Add errorOption example to Readme

Added example for errorOption conversion in Readme.

Readme.md

@@ -151,7 +151,7 @@ have to explicitly import the `questionable/results` module:
 import questionable/results
 ```
 
-You can use `?!` make a Result type. These Result types either hold a value or
+You can use `?!` to 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]`.
 
@@ -226,6 +226,7 @@ Any Result can be converted to an Option:
 
 ```nim
 let converted = works().option  # equals @[1, 1, 2, 2, 2].some
+let errOption = fails().errorOption # option that is set when the Result holds an error
 ```
 
 [1]: https://nim-lang.org/docs/options.html

questionable/private/binderror.nim

@@ -23,16 +23,16 @@ macro captureBindError*(error: var ref CatchableError, expression): auto =
     # return the evaluated result
     `evaluated`
 
-func unsafeError[T](_: Option[T]): ref CatchableError =
+func unsafeCatchableError[T](_: Option[T]): ref CatchableError =
   newException(ValueError, "Option is set to `none`")
 
-func unsafeError[T](_: ref T): ref CatchableError =
+func unsafeCatchableError[T](_: ref T): ref CatchableError =
   newException(ValueError, "ref is nil")
 
-func unsafeError[T](_: ptr T): ref CatchableError =
+func unsafeCatchableError[T](_: ptr T): ref CatchableError =
   newException(ValueError, "ptr is nil")
 
-func unsafeError[Proc: proc | iterator](_: Proc): ref CatchableError =
+func unsafeCatchableError[Proc: proc | iterator](_: Proc): ref CatchableError =
   newException(ValueError, "proc or iterator is nil")
 
 macro bindFailed*(expression: typed) =
@@ -51,4 +51,4 @@ macro bindFailed*(expression: typed) =
       # check that the error variable is in scope
       when compiles(`errorVariable`):
         # assign bind error to error variable
-        `errorVariable` = `expression`.unsafeError
+        `errorVariable` = `expression`.unsafeCatchableError

questionable/results.nim

@@ -123,6 +123,18 @@ template toOption*[T, E](value: Result[T, E]): ?T =
 
   value.option
 
+proc unsafeCatchableError*[T, E](value: Result[T, E]): ref CatchableError =
+  ## Returns the error from the Result, converted to `ref CatchableError` if
+  ## necessary. Behaviour is undefined when the result holds a value instead of
+  ## an error.
+  when E is ref CatchableError:
+    value.unsafeError
+  else:
+    when compiles($value.unsafeError):
+      newException(ResultFailure, $value.unsafeError)
+    else:
+      newException(ResultFailure, "Result is an error")
+
 proc errorOption*[T, E](value: Result[T, E]): ?E =
   ## Returns an Option that contains the error from the Result, if it has one.
 

testmodules/results/test.nim

@@ -663,7 +663,7 @@ suite "result compatibility":
 
   type R = Result[int, string]
   let good = R.ok 42
-  let bad = R.err "error"
+  let bad = R.err "some error"
 
   test "|?, =? and .option work on other types of Result":
     check bad |? 43 == 43
@@ -681,3 +681,13 @@ suite "result compatibility":
       fail
     without b =? good:
       fail
+
+  test "without statement with error works on other type of Result":
+    without value =? bad, error:
+      check error of ResultFailure
+      check error.msg == "some error"
+
+  test "without statement with error works on Result[T, void]":
+    without value =? Result[int, void].err, error:
+      check error of ResultFailure
+      check error.msg == "Result is an error"