Compilation breaks silently

[gusty]

Here's a standalone repro, including code from F#+

It prints the first message, and everything behind the following line doesn't seem to execute at all.

open System

let inline konst (k: 'T) = fun (_: 'Ignored) -> k
let inline either fOk fError (source: Result<'T,'Error>) : 'U = match source with Ok x -> fOk x | Error x -> fError x
let inline option f n = function Some x -> f x | None -> n

type Return = Return with
    static member inline InvokeOnInstance (x: 'T) = (^``Applicative<'T>`` : (static member Return : ^T -> ^``Applicative<'T>``) x)

type Map = Map with
    static member inline InvokeOnInstance (mapping: 'T->'U) (source: '``Functor<'T>``) : '``Functor<'U>`` = 
        (^``Functor<'T>`` : (static member Map : _ * _ -> _) source, mapping)

[<Struct>]
type Identity<'t> = Identity of 't with
    static member Return x = Identity x                                             : Identity<'T>
    static member (>>=) (Identity x, f :'T -> Identity<'U>) = f x                   : Identity<'U>
    static member (<*>) (Identity (f : 'T->'U), Identity (x : 'T)) = Identity (f x) : Identity<'U>
    static member Lift2 (f, Identity (x: 'T), Identity (y: 'U)) = Identity (f x y)  : Identity<'V>
    static member Map   (Identity x, f : 'T->'U) = Identity (f x)                   : Identity<'U>
    static member Zip   (Identity x, Identity y) = Identity (x, y)                  : Identity<'T * 'U>

/// Basic operations on Identity
[<RequireQualifiedAccess>]
module Identity = 
    let run (Identity x) = x  : 'T

/// Boolean monoid under conjunction.
[<Struct>]
type All = All of bool with
    static member Zero = All true
    static member (+) (All x, All y) = All (x && y)

/// Boolean monoid under disjunction.
[<Struct>]
type Any = Any of bool with
    static member Zero = Any false
    static member (+) (Any x, Any y) = Any (x || y)

[<Struct>]
type Const<'t,'u> = Const of 't with

    // Functor
    static member Map (Const x: Const<_,'T>, _: 'T->'U) = Const x : Const<'C,'U>
    static member inline Return (_: 'U) = Const LanguagePrimitives.GenericZero : Const<'T,'U>

/// Basic operations on Const
[<RequireQualifiedAccess>]
module Const =
    let run (Const t) = t

/// Option<'T> monoid returning the leftmost non-None value.
[<Struct>]
type First<'t> = First of Option<'t> with
    static member get_Zero () = First None                                    : First<'t>
    static member (+) (x, y) = match x, y with First None, r -> r | l, _ -> l : First<'t>
    static member run (First a) = a                                           : 't option

/// Lens functions and operators
module Lens =

    let dimap' ab cd  p = ab >> p >> cd

    let setl optic value (source: 's) : 't = Identity.run (optic (fun _ -> Identity value) source)

    let over optic updater (source: 's) : 't = Identity.run (optic (Identity << updater) source)

    let view (optic: ('a -> Const<_,'b>) -> _ -> Const<_,'t>) (source: 's) : 'a = Const.run (optic Const source)

    let preview (optic: ('a -> Const<_,'b>) -> _ -> Const<_,'t>) (source: 's) : 'a option = source |> optic (fun x -> Const (First (Some x))) |> Const.run |> First.run

    let inline lens (getter: 's -> 'a) (setter: 's -> 'b -> 't) (f: 'a -> '``F<'b>``) = fun s -> setter s </Map.InvokeOnInstance/> f (getter s) : '``F<'t>``

    let inline prism (constructor: 'b -> 't) (getter: 's -> Result<'a,'t>) (f: 'a -> '``F<'b>``) = f |> (fun g -> either (Ok << g) Error) |> dimap' getter (either (Map.InvokeOnInstance constructor) Return.InvokeOnInstance) : 's -> '``F<'t>``

    let inline prism' (constructor: 'b -> 's) (getter: 's -> Option<'a>) (f: 'a -> '``F<'b>``) = prism constructor (fun s -> option Ok (Error s) (getter s)) f : 's -> '``F<'t>``

    // Prism

    /// Prism providing a Traversal for targeting the 'Ok' part of a Result<'T,'Error>
    let inline _Ok    x = (prism Ok    <| either Ok (Error << Error)) x

    /// Prism providing a Traversal for targeting the 'Error' part of a Result<'T,'Error>
    let inline _Error x = (prism Error <| either (Error << Ok) Ok) x

    /// Prism providing a Traversal for targeting the 'Some' part of an Option<'T>
    let inline _Some x = (prism Some <| option Ok (Error None)) x

    /// Prism providing a Traversal for targeting the 'None' part of an Option<'T>
    let inline _None x = (prism' (konst None) <| option (konst None) (Some ())) x

    let inline (<&>) (x: '``F<'t>``) (f: 't -> 'u) : '``F<'u>`` = Map.InvokeOnInstance f x

open Lens

let equal x  y = x = y

let t3 = equal (Some 1) (preview _Ok (Ok 1))
let t4 = equal (Some 1) (preview _Error (Error 1))

printfn "So far so good"

let t2 = equal None (preview _Ok (Error 1))

printfn "It doesn't print anything"

let t5 = equal None (preview _Error (Ok 1))

let t6 = equal true (Option.isNone (preview _Some None))
let t7 = equal (Some 1) (preview _Some (Some 1))
let t8 = equal (Some ()) (preview _None None)
let t9 = equal true (Option.isNone (preview _None (Some 1)))

printfn "%A" t2
printfn "%A" t3
printfn "%A" t4
printfn "%A" t5
printfn "%A" t6
printfn "%A" t7
printfn "%A" t8
printfn "%A" t9

[gusty]

I just came with a way smaller repro:

type First<'t> = First of Option<'t> with
    static member get_Zero () = First None : First<'t>
    static member run (First a) = a        : 't option

type Const<'t,'u> = Const of 't with
    static member inline Return (_: 'U) = Const LanguagePrimitives.GenericZero : Const<'T,'U>
    static member run (Const a) = a

let t2: int option =
    Error 1
    |> fun x ->  (^``Applicative<'T>`` : (static member Return : ^T -> ^``Applicative<'T>``) x)
    |> Const.run 
    |> First.run

printfn "t2"

If you exchange the trait-call line for |> Const.Return it works, but otherwise compilation seems to break silently as the last line (the printfn) is not executed.

[MangelMaxime]

If you open the console of your browser you will see that an error is reported there.

In the "Console" section of the REPL, we only report calls to error/warn/log that are called with strings. If you have an uncatch exception at runtime like you does here you will see it in your browser console.

[gusty]

Fair enough, now can you tell me why do I have that runtime error? This code works just fine in fsi.

[gusty]

Looking at the javascript being generated, there is a difference right at the end:

This is the version with the trait-call:

export const t2 = (() => {
  let arg00$$1;
  let arg00;
  const x = new Result(1, "Error", 1);
  arg00 = new Const$00602(0, "Const", null);
  arg00$$1 = Const$00602$$$run$$43814500(arg00);
  return First$00601$$$run$$Z4DCCCC0(arg00$$1);
})();
toConsole(printf("t2"));

And this is the version replacing the trait-call with the actual call

export const t2 = (() => {
  let arg00$$2;
  let arg00$$1;
  const arg00 = new Result(1, "Error", 1);
  arg00$$1 = new Const$00602(0, "Const", First$00601$$$get_Zero());
  arg00$$2 = Const$00602$$$run$$43814500(arg00$$1);
  return First$00601$$$run$$Z4DCCCC0(arg00$$2);
})();
toConsole(printf("t2"));

Note that the trait-call generates new Const$00602(0, "Const", null); instead of new Const$00602(0, "Const", First$00601$$$get_Zero());

That null seems to be the problem.

I guess this should be reported to the Fable compiler, right?

[MangelMaxime]

Yes, you did right reporting the problem in Fable repository.

I am closing this issue as the bug is in Fable compiler.