Questionable 🤔
==============

[Option][1] and [Result][2] are two powerful abstractions that can be used
instead of raising errors. They can be a bit unwieldy though. This library is an
attempt at making their use a bit more elegant.

Installation
------------

Use the [Nimble][3] package manager to add `questionable` to an existing
project. Add the following to its .nimble file:

```nim
requires "questionable >= 0.4.0 & < 0.5.0"
```

If you want to make use of Result types, then you also have to add either the
[result][2] package, or the [stew][4] package:

```nim
requires "result" # either this
requires "stew"   # or this
```

Options
-------

You can use `?` to make a type optional. For example, the type `?int` is just
short for [`Option[int]`][1].

```nim
import questionable

var x: ?int
```

Assigning values is done using the `some` and `none` procs from the standard library:

```nim
x = 42.some   # Option x now holds the value 42
x = int.none  # Option x no longer holds a value
```
### Option binding

The `=?` operator lets you bind the value inside an Option to a new variable. It
can be used inside of a conditional expression, for instance in an `if`
statement:

```nim
x = 42.some

if y =? x:
  # y equals 42 here
else:
  # this is never reached

x = int.none

if y =? x:
  # this is never reached
else:
  # this is reached, and y is not defined
```

### Option chaining

To safely access fields and call procs, you can use the `?.` operator:

> Note: before version 0.3.0, the operator was `.?` instead of `?.`

```nim
var numbers: ?seq[int]
var amount: ?int

numbers = @[1, 2, 3].some
amount = numbers?.len
# amount now holds the integer 3

numbers = seq[int].none
amount = numbers?.len
# amount now equals int.none
```

Invocations of the `?.` operator can be chained:
```nim
import sequtils

numbers = @[1, 1, 2, 2, 2].some
amount = numbers?.deduplicate?.len
# amount now holds the integer 2
```

### Fallback values

Use the `|?` operator to supply a fallback value when the Option does not hold
a value:

```nim
x = int.none

let z = x |? 3
# z equals 3
```

### Operators

The operators `[]`, `-`, `+`, `@`, `*`, `/`, `div`, `mod`, `shl`, `shr`, `&`,
`<=`, `<`, `>=`, `>` are all lifted, so they can be used directly on Options:

```nim
numbers = @[1, 2, 3].some
x = 39.some

let indexed = numbers[0]  # equals 1.some
let sum = x + 3           # equals 42.some
```

Results
-------

Support for `Result` is considered experimental. If you want to use them you
have to explicitly import the `questionable/results` module:

```nim
import questionable/results
```

You can use `?!` make a Result type. These Result types either hold a value or
an error. For example the type `?!int` is short for `Result[int, ref
CatchableError]`.

```nim
proc example: ?!int =
  # either return an integer or an error
```

Assigning values is done using the `success` and `failure` procs:

```nim
proc works: ?!seq[int] =
  # always returns a Result holding a sequence
  @[1, 1, 2, 2, 2].success

proc fails: ?!seq[int] =
  # always returns a Result holding a ValueError
  seq[int].failure newException(ValueError, "something went wrong")
```

### Binding, chaining, fallbacks and operators

Binding with the `=?` operator, chaining with the `?.` operator, fallbacks with
the `|?` operator, and all the other operators that work with Options also work
for Results:
```nim
import sequtils

# binding:
if x =? works():
  # use x

# chaining:
let amount = works()?.deduplicate?.len

# fallback values:
let value = fails() |? @[]

# lifted operators:
let sum = works()[3] + 40
```

### Catching errors

When you want to use Results, but need to call a proc that may raise an
error, you can use `catch`:

```nim
import strutils

let x = parseInt("42").catch  # equals 42.success
let y = parseInt("XX").catch  # equals int.failure(..)
```

### Conversion to Option

Any Result can be converted to an Option:

```nim
let converted = works().option  # equals @[1, 1, 2, 2, 2].some
```

Banning Errors
--------------

Should you decide to use Options and Results instead of the standard exception
handling in Nim, you may want to check that your procs are not accidently
raising errors. You can use the following include for this:

```nim
include questionable/errorban
```

Proc definitions below the error ban are checked by the compiler to ensure that
they do not raise errors.

[1]: https://nim-lang.org/docs/options.html
[2]: https://github.com/arnetheduck/nim-result
[3]: https://github.com/nim-lang/nimble
[4]: https://github.com/status-im/nim-stew