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Improved union/intersection type inference#5738

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ahejlsberg merged 4 commits into
mastermicrosoft/TypeScript:masterfrom
unionIntersectionTypeInferencemicrosoft/TypeScript:unionIntersectionTypeInferenceCopy head branch name to clipboard
Nov 24, 2015
Merged

Improved union/intersection type inference#5738
ahejlsberg merged 4 commits into
mastermicrosoft/TypeScript:masterfrom
unionIntersectionTypeInferencemicrosoft/TypeScript:unionIntersectionTypeInferenceCopy head branch name to clipboard

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@ahejlsberg

@ahejlsberg ahejlsberg commented Nov 20, 2015

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This PR improves type inference involving source and target sides that are both union or intersection types. When inferring from a type S to a type T, if S and T are both union types or both intersection types, we first reduce S and T by removing constituents that are matched by a constituent in the other type. For example, when inferring from string | string[] to string | T, we reduce the types to string[] and T, thus inferring string[] for T.

An example:

type Maybe<T> = T | void;

function isDefined<T>(x: Maybe<T>): x is T {
    return x !== undefined && x !== null;
}

function isUndefined<T>(x: Maybe<T>): x is void {
    return x === undefined || x === null;
}

function getOrElse<T>(x: Maybe<T>, defaultValue: T): T {
    return isDefined(x) ? x : defaultValue;
}

function test1(x: Maybe<string>) {
    let x1 = getOrElse(x, "Undefined");         // string
    let x2 = isDefined(x) ? x : "Undefined";    // string
    let x3 = isUndefined(x) ? "Undefined" : x;  // string
}

function test2(x: Maybe<number>) {
    let x1 = getOrElse(x, -1);         // number
    let x2 = isDefined(x) ? x : -1;    // number
    let x3 = isUndefined(x) ? -1 : x;  // number
}

Fixes #2264.
Fixes #4212.
Fixes #5417.
Fixes #5456.

sandersn
sandersn reviewed Nov 20, 2015
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Comment thread src/compiler/checker.ts

@sandersn sandersn Nov 20, 2015

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How about

return forEach(target.types, t => isTypeIdenticalTo(source, t) || undefined);

@ahejlsberg ahejlsberg Nov 24, 2015

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Definitely an option, but not quite as efficient. I will keep what's there.

@sandersn

sandersn commented Nov 20, 2015

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👍

DanielRosenwasser
DanielRosenwasser reviewed Nov 23, 2015
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Comment thread src/compiler/checker.ts

@DanielRosenwasser DanielRosenwasser Nov 23, 2015

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Can you break this into multiple lines?

@ffMathy ffMathy Dec 7, 2015

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Use wordwrapping man.

ahejlsberg added a commit that referenced this pull request Nov 24, 2015
@ahejlsberg
Merge pull request #5738 from Microsoft/unionIntersectionTypeInference
ea0cc79 
Improved union/intersection type inference
@ahejlsberg
ahejlsberg merged commit ea0cc79 into master Nov 24, 2015
This was referenced Nov 24, 2015
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@RyanCavanaugh
RyanCavanaugh deleted the unionIntersectionTypeInference branch November 24, 2015 22:23
@mariusschulz

mariusschulz commented Nov 29, 2015

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@ahejlsberg You've got a little typo in your code example: isUndefined<T> should use or rather than and, shouldn't it? Just trying to avoid confusion here.

function isUndefined<T>(x: Maybe<T>): x is void {
    return x === undefined || x === null;
}

@ahejlsberg

ahejlsberg commented Nov 29, 2015

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@mariusschulz Yes indeed, thanks for catching that.

@vladima vladima mentioned this pull request Dec 1, 2015
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@ahejlsberg ahejlsberg mentioned this pull request Dec 2, 2015
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@david-driscoll david-driscoll mentioned this pull request Dec 3, 2015
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@vladima vladima mentioned this pull request Dec 3, 2015
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@ENikS

ENikS commented Dec 7, 2015

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Will this address issue mentioned in #3215 ?

@ahejlsberg

ahejlsberg commented Dec 8, 2015

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@ENikS No, the two are unrelated.

@HerringtonDarkholme HerringtonDarkholme mentioned this pull request Dec 11, 2015
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@masaeedu

masaeedu commented Feb 9, 2016

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@ahejlsberg Would inferring T more accurately in the following scenario be worthwhile:

interface A { a: any; }

function test<T>(arg: A & T): T { ... }

let result = test({a: "", b: 10});

The type of result is inferred as {}, although I would expect it to be inferred as {b: number}. Admittedly I don't have a very clear use case for this at the moment.

@ahejlsberg

ahejlsberg commented Feb 9, 2016

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@masaeedu We could potentially explore that, but it could get complex and would require type inference to manufacture new types, neither of which I'm crazy about. Specifically, in your example, we'd have to manufacture a new type { b: number } and infer that for T. I think it is desirable to have the property that type inference never infers types that don't already occur in your code.

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8 participants

@ahejlsberg @sandersn @mariusschulz @ENikS @masaeedu @DanielRosenwasser @ffMathy @msftclas
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