Created
August 9, 2017 13:57
-
-
Save nojaf/aa8b191e2f21c291fa58e80076f3c92f to your computer and use it in GitHub Desktop.
Computation Expressions question
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
| // Learn more about F# at http://fsharp.org | |
| // See the 'F# Tutorial' project for more help. | |
| open System | |
| let toInt a = | |
| match (Int32.TryParse(a)) with | |
| | (true, x) -> Some x | |
| | (false, _) -> None | |
| type MaybeBuilder() = | |
| member this.Bind(x, f) = | |
| printfn "x: %A, f: %A" x f | |
| match x with | |
| | None -> None | |
| | Some a -> f a | |
| member this.Return(x) = | |
| Some x | |
| let maybe = new MaybeBuilder() | |
| [<EntryPoint>] | |
| let main argv = | |
| printfn "%A" argv | |
| let parsed = | |
| maybe { | |
| let! head = Array.tryHead argv | |
| let! v = head |> toInt | |
| return v | |
| } | |
| printfn "parsed is %A" parsed | |
| 0 // return an integer exit code |
PS: it's not you. This is as tricky as functional programming in F# gets and the official documentation is terrible :)
Author
Thanks for the quick responses!
So if I change the maybe block to use the maybe instance I get:
let parsedTwo =
maybe.Bind
(tryHead argv,
fun head ->
maybe.Bind(toInt head, fun result ->
maybe.Return(result)
)
)if tryHead argv is None then the fun head -> ... never get's executed right?
@nojaf correct!
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
As @mavnn mentioned, things might make more sense if you de-sugar the expression.
That f argument that gets passed into Bind is called a continuation, which represents "the next steps after this statement" (I first encountered this pattern when working with async code, it's similar to that). F# for fun and profit has a nice explanation on continuations and Bind which might help clarify things.
In short: every let! is translated to a call to Bind, with the first argument being the option value (x) and the second argument a function that represents all the next steps in the expression (f). Not all those Bind calls get evaluated, so not all those continuations get evaluated either. As soon as you hit a None case in a call to Bind, that Bind call will just return None and not even try to evaluate the continuation, i.e. no additional printfs will be evaluated.