect
|
||||
## Defect raised when accessing value when error is set and vice versa
|
||||
## See also Exception bridge mode
|
||||
|
||||
Result*[T, E] = object
|
||||
## Result type that can hold either a value or an error, but not both
|
||||
##
|
||||
## # Example
|
||||
##
|
||||
## ```
|
||||
## # It's convenient to create an alias - most likely, you'll do just fine
|
||||
## # with strings or cstrings as error
|
||||
##
|
||||
## type R = Result[int, string]
|
||||
##
|
||||
## # Once you have a type, use `ok` and `err`:
|
||||
##
|
||||
## func works(): R =
|
||||
## # ok says it went... ok!
|
||||
## R.ok 42
|
||||
## func fails(): R =
|
||||
## # or type it like this, to not repeat the type!
|
||||
## result.err "bad luck"
|
||||
##
|
||||
## if (let w = works(); w.isOk):
|
||||
## echo w[], " or use value: ", w.value
|
||||
##
|
||||
## # In case you think your callers want to differentiate between errors:
|
||||
## type
|
||||
## Error = enum
|
||||
## a, b, c
|
||||
## type RE[T] = Result[T, Error]
|
||||
##
|
||||
## # In the expriments corner, you'll find the following syntax for passing
|
||||
## # errors up the stack:
|
||||
## func f(): R =
|
||||
## let x = ?works() - ?fails()
|
||||
## assert false, "will never reach"
|
||||
##
|
||||
## # If you provide this exception converter, this exception will be raised
|
||||
## # on dereference
|
||||
## func toException(v: Error): ref CatchableError = (ref CatchableError)(msg: $v)
|
||||
## try:
|
||||
## RE[int].err(a)[]
|
||||
## except CatchableError:
|
||||
## echo "in here!"
|
||||
##
|
||||
## ```
|
||||
##
|
||||
## See the tests for more practical examples, specially when working with
|
||||
## back and forth with the exception world!
|
||||
##
|
||||
## # Potential benefits:
|
||||
##
|
||||
## * Handling errors becomes explicit and mandatory at the call site -
|
||||
## goodbye "out of sight, out of mind"
|
||||
## * Errors are a visible part of the API - when they change, so must the
|
||||
## calling code and compiler will point this out - nice!
|
||||
## * Errors are a visible part of the API - your fellow programmer is
|
||||
## reminded that things actually can go wrong
|
||||
## * Jives well with Nim `discard`
|
||||
## * Jives well with the new Defect exception hierarchy, where defects
|
||||
## are raised for unrecoverable errors and the rest of the API uses
|
||||
## results
|
||||
## * Error and value return have similar performance characteristics
|
||||
## * Caller can choose to turn them into exceptions at low cost - flexible
|
||||
## for libraries!
|
||||
## * Mostly relies on simple Nim features - though this library is no
|
||||
## exception in that compiler bugs were discovered writing it :)
|
||||
##
|
||||
## # Potential costs:
|
||||
##
|
||||
## * Handling errors becomes explicit and mandatory - if you'd rather ignore
|
||||
## them or just pass them to some catch-all, this is noise
|
||||
## * When composing operations, value must be lifted before processing,
|
||||
## adding potential verbosity / noise (fancy macro, anyone?)
|
||||
## * There's no call stack captured by default (see also `catch` and
|
||||
## `capture`)
|
||||
## * The extra branching may be more expensive for the non-error path
|
||||
## (though this can be minimized with PGO)
|
||||
##
|
||||
## The API visibility issue of exceptions can also be solved with
|
||||
## `{.raises.}` annotations - as of now, the compiler doesn't remind
|
||||
## you to do so, even though it knows what the right annotation should be.
|
||||
## `{.raises.}` does not participate in generic typing, making it just as
|
||||
## verbose but less flexible in some ways, if you want to type it out.
|
||||
##
|
||||
## Many system languages make a distinction between errors you want to
|
||||
## handle and those that are simply bugs or unrealistic to deal with..
|
||||
## handling the latter will often involve aborting or crashing the funcess -
|
||||
## reliable systems like Erlang will try to relaunch it.
|
||||
##
|
||||
## On the flip side we have dynamic languages like python where there's
|
||||
## nothing exceptional about exceptions (hello StopIterator). Python is
|
||||
## rarely used to build reliable systems - its strengths lie elsewhere.
|
||||
##
|
||||
## # Exception bridge mode
|
||||
##
|
||||
## When the error of a `Result` is an `Exception`, or a `toException` helper
|
||||
## is present for your error type, the "Exception bridge mode" is
|
||||
## enabled and instead of raising `Defect`, we will raise the given
|
||||
## `Exception` on access.
|
||||
##
|
||||
## This is an experimental feature that may be removed.
|
||||
##
|
||||
## # Other languages
|
||||
##
|
||||
## Result-style error handling seems pretty popular lately, specially with
|
||||
## statically typed languages:
|
||||
## Haskell: https://hackage.haskell.org/package/base-4.11.1.0/docs/Data-Either.html
|
||||
## Rust: https://doc.rust-lang.org/std/result/enum.Result.html
|
||||
## Modern C++: https://github.com/viboes/std-make/tree/master/doc/proposal/expected
|
||||
## More C++: https://github.com/ned14/outcome
|
||||
##
|
||||
## Swift is interesting in that it uses a non-exception implementation but
|
||||
## calls errors exceptions and has lots of syntactic sugar to make them feel
|
||||
## that way by implicitly passing them up the call chain - with a mandatory
|
||||
## annotation that function may throw:
|
||||
## https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/ErrorHandling.html
|
||||
##
|
||||
## # Considerations for the error type
|
||||
##
|
||||
## * Use a `string` or a `cstring` if you want to provide a diagnostic for
|
||||
## the caller without an expectation that they will differentiate between
|
||||
## different errors. Callers should never parse the given string!
|
||||
## * Use an `enum` to provide in-depth errors where the caller is expected
|
||||
## to have different logic for different errors
|
||||
## * Use a complex type to include error-specific meta-data - or make the
|
||||
## meta-data collection a visible part of your API in another way - this
|
||||
## way it remains discoverable by the caller!
|
||||
##
|
||||
## A natural "error API" progression is starting with `Option[T]`, then
|
||||
## `Result[T, cstring]`, `Result[T, enum]` and `Result[T, object]` in
|
||||
## escalating order of complexity.
|
||||
##
|
||||
## # Other implemenations in nim
|
||||
##
|
||||
## There are other implementations in nim that you might prefer:
|
||||
## * Either from nimfp: https://github.com/vegansk/nimfp/blob/master/src/fp/either.nim
|
||||
## * result_type: https://github.com/kapralos/result_type/
|
||||
##
|
||||
## # Implementation notes
|
||||
##
|
||||
## This implementation is mostly based on the one in rust. Compared to it,
|
||||
## there are a few differences - if know of creative ways to improve things,
|
||||
## I'm all ears.
|
||||
##
|
||||
## * Rust has the enum variants which lend themselves to nice construction
|
||||
## where the full Result type isn't needed: `Err("some error")` doesn't
|
||||
## need to know value type - maybe some creative converter or something
|
||||
## can deal with this?
|
||||
## * Nim templates allow us to fail fast without extra effort, meaning the
|
||||
## other side of `and`/`or` isn't evaluated unless necessary - nice!
|
||||
## * Rust uses From traits to deal with result translation as the result
|
||||
## travels up the call stack - needs more tinkering - some implicit
|
||||
## conversions would be nice here
|
||||
## * Pattern matching in rust allows convenient extraction of value or error
|
||||
## in one go.
|
||||
##
|
||||
## # Performance considerations
|
||||
##
|
||||
## When returning a Result instead of a simple value, there are a few things
|
||||
## to take into consideration - in general, we are returning more
|
||||
## information directly to the caller which has an associated cost.
|
||||
##
|
||||
## Result is a value type, thus its performance characteristics
|
||||
## generally follow the performance of copying the value or error that
|
||||
## it stores. `Result` would benefit greatly from "move" support in the
|
||||
## language.
|
||||
##
|
||||
## In many cases, these performance costs are negligeable, but nonetheless
|
||||
## they are important to be aware of, to structure your code in an efficient
|
||||
## manner:
|
||||
##
|
||||
## * Memory overhead
|
||||
## Result is stored in memory as a union with a `bool` discriminator -
|
||||
## alignment makes it somewhat tricky to give an exact size, but in
|
||||
## general, `Result[int, int]` will take up `2*sizeof(int)` bytes:
|
||||
## 1 `int` for the discriminator and padding, 1 `int` for either the value
|
||||
## or the error. The additional size means that returning may take up more
|
||||
## registers or spill onto the stack.
|
||||
## * Loss of RVO
|
||||
## Nim does return-value-optimization by rewriting `proc f(): X` into
|
||||
## `proc f(result: var X)` - in an expression like `let x = f()`, this
|
||||
## allows it to avoid a copy from the "temporary" return value to `x` -
|
||||
## when using Result, this copy currently happens always because you need
|
||||
## to fetch the value from the Result in a second step: `let x = f().value`
|
||||
## * Extra copies
|
||||
## To avoid spurious evaluation of expressions in templates, we use a
|
||||
## temporary variable sometimes - this means an unnecessary copy for some
|
||||
## types.
|
||||
## * Bad codegen
|
||||
## When doing RVO, Nim generates poor and slow code: it uses a construct
|
||||
## called `genericReset` that will zero-initialize a value using dynamic
|
||||
## RTTI - a process that the C compiler subsequently is unable to
|
||||
## optimize. This applies to all types, but is exacerbated with Result
|
||||
## because of its bigger footprint - this should be fixed in compiler.
|
||||
## * Double zero-initialization bug
|
||||
## Nim has an initialization bug that causes additional poor performance:
|
||||
## `var x = f()` will be expanded into `var x; zeroInit(x); f(x)` where
|
||||
## `f(x)` will call the slow `genericReset` and zero-init `x` again,
|
||||
## unnecessarily.
|
||||
##
|
||||
## Comparing `Result` performance to exceptions in Nim is difficult - the
|
||||
## specific performance will depend on the error type used, the frequency
|
||||
## at which exceptions happen, the amount of error handling code in the
|
||||
## application and the compiler and backend used.
|
||||
##
|
||||
## * the default C backend in nim uses `setjmp` for exception handling -
|
||||
## the relative performance of the happy path will depend on the structure
|
||||
## of the code: how many exception handlers there are, how much unwinding
|
||||
## happens. `setjmp` works by taking a snapshot of the full CPU state and
|
||||
## saving it in memory when enterting a try block (or an implict try
|
||||
## block, such as is introduced with `defer` and similar constructs) which
|
||||
## is an expensive operation.
|
||||
## * an efficient exception handling mechanism (like the C++ backend or
|
||||
## `nlvm`) will usually have a lower cost on the happy path because the
|
||||
## value can be returned more efficiently. However, there is still a code
|
||||
## and data size increase depending on the specific situation, as well as
|
||||
## loss of optimization opportunities to consider.
|
||||
## * raising an exception is usually (a lot) slower than returning an error
|
||||
## through a Result - at raise time, capturing a call stack and allocating
|
||||
## memory for the Exception is expensive, so the performance difference
|
||||
## comes down to the complexity of the error type used.
|
||||
## * checking for errors with Result is local branching operation that
|
||||
## happens on the happy path - even if all that is done is passing the
|
||||
## error to the next layer - when errors happen rarely, this may be a cost.
|
||||
##
|
||||
## An accurate summary might be that Exceptions are at its most efficient
|
||||
## when errors are not handled and don't happen.
|
||||
##
|
||||
## # Relevant nim bugs
|
||||
##
|
||||
## https://github.com/nim-lang/Nim/issues/13799 - type issues
|
||||
## https://github.com/nim-lang/Nim/issues/8745 - genericReset slow
|
||||
## https://github.com/nim-lang/Nim/issues/13879 - double-zero-init slow
|
||||
|
||||
case o: bool
|
||||
of false:
|
||||
e: E
|
||||
of true:
|
||||
v: T
|
||||
|
||||
func raiseResultError[T, E](self: Result[T, E]) {.noreturn, noinline.} =
|
||||
# noinline because raising should take as little space as possible at call
|
||||
# site
|
||||
mixin toException
|
||||
|
||||
when E is ref Exception:
|
||||
if self.e.isNil: # for example Result.default()!
|
||||
raise (ref ResultError[void])(msg: "Trying to access value with err (nil)")
|
||||
raise self.e
|
||||
elif compiles(toException(self.e)):
|
||||
raise toException(self.e)
|
||||
elif compiles($self.e):
|
||||
raise (ref ResultError[E])(
|
||||
error: self.e, msg: "Trying to access value with err: " & $self.e)
|
||||
else:
|
||||
raise (res ResultError[E])(msg: "Trying to access value with err", error: self.e)
|
||||
|
||||
func raiseResultDefect(m: string, v: auto) {.noreturn, noinline.} =
|
||||
if compiles($v): raise (ref ResultDefect)(msg: m & ": " & $v)
|
||||
else: raise (ref ResultDefect)(msg: m)
|
||||
|
||||
template checkOk(self: Result) =
|
||||
# TODO This condition is a bit odd in that it raises different exceptions
|
||||
# depending on the type of E - this is done to support using Result as a
|
||||
# bridge type that can transport Exceptions
|
||||
mixin toException
|
||||
if not self.isOk:
|
||||
when E is ref Exception or compiles(toException(self.e)):
|
||||
raiseResultError(self)
|
||||
else:
|
||||
raiseResultDefect("Trying to acces value with err Result", self.e)
|
||||
|
||||
template ok*[T, E](R: type Result[T, E], x: auto): R =
|
||||
## Initialize a result with a success and value
|
||||
## Example: `Result[int, string].ok(42)`
|
||||
R(o: true, v: x)
|
||||
|
||||
template ok*[T, E](self: var Result[T, E], x: auto) =
|
||||
## Set the result to success and update value
|
||||
## Example: `result.ok(42)`
|
||||
self = ok(type self, x)
|
||||
|
||||
template err*[T, E](R: type Result[T, E], x: auto): R =
|
||||
## Initialize the result to an error
|
||||
## Example: `Result[int, string].err("uh-oh")`
|
||||
R(o: false, e: x)
|
||||
|
||||
template err*[T, E](self: var Result[T, E], x: auto) =
|
||||
## Set the result as an error
|
||||
## Example: `result.err("uh-oh")`
|
||||
self = err(type self, x)
|
||||
|
||||
template ok*(v: auto): auto = ok(typeof(result), v)
|
||||
template err*(v: auto): auto = err(typeof(result), v)
|
||||
|
||||
template isOk*(self: Result): bool = self.o
|
||||
template isErr*(self: Result): bool = not self.o
|
||||
|
||||
func map*[T, E, A](
|
||||
self: Result[T, E], f: proc(x: T): A): Result[A, E] {.inline.} =
|
||||
## Transform value using f, or return error
|
||||
if self.isOk: result.ok(f(self.v))
|
||||
else: result.err(self.e)
|
||||
|
||||
func flatMap*[T, E, A](
|
||||
self: Result[T, E], f: proc(x: T): Result[A, E]): Result[A, E] {.inline.} =
|
||||
if self.isOk: f(self.v)
|
||||
else: Result[A, E].err(self.e)
|
||||
|
||||
func mapErr*[T: not void, E, A](
|
||||
self: Result[T, E], f: proc(x: E): A): Result[T, A] {.inline.} =
|
||||
## Transform error using f, or return value
|
||||
if self.isOk: result.ok(self.v)
|
||||
else: result.err(f(self.e))
|
||||
|
||||
func mapConvert*[T0, E0](
|
||||
self: Result[T0, E0], T1: type): Result[T1, E0] {.inline.} =
|
||||
## Convert result value to A using an conversion
|
||||
# Would be nice if it was automatic...
|
||||
if self.isOk: result.ok(T1(self.v))
|
||||
else: result.err(self.e)
|
||||
|
||||
func mapCast*[T0, E0](
|
||||
self: Result[T0, E0], T1: type): Result[T1, E0] {.inline.} =
|
||||
## Convert result value to A using a cast
|
||||
## Would be nice with nicer syntax...
|
||||
if self.isOk: result.ok(cast[T1](self.v))
|
||||
else: result.err(self.e)
|
||||
|
||||
template `and`*[T0, E, T1](self: Result[T0, E], other: Result[T1, E]): Result[T1, E] =
|
||||
## Evaluate `other` iff self.isOk, else return error
|
||||
## fail-fast - will not evaluate other if a is an error
|
||||
let s = self
|
||||
if s.isOk:
|
||||
other
|
||||
else:
|
||||
type R = type(other)
|
||||
R.err(s.e)
|
||||
|
||||
template `or`*[T, E](self, other: Result[T, E]): Result[T, E] =
|
||||
## Evaluate `other` iff not self.isOk, else return self
|
||||
## fail-fast - will not evaluate other if a is a value
|
||||
let s = self
|
||||
if s.isOk: s
|
||||
else: other
|
||||
|
||||
template catch*(body: typed): Result[type(body), ref CatchableError] =
|
||||
## Catch exceptions for body and store them in the Result
|
||||
##
|
||||
## ```
|
||||
## let r = catch: someFuncThatMayRaise()
|
||||
## ```
|
||||
type R = Result[type(body), ref CatchableError]
|
||||
|
||||
try:
|
||||
R.ok(body)
|
||||
except CatchableError as e:
|
||||
R.err(e)
|
||||
|
||||
template capture*[E: Exception](T: type, someExceptionExpr: ref E): Result[T, ref E] =
|
||||
## Evaluate someExceptionExpr and put the exception into a result, making sure
|
||||
## to capture a call stack at the capture site:
|
||||
##
|
||||
## ```
|
||||
## let e: Result[void, ValueError] = void.capture((ref ValueError)(msg: "test"))
|
||||
## echo e.error().getStackTrace()
|
||||
## ```
|
||||
type R = Result[T, ref E]
|
||||
|
||||
var ret: R
|
||||
try:
|
||||
# TODO is this needed? I think so, in order to grab a call stack, but
|
||||
# haven't actually tested...
|
||||
if true:
|
||||
# I'm sure there's a nicer way - this just works :)
|
||||
raise someExceptionExpr
|
||||
except E as caught:
|
||||
ret = R.err(caught)
|
||||
ret
|
||||
|
||||
func `==`*[T0, E0, T1, E1](lhs: Result[T0, E0], rhs: Result[T1, E1]): bool {.inline.} =
|
||||
if lhs.isOk != rhs.isOk:
|
||||
false
|
||||
elif lhs.isOk:
|
||||
lhs.v == rhs.v
|
||||
else:
|
||||
lhs.e == rhs.e
|
||||
|
||||
func get*[T: not void, E](self: Result[T, E]): T {.inline.} =
|
||||
## Fetch value of result if set, or raise Defect
|
||||
## Exception bridge mode: raise given Exception instead
|
||||
## See also: Option.get
|
||||
checkOk(self)
|
||||
self.v
|
||||
|
||||
func tryGet*[T: not void, E](self: Result[T, E]): T {.inline.} =
|
||||
## Fetch value of result if set, or raise
|
||||
## When E is an Exception, raise that exception - otherwise, raise a ResultError[E]
|
||||
if not self.isOk: self.raiseResultError
|
||||
self.v
|
||||
|
||||
func get*[T, E](self: Result[T, E], otherwise: T): T {.inline.} =
|
||||
## Fetch value of result if set, or return the value `otherwise`
|
||||
if self.isErr: otherwise
|
||||
else: self.v
|
||||
|
||||
func get*[T, E](self: var Result[T, E]): var T {.inline.} =
|
||||
## Fetch value of result if set, or raise Defect
|
||||
## Exception bridge mode: raise given Exception instead
|
||||
## See also: Option.get
|
||||
checkOk(self)
|
||||
self.v
|
||||
|
||||
template `[]`*[T, E](self: Result[T, E]): T =
|
||||
## Fetch value of result if set, or raise Defect
|
||||
## Exception bridge mode: raise given Exception instead
|
||||
self.get()
|
||||
|
||||
template `[]`*[T, E](self: var Result[T, E]): var T =
|
||||
## Fetch value of result if set, or raise Defect
|
||||
## Exception bridge mode: raise given Exception instead
|
||||
self.get()
|
||||
|
||||
template unsafeGet*[T, E](self: Result[T, E]): T =
|
||||
## Fetch value of result if set, undefined behavior if unset
|
||||
## See also: Option.unsafeGet
|
||||
assert isOk(self)
|
||||
self.v
|
||||
|
||||
func expect*[T: not void, E](self: Result[T, E], m: string): T =
|
||||
## Return value of Result, or raise a `Defect` with the given message - use
|
||||
## this helper to extract the value when an error is not expected, for example
|
||||
## because the program logic dictates that the operation should never fail
|
||||
##
|
||||
## ```nim
|
||||
## let r = Result[int, int].ok(42)
|
||||
## # Put here a helpful comment why you think this won't fail
|
||||
## echo r.expect("r was just set to ok(42)")
|
||||
## ```
|
||||
if not self.isOk():
|
||||
raiseResultDefect(m, self.error)
|
||||
self.v
|
||||
|
||||
func expect*[T: not void, E](self: var Result[T, E], m: string): var T =
|
||||
if not self.isOk():
|
||||
raiseResultDefect(m, self.error)
|
||||
self.v
|
||||
|
||||
func `$`*(self: Result): string =
|
||||
## Returns string representation of `self`
|
||||
if self.isOk: "Ok(" & $self.v & ")"
|
||||
else: "Err(" & $self.e & ")"
|
||||
|
||||
func error*[T, E](self: Result[T, E]): E =
|
||||
## Fetch error of result if set, or raise Defect
|
||||
if not self.isErr:
|
||||
when T is not void:
|
||||
raiseResultDefect("Trying to access error when value is set", self.v)
|
||||
else:
|
||||
raise (ref ResultDefect)(msg: "Trying to access error when value is set")
|
||||
|
||||
self.e
|
||||
|
||||
template value*[T, E](self: Result[T, E]): T = self.get()
|
||||
template value*[T, E](self: var Result[T, E]): T = self.get()
|
||||
|
||||
template valueOr*[T, E](self: Result[T, E], def: T): T =
|
||||
## Fetch value of result if set, or supplied default
|
||||
## default will not be evaluated iff value is set
|
||||
self.get(def)
|
||||
|
||||
# void support
|
||||
|
||||
template ok*[E](R: type Result[void, E]): auto =
|
||||
## Initialize a result with a success and value
|
||||
## Example: `Result[int, string].ok(42)`
|
||||
R(o: true)
|
||||
|
||||
template ok*[E](self: var Result[void, E]) =
|
||||
## Set the result to success and update value
|
||||
## Example: `result.ok(42)`
|
||||
self = (type self).ok()
|
||||
|
||||
template ok*(): auto = ok(typeof(result))
|
||||
template err*(): auto = err(typeof(result))
|
||||
|
||||
# TODO:
|
||||
# Supporting `map` and `get` operations on a `void` result is quite
|
||||
# an unusual API. We should provide some motivating examples.
|
||||
|
||||
func map*[E, A](
|
||||
self: Result[void, E], f: proc(): A): Result[A, E] {.inline.} =
|
||||
## Transform value using f, or return error
|
||||
if self.isOk: result.ok(f())
|
||||
else: result.err(self.e)
|
||||
|
||||
func flatMap*[E, A](
|
||||
self: Result[void, E], f: proc(): Result[A, E]): Result[A, E] {.inline.} =
|
||||
if self.isOk: f(self.v)
|
||||
else: Result[A, E].err(self.e)
|
||||
|
||||
func mapErr*[E, A](
|
||||
self: Result[void, E], f: proc(x: E): A): Result[void, A] {.inline.} =
|
||||
## Transform error using f, or return value
|
||||
if self.isOk: result.ok()
|
||||
else: result.err(f(self.e))
|
||||
|
||||
func map*[T, E](
|
||||
self: Result[T, E], f: proc(x: T)): Result[void, E] {.inline.} =
|
||||
## Transform value using f, or return error
|
||||
if self.isOk: f(self.v); result.ok()
|
||||
else: result.err(self.e)
|
||||
|
||||
func get*[E](self: Result[void, E]) {.inline.} =
|
||||
## Fetch value of result if set, or raise
|
||||
## See also: Option.get
|
||||
checkOk(self)
|
||||
|
||||
func tryGet*[E](self: Result[void, E]) {.inline.} =
|
||||
## Fetch value of result if set, or raise a CatchableError
|
||||
if not self.isOk: self.raiseResultError
|
||||
|
||||
template `[]`*[E](self: Result[void, E]) =
|
||||
## Fetch value of result if set, or raise
|
||||
self.get()
|
||||
|
||||
template unsafeGet*[E](self: Result[void, E]) =
|
||||
## Fetch value of result if set, undefined behavior if unset
|
||||
## See also: Option.unsafeGet
|
||||
assert not self.isErr
|
||||
|
||||
func expect*[E](self: Result[void, E], msg: string) =
|
||||
if not self.isOk():
|
||||
raise (ref ResultDefect)(msg: msg)
|
||||
|
||||
func `$`*[E](self: Result[void, E]): string =
|
||||
## Returns string representation of `self`
|
||||
if self.isOk: "Ok()"
|
||||
else: "Err(" & $self.e & ")"
|
||||
|
||||
template value*[E](self: Result[void, E]) = self.get()
|
||||
template value*[E](self: var Result[void, E]) = self.get()
|
||||
|
||||
template `?`*[T, E](self: Result[T, E]): T =
|
||||
## Early return - if self is an error, we will return from the current
|
||||
## function, else we'll move on..
|
||||
##
|
||||
## ```
|
||||
## let v = ? funcWithResult()
|
||||
## echo v # prints value, not Result!
|
||||
## ```
|
||||
## Experimental
|
||||
# TODO the v copy is here to prevent multiple evaluations of self - could
|
||||
# probably avoid it with some fancy macro magic..
|
||||
let v = (self)
|
||||
if v.isErr: return err(typeof(result), v.error)
|
||||
|
||||
v.value
|
||||
|
|
@ -18,5 +18,5 @@ import
|
|||
test_endians2,
|
||||
test_objects,
|
||||
test_ptrops,
|
||||
test_result,
|
||||
test_results,
|
||||
test_varints
|
||||
|
|
|
|||
|
|
@ -1,6 +1,6 @@
|
|||
# nim-result is also available stand-alone from https://github.com/arnetheduck/nim-result/
|
||||
|
||||
import ../stew/result
|
||||
import ../stew/results
|
||||
type R = Result[int, string]
|
||||
|
||||
# Basic usage, producer
|
||||
Loading…
Reference in New Issue