Conditional continuation operator for options #9161
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| Original file line number | Diff line number | Diff line change | ||||||
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| # | ||||||||
| # | ||||||||
| # Nim's Runtime Library | ||||||||
| # (c) Copyright 2015 Nim Contributors | ||||||||
| # | ||||||||
| # See the file "copying.txt", included in this | ||||||||
| # distribution, for details about the copyright. | ||||||||
| # | ||||||||
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| ## This module, previously a part of ``options``, implements some more advanced | ||||||||
| ## ways to interact with options. It includes conditional mapping of procedures | ||||||||
| ## over an option, flattening of nested options, and filtering of values within | ||||||||
| ## options. It also includes an existential operator that works like regular | ||||||||
| ## dot-chaining but stops if the left hand side is a none-option. | ||||||||
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| import options, macros | ||||||||
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| proc map*[T](self: Option[T], callback: proc (input: T)) = | ||||||||
| ## Applies a callback to the value in this Option | ||||||||
| if self.isSome: | ||||||||
| callback(self.unsafeGet) | ||||||||
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| proc map*[T, R](self: Option[T], callback: proc (input: T): R): Option[R] = | ||||||||
| ## Applies a callback to the value in this Option and returns an option | ||||||||
| ## containing the new value. If this option is None, None will be returned | ||||||||
| if self.isSome: | ||||||||
| some[R]( callback(self.unsafeGet) ) | ||||||||
| else: | ||||||||
| none(R) | ||||||||
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| proc flatten*[A](self: Option[Option[A]]): Option[A] = | ||||||||
| ## Remove one level of structure in a nested Option. | ||||||||
| if self.isSome: | ||||||||
| self.unsafeGet | ||||||||
| else: | ||||||||
| none(A) | ||||||||
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| proc flatMap*[A, B](self: Option[A], callback: proc (input: A): Option[B]): Option[B] = | ||||||||
| ## Applies a callback to the value in this Option and returns an | ||||||||
| ## option containing the new value. If this option is None, None will be | ||||||||
| ## returned. Similar to ``map``, with the difference that the callback | ||||||||
| ## returns an Option, not a raw value. This allows multiple procs with a | ||||||||
| ## signature of ``A -> Option[B]`` (including A = B) to be chained together. | ||||||||
| map(self, callback).flatten() | ||||||||
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| proc filter*[T](self: Option[T], callback: proc (input: T): bool): Option[T] = | ||||||||
| ## Applies a callback to the value in this Option. If the callback returns | ||||||||
| ## `true`, the option is returned as a Some. If it returns false, it is | ||||||||
| ## returned as a None. | ||||||||
| if self.isSome and not callback(self.unsafeGet): | ||||||||
| none(T) | ||||||||
| else: | ||||||||
| self | ||||||||
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| macro `?.`*(option: untyped, statements: untyped): untyped = | ||||||||
| ## Existential operator. Works like regular dot-chaining, but if | ||||||||
| ## the left had side is a ``none`` then the right hand side is not evaluated. | ||||||||
| ## In the case that ``statements`` return something the return type of this | ||||||||
| ## will be ``Option[T]`` where ``T`` is the returned type of ``statements``. | ||||||||
| ## If nothing is returned from ``statements`` this returns nothing. | ||||||||
| ## | ||||||||
| ## .. code-block:: nim | ||||||||
| ## echo some("Hello")?.find('l') ## Prints out Some(2) | ||||||||
| ## some("Hello")?.find('l').echo # Prints out 2 | ||||||||
| ## none(string)?.find('l').echo # Doesn't print out anything | ||||||||
| ## echo none(string)?.find('l') # Prints out None[int] (return type of find) | ||||||||
| ## # These also work in things like ifs | ||||||||
| ## if some("Hello")?.find('l') == 2: | ||||||||
| ## echo "This prints" | ||||||||
| ## if none(string)?.find('l') == 2: | ||||||||
| ## echo "This doesn't" | ||||||||
| let opt = genSym(nskLet) | ||||||||
| var | ||||||||
| injected = statements | ||||||||
| firstBarren = statements | ||||||||
| if firstBarren.len != 0: | ||||||||
| while true: | ||||||||
| if firstBarren[0].len == 0: | ||||||||
| firstBarren[0] = nnkDotExpr.newTree( | ||||||||
| nnkDotExpr.newTree(opt, newIdentNode("unsafeGet")), firstBarren[0]) | ||||||||
| break | ||||||||
| firstBarren = firstBarren[0] | ||||||||
| else: | ||||||||
| injected = nnkDotExpr.newTree( | ||||||||
| nnkDotExpr.newTree(opt, newIdentNode("unsafeGet")), firstBarren) | ||||||||
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| result = quote do: | ||||||||
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There was a problem hiding this comment. That it's wrapped in a proc? Any idea how to avoid it? There was a problem hiding this comment. this pattern (https://en.wikipedia.org/wiki/Immediately-invoked_function_expression) in Nim can use block expressions: There was a problem hiding this comment. The logic in the current procedure is not that simple though, this is what would be generated for something that returns a value: (proc (): auto =
let :tmp130615 = x
if :tmp130615.isSome:
return some(:tmp130615.unsafeGet.slice(3)[0]))()As you can see it uses auto as the type and relies on Nim to create the default There was a problem hiding this comment.
Suggested change
There was a problem hiding this comment. Yeah making it inline is probably a good idea, but if I apply your suggested cange it would be a double proc wouldn't it? Not sure how this new, fancy, GitHub feature works :P There was a problem hiding this comment. oh i see, guess I used that new feature wrong myself; maybe just apply the change manually then There was a problem hiding this comment. |
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| (proc (): auto {.inline.} = | ||||||||
| let `opt` = `option` | ||||||||
| if `opt`.isSome: | ||||||||
| when compiles(`injected`) and not compiles(some(`injected`)): | ||||||||
| `injected` | ||||||||
| else: | ||||||||
| return some(`injected`) | ||||||||
| )() | ||||||||
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| when isMainModule: | ||||||||
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| import unittest, sequtils | ||||||||
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| suite "optionsutils": | ||||||||
| # work around a bug in unittest | ||||||||
| let intNone = none(int) | ||||||||
| let stringNone = none(string) | ||||||||
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| test "map with a void result": | ||||||||
| var procRan = 0 | ||||||||
| some(123).map(proc (v: int) = procRan = v) | ||||||||
| check procRan == 123 | ||||||||
| intNone.map(proc (v: int) = check false) | ||||||||
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| test "map": | ||||||||
| check(some(123).map(proc (v: int): int = v * 2) == some(246)) | ||||||||
| check(intNone.map(proc (v: int): int = v * 2).isNone) | ||||||||
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| test "filter": | ||||||||
| check(some(123).filter(proc (v: int): bool = v == 123) == some(123)) | ||||||||
| check(some(456).filter(proc (v: int): bool = v == 123).isNone) | ||||||||
| check(intNone.filter(proc (v: int): bool = check false).isNone) | ||||||||
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| test "flatMap": | ||||||||
| proc addOneIfNotZero(v: int): Option[int] = | ||||||||
| if v != 0: | ||||||||
| result = some(v + 1) | ||||||||
| else: | ||||||||
| result = none(int) | ||||||||
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| check(some(1).flatMap(addOneIfNotZero) == some(2)) | ||||||||
| check(some(0).flatMap(addOneIfNotZero) == none(int)) | ||||||||
| check(some(1).flatMap(addOneIfNotZero).flatMap(addOneIfNotZero) == some(3)) | ||||||||
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| proc maybeToString(v: int): Option[string] = | ||||||||
| if v != 0: | ||||||||
| result = some($v) | ||||||||
| else: | ||||||||
| result = none(string) | ||||||||
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| check(some(1).flatMap(maybeToString) == some("1")) | ||||||||
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| proc maybeExclaim(v: string): Option[string] = | ||||||||
| if v != "": | ||||||||
| result = some v & "!" | ||||||||
| else: | ||||||||
| result = none(string) | ||||||||
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| check(some(1).flatMap(maybeToString).flatMap(maybeExclaim) == some("1!")) | ||||||||
| check(some(0).flatMap(maybeToString).flatMap(maybeExclaim) == none(string)) | ||||||||
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| test "existential operator": | ||||||||
| when not compiles(some("Hello world")?.find('w').echo): | ||||||||
| check false | ||||||||
| check (some("Hello world")?.find('w')).unsafeGet == 6 | ||||||||
| var evaluated = false | ||||||||
| if (some("team")?.find('i')).unsafeGet == -1: | ||||||||
| evaluated = true | ||||||||
| check evaluated == true | ||||||||
| evaluated = false | ||||||||
| if (none(string)?.find('i')).isSome: | ||||||||
| evaluated = true | ||||||||
| check evaluated == false | ||||||||
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runnableExamples + doAssert's instead of echo's ? (otherwise nothing guarantees it stays in sync or keeps compiling)