Invalid TypeScript code generated when running Fable in watch mode / release mode

[MangelMaxime]

Description

1. dotnet run -c Release --project /Users/mmangel/Workspaces/Github/fable-compiler/fable/src/Fable.Cli -- tests/TypeScript --outDir build/tests/TypeScript --lang typescript --exclude Fable.Core --noCache
2. npx tsc -p build/tests/TypeScript --outDir build/tests/TypeScriptCompiled

See that no errors are reported.

1. dotnet run -c Release --project /Users/mmangel/Workspaces/Github/fable-compiler/fable/src/Fable.Cli -- tests/TypeScript --outDir build/tests/TypeScript --lang typescript --exclude Fable.Core --noCache --watch
2. Kill Fable
3. npx tsc -p build/tests/TypeScript --outDir build/tests/TypeScriptCompiled

See that errors are reported.

Fable git:(infra_clean_up) ✗ npx tsc -p build/tests/TypeScript --outDir build/tests/TypeScriptCompiled
build/tests/TypeScript/Js/Main/ApplicativeTests.ts:928:29 - error TS2322: Type '(a2: number) => number' is not assignable to type 'number'.

928         const f1_1: int32 = curry2(f_3)(x_4);
                                ~~~~~~~~~~~~~~~~

  build/tests/TypeScript/Js/Main/ApplicativeTests.ts:928:29
    928         const f1_1: int32 = curry2(f_3)(x_4);
                                    ~~~~~~~~~~~~~~~~
    Did you mean to call this expression?

build/tests/TypeScript/Js/Main/ApplicativeTests.ts:930:32 - error TS2349: This expression is not callable.
  Type 'Number' has no call signatures.

930             const y_2: int32 = f1_1(arg_1) | 0;
                                   ~~~~

build/tests/TypeScript/Js/Main/ApplicativeTests.ts:1948:104 - error TS2322: Type '(a3: (arg0: unknown) => number) => (a4: unknown) => [number, number]' is not assignable to type '(arg0: (arg0: $a) => number) => (arg0: $a) => [number, number]'.
  Types of parameters 'a3' and 'arg0' are incompatible.
    Types of parameters 'arg0' and 'arg0' are incompatible.
      Type 'unknown' is not assignable to type '$a'.
        '$a' could be instantiated with an arbitrary type which could be unrelated to 'unknown'.

1948     const bar = <$a>(a_1: int32): ((arg0: ((arg0: $a) => int32)) => ((arg0: $a) => [int32, int32])) => curry4(foo)(1)(a_1);
                                                                                                            ~~~~~~~~~~~~~~~~~~~

build/tests/TypeScript/Js/Main/ApplicativeTests.ts:1949:35 - error TS2554: Expected 1 arguments, but got 0.

1949     const baz_1: [int32, int32] = bar(2)((): int32 => 3)();
                                       ~~~~~~~~~~~~~~~~~~~~~~~~

  build/tests/TypeScript/Js/Main/ApplicativeTests.ts:1948:71
    1948     const bar = <$a>(a_1: int32): ((arg0: ((arg0: $a) => int32)) => ((arg0: $a) => [int32, int32])) => curry4(foo)(1)(a_1);
                                                                               ~~~~~~~~
    An argument for 'arg0' was not provided.

build/tests/TypeScript/Js/Main/MiscTests.ts:1311:28 - error TS2554: Expected 0 arguments, but got 1.

1311     equal<int32>(9, func2b(void 0));
                                ~~~~~~

build/tests/TypeScript/Js/Main/ReflectionTests.ts:966:27 - error TS2367: This comparison appears to be unintentional because the types '"Maybe`1"' and '""' have no overlap.

966     equal<boolean>(false, name === "");
                              ~~~~~~~~~~~

build/tests/TypeScript/Js/Main/ReflectionTests.ts:969:27 - error TS2367: This comparison appears to be unintentional because the types '"Maybe`1"' and '""' have no overlap.

969     equal<boolean>(false, name_1 === "");
                              ~~~~~~~~~~~~~

build/tests/TypeScript/Js/Main/StringTests.ts:165:5 - error TS2571: Object is of type 'unknown'.

165     linef(printf("open %s"))("Foo");
        ~~~~~~~~~~~~~~~~~~~~~~~~

build/tests/TypeScript/Js/Main/TailCallTests.ts:586:23 - error TS2345: Argument of type '<$a, $b>(model_1: FSharpMap<$a, $b>, dispatch: (arg0: $a) => void) => Element$' is not assignable to parameter of type '(a1: FSharpMap<number, boolean>, a2: (arg0: never) => void) => Element$'.
  Types of parameters 'dispatch' and 'a2' are incompatible.
    Types of parameters 'arg0' and 'arg0' are incompatible.
      Type 'number' is not assignable to type 'never'.

586     main(init, curry2(view), curry2(update));
                          ~~~~

build/tests/TypeScript/Js/Main/TypeTests.ts:1696:27 - error TS2741: Property 'name' is missing in type 'MyEx' but required in type 'Error'.

1696                     const e_1: Error = matchValue;
                               ~~~

  node_modules/typescript/lib/lib.es5.d.ts:1054:5
    1054     name: string;
             ~~~~
    'name' is declared here.

build/tests/TypeScript/Js/Main/TypeTests.ts:1701:27 - error TS2322: Type 'MyEx' is not assignable to type 'Error'.

1701                     const e_2: Error = matchValue;
                               ~~~


Found 11 errors in 6 files.

Errors  Files
     4  build/tests/TypeScript/Js/Main/ApplicativeTests.ts:928
     1  build/tests/TypeScript/Js/Main/MiscTests.ts:1311
     2  build/tests/TypeScript/Js/Main/ReflectionTests.ts:966
     1  build/tests/TypeScript/Js/Main/StringTests.ts:165
     1  build/tests/TypeScript/Js/Main/TailCallTests.ts:586
     2  build/tests/TypeScript/Js/Main/TypeTests.ts:1696

Expected and actual results

The reason for this difference between production and watch mode is that the generated code is not the same.

In release mode

testCase("Arity is checked also when constructing records", (): void => {
    equal<string>("foo23", (new ArityRecord((x_4: int32, arg_1: int32): string => {
        const y_2: int32 = ((j_1: int32): int32 => ((x_4 * 2) + (j_1 * 3)))(arg_1) | 0;
        return toText(printf("foo%i"))(y_2);
    })).arity2(4, 5));
})

In watch mode

testCase("Arity is checked also when constructing records", (): void => {
    const f_3 = (i_1: int32, j_1: int32): int32 => ((i_1 * 2) + (j_1 * 3));
    const r: ArityRecord = new ArityRecord(uncurry2((x_4: int32): ((arg0: int32) => string) => {
        const f1_1: int32 = curry2(f_3)(x_4);
        return (arg_1: int32): string => {
            const y_2: int32 = f1_1(arg_1) | 0;
            return toText(printf("foo%i"))(y_2);
        };
    }));
    equal<string>("foo23", r.arity2(4, 5));
})

I suspect this is because of different optimisation done by Fable or the F# compiler, which I guess is not problematic per se.

However, the generated code in watch mode is wrongly type.

For example, const f1_1: int32 = curry2(f_3)(x_4); should be written as const f1_1: ((x : int32) => int32) = curry2(f_3)(x_4); instead.

In release mode

testCase("Name can be extracted from RecWithGenDU", (): void => {
    equal<boolean>(false, false);
})

In watch mode

testCase("Name can be extracted from RecWithGenDU", (): void => {
    const name = "Maybe`1";
    equal<boolean>(false, name === "");
})

In this case, the correct code seems to be the watch mode, because release mode just seems to not generate anything at all...

Related information

• Fable version: 4.1.4
• Operating system

[github-actions]

🤖 This is an automated response from Repo Assist.

Thanks for the detailed report! After reading the TypeScript output and the Fable2Babel type annotation pipeline, here's my analysis of the root cause:

Root Cause

The incorrect type annotation (int32 instead of (a2: number) => number) for f1_1 originates in multiVarDeclaration in Fable2Babel.fs (around line 1255):

let ta, tp = makeTypeAnnotationWithParametersIfTypeScript com ctx id.Type value

The TypeScript type annotation is derived from id.Type — the Fable AST type of the identifier. For f1_1 the Fable type appears to be int32 (the final return type) rather than LambdaType(int32, int32) (the intermediate curried function type).

In watch mode, the local function f is not inlined, so the code becomes:

const f1_1: int32 = curry2(f_3)(x_4);   // f1_1 is actually int32 → int32

In release mode, f is inlined away, so f1_1 is never emitted as a separate binding — hiding the bug.

Likely Location

The wrong id.Type probably originates in FSharp2Fable.fs when computing the Fable type for a partial application of a multi-argument F# function. When curry2(f_3) is applied to x_4, the result should have Fable type LambdaType(int32, int32), but it appears to be getting simplified to just int32 (the final return type).

A potential secondary fix point: makeTypeAnnotationIfTypeScript in Fable2Babel.fs already skips annotation for FunctionExpression/ArrowFunctionExpression values (it returns None). It doesn't, however, skip annotation for curry/uncurry call expressions that return functions — adding a case for those would be a targeted workaround, though fixing the id.Type upstream would be cleaner.

Reproducing the Pattern

The simplest reproduction is probably any code like:

let f (i: int) (j: int) = i + j
let f1 = f 5  // f1: int -> int

If this generates const f1: int = ... in TypeScript watch mode (without inlining), that confirms the hypothesis.

Note

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[MangelMaxime]

It seems like I can't reproduce this error anymore