Fable: Error when composing 2 functions

[durkiewicz]

Description

There is a runtime error when composing 2 functions. The same code works fine when composition operator is avoided (e.g. let foo = squarePromise 4 |> createPromise)

Repro code

open Fable.Core

let squarePromise a resolve reject =
    resolve (a*a)

[<Emit("new Promise($0)")>]
let createPromise (f: ('a -> unit) -> (System.Exception -> unit) -> unit): Fable.Import.JS.Promise<'a> = jsNative

let foo =
    (squarePromise >> createPromise) 4

Expected and actual results

Expected: foo should contain a promise that is resolved to 16
Actual: Uncaught TypeError: Promise resolver undefined is not a function

Related information

• Fable version (dotnet fable --version): 1.3.10
• Operating system: Ubuntu 16.0.4
• The bug is also reproducible in the REPL: (http://fable.io/repl/#?code=open%20Fable.Core%0A%0Alet%20squarePromise%20a%20resolve%20reject%20%3D%0A%20%20%20%20resolve%20(a*a)%0A%0A%5B%3CEmit(%22new%20Promise(%240)%22)%3E%5D%0Alet%20createPromise%20(f%3A%20('a%20-%3E%20unit)%20-%3E%20(System.Exception%20-%3E%20unit)%20-%3E%20unit)%3A%20Fable.Import.JS.Promise%3C'a%3E%20%3D%20jsNative%0A%0Alet%20foo%20%3D%0A%20%20%20%20(squarePromise%20%3E%3E%20createPromise)%204%0A&html=%3Chtml%3E%3Cbody%3E%3C%2Fbody%3E%3C%2Fhtml%3E)

[alfonsogarciacaro]

Thanks for the report @durkiewicz and sorry for replying late. This seems due to the current way in Fable 1 to "uncurry" lambdas which relies mainly in runtime checks and it fails in some cases. I've tested with Fable 2 (dev2.0 branch) and the error is not happening. I am trying to release the next version soon, would it be possible for you to use the work around (|> operator) until the next version is ready? If necessary, I can also try to fix it for the current version.

[durkiewicz]

Hi Alfonso, thanks for replying. It's nice to hear that the bug is going to be fixed in Fable 2. I'm fine with using the workaround - I just need to be careful.

BTW, the fable.io page should have a donation button!

[alfonsogarciacaro]

Great, thanks for your understanding! For me personally, the best donation is you keep using Fable, post about it and help make it better everyday, so companies trust the tool I (and others) can get professional projects using Fable :)

[alfonsogarciacaro]

Fixed in Fable 2 so we can close this :)