Various issues when trying to use `pipe` function

[OliverJAsh]

TypeScript Version: 3.3.1

Search Terms: pipe compose functional programming composition generics overloads

Code

It goes without saying that pipe is a very common utility function for composing functions, used often in functional programming. I've been using a pipe function in TypeScript for about 2 years, and over time I've collected numerous issues.

I wanted to create an issue to collect all of this information, to help others who want to use pipe so they are aware of the various footguns—and also for the TypeScript team to help their visibility of these issues.

To the best of my ability, I have narrowed these bugs down to the simplest examples, and provided any interesting (and sometimes useful) workarounds. I would appreciate any help in narrowing these examples further—perhaps even combining them where perceived issues are artefacts of the same underlying problems.

In my experience, 80% of the time pipe just works. These issues cover the remaining 20% of the time. Issue 1 is by far the most significant. The rest are unordered.

I have linked to sub issues where I'm aware they exist. For those without linked issues, we may want to create new issues to trick them independently.

1. Generics are lost when first composed function is generic

Related issues:

• Losing generics (composition/currying) #12838
• Function composition challenge for type system #10247

declare const pipe: {
    <A, B, C>(ab: (a: A) => B, bc: (b: B) => C): (a: A) => C;
};

type Component<P> = (props: P) => {};

declare const myHoc1: <P>(C: Component<P>) => Component<P>;
declare const myHoc2: <P>(C: Component<P>) => Component<P>;

declare const MyComponent1: Component<{ foo: 1 }>;

// When `strictFunctionTypes` disabled:
// Expected type: `(a: Component<{ foo: 1 }>) => Component<{ foo: 1 }>`
// Actual type: `(a: {}) => Component<{}>`
const enhance = pipe(
    /*
    When `strictFunctionTypes` enabled, unexpected type error::
    Argument of type '<P>(C: Component<P>) => Component<P>' is not assignable to parameter of type '(a: {}) => Component<{}>'.
        Types of parameters 'C' and 'a' are incompatible.
            Type '{}' is not assignable to type 'Component<{}>'.
                Type '{}' provides no match for the signature '(props: {}): {}'.
    */
    myHoc1,
    myHoc2,
);
// Expected type: `Component<{ foo: 1 }>`
// Actual type: `Component<{}>`
const MyComponent2 = enhance(MyComponent1);

// Workaround:
const enhance2 = pipe(
    () => myHoc1(MyComponent1),
    myHoc2,
);
const MyComponent3 = enhance2({});

With the option suggested in #27288, TypeScript would at least alert the developer to change the code to workaround this problem.

2. Incorrect overload is used for pipe

• when strictFunctionTypes is disabled
• when first composed function parameter is optional
• when first pipe overload is zero parameters for first function

Related issues: #29913

declare const pipe: {
    // 0-argument first function
    // Workaround: disable this overload
    <A>(a: () => A): () => A;

    // 1-argument first function
    <A, B>(ab: (a: A) => B): (a: A) => B;
};

// Expected type: `(a: {} | undefined) => number`
// Actual type: `() => number`
const fn = pipe((_a?: {}) => 1);

3. Inference does not work when first pipe overload is zero parameters for first function

Related issues:

• Generic not inferred from contextual types #25293
• Problem explanation: Generic not inferred from contextual types #25293 (comment)

declare const pipe: {
    // 0-argument first function
    // Workaround: disable this overload
    <A, B>(a: () => A, ab: (a: A) => B): () => B;

    // 1-argument first function
    <A, B, C>(ab: (a: A) => B, bc: (b: B) => C): (a: A) => C;
};

// Example 1

type Fn = (n: number) => number;
const fn: Fn = pipe(
    // Expected param `x` type to be inferred as `number`
    // Actual type: any
    x => x + 1,
    x => x * 2,
);

// Example 2

const promise = Promise.resolve(1);
promise.then(
    pipe(
        // Expected param `x` type to be inferred as `number`
        // Actual type: any
        x => x + 1,
        x => x * 2,
    ),
);

4. Untitled

Related issues:

• Generic inference inside of pipe is incorrect when strict mode is disabled #25826
• Problem explanation: Generic inference inside of pipe is incorrect when strict mode is disabled #25826 (comment)

declare const pipe: {
    // Workaround 1: enable this overload
    // <A, B, C>(ab: (a: A) => B, bc: (b: B) => C): (a: A) => C;

    <A, B, C, D>(ab: (a: A) => B, bc: (b: B) => C, cd: (c: C) => D): (a: A) => D;
};

declare const getString: () => string;
declare const orUndefined: (name: string) => string | undefined;
declare const identity: <T>(value: T) => T;

const fn = pipe(
    getString,

    /*
    Unexpected type error:
    Type 'string | undefined' is not assignable to type '{}'.
        Type 'undefined' is not assignable to type '{}'.
    */
    string => orUndefined(string),

    // Workaround 2: pass the function directly, instead of wrapping:
    // get,

    identity,
);

5. Incorrect overload is used for composed function

Related issues:

• Incorrect overload is used for composed function inside pipe #25637
• Incorrect overload is used for composed function inside pipe #25637 (comment)

declare const pipe: {
    <A, B, C>(ab: (a: A) => B, bc: (b: B) => C): (a: A) => C;
};

declare const myFn: {
    (p: string): string;
    (p: any): number;
};

declare const getString: () => string;

// Expected type: `(a: {}) => string`
// Actual type: `(a: {}) => number`
// Note: if we comment out the last overload for `myFn`, we get the expected
// type.
const fn = pipe(
    getString,
    myFn,
);

6. Untitled

Related issues:

• Unexpected error/bad inference for generic composed function inside pipe #25791
• Unexpected error/bad inference for generic composed function inside pipe #25791 (comment)

declare const pipe: {
    <A, B, C, D>(ab: (a: A) => B, bc: (b: B) => C, cd: (c: C) => D): (a: A) => D;
};

declare const getArray: () => string[];
declare const first: <T>(ts: T[]) => T;

// When `strictFunctionTypes` disabled:
// Expected type: `(a: {}) => string`
// Actual type: `(a: {}) => {}`
const fn = pipe(
    getArray,
    x => x,
    /*
    When `strictFunctionTypes` enabled, unexpected type error:
    Argument of type '<T>(ts: T[]) => T' is not assignable to parameter of type '(c: {}) => {}'.
        Types of parameters 'ts' and 'c' are incompatible.
            Type '{}' is missing the following properties from type '{}[]': length, pop, push, concat, and 25 more.
    */
    first,
);

// Workaround 1: use `identity` function
declare const identity: <T>(value: T) => T;
const fn2 = pipe(
    getArray,
    identity,
    first,
);

// Workaround 2: wrap last function
const fn3 = pipe(
    getArray,
    x => x,
    x => first(x),
);

[ahejlsberg]

@OliverJAsh Appreciate the write-up. See #30114 for an improvement that will allow you to use a generic rest parameter to represent the parameter list of the first function argument passed to pipe. This alleviates the need for overloads which in turn helps with some of the issues.

Other issues remain that we are aware of, such as support for inferring higher-order functions (the first issue) and issues relating to ordering of inferences when piping through combinations of contextually sensitive and contextually insensitive arguments (e.g. combinations of arrow functions and declared functions). We continue to contemplate possible solutions.

[ahejlsberg]

With #30114 and #30193 all but issue 1 (inferring higher order function types) and issue 5 (overload resolution in inference and type relationships) above are now fixed. I'm definitely eager to address higher order function type inference (tracked by #9366) and I think we're getting closer to a workable solution.

[ahejlsberg]

Higher order function type inference now implemented in #30215.

[weswigham]

With all these PRs merged, can we call this closed?

[OliverJAsh]

😮. Thank you so much, this is going to make a massive difference.

I'm not sure if all of the issues I listed above are fixed, so I'll defer to @ahejlsberg.

[ahejlsberg]

Everything but issue 5 (overload resolution in type inference) has been fixed. That issue is already tracked elsewhere (e.g. #26591 and #29732) so I think we can safely call this closed.

[OliverJAsh]

Presuming these fixes are now merged and released in next, I just tested all of the above examples with typescript@next.

1, 4, 6 are now working as expected. 🎉 (Thank you!)

2, 3, 5 however do not seem to have changed. IIUC, 5 is not expected to be fixed yet, but what about 2 and 3?

[ahejlsberg]

The fix for 2 and 3 is to change your declarations of pipe to use generic rest parameters:

declare function pipe<A extends any[], B>(ab: (...args: A) => B): (...args: A) => B;
declare function pipe<A extends any[], B, C>(ab: (...args: A) => B, bc: (b: B) => C): (...args: A) => C;
declare function pipe<A extends any[], B, C, D>(ab: (...args: A) => B, bc: (b: B) => C, cd: (c: C) => D): (...args: A) => D;
// ...

These declarations will more accurately capture the parameter list of the first function and will work for any number of parameters in the first function.

[millsp]

Instead of writing overloads, we can write pipe & compose generically.