Function parameters are checked one at a time, with the type in each corresponding parameter position checked against each other. I'm trying to create something similar to native ReturnType type. The algorithm is as follows. That says to TypeScript: "I want to take whatever TypeScript infers to be at this position and assign it to the name returnType".It just so happens that the thing at that position is the return type of a given function, that we have … The dream of inference from function calls is really not clear as some people would imply. Consider this code, which is a totally legal JavaScript program: Flow says this program has one error, TypeScript says this program has two errors, H-M I believe says this program has zero errors (x ends up with as { kind: string, type: string } | { knd: string, type: string }). Some JavaScript functions can be called in a variety of argument counts and types. Obviously easier said than done. Have a question about this project? I think one vs two is arguable, but it's almost certainly not zero (despite being a program that does not throw exceptions or observably mistype a value). Today we’re proud to release TypeScript 4.1! Since the function body is well scoped and doesn't belong to multiple files. Press question mark to learn the rest of the keyboard shortcuts. Instead of manually copying list of parameters with their types from b() to a(), there should be a way to assign types of parameters to a() from b(). The inference is in unidirectional (barring edge cases like recursion), and in inverse direction to the call graph. Haskell infers x as Num, which compiles: @bcherny well, this works in Haskell because of type classes, but they are not a part of H-M. So I have a situation I can't seem to figure out … What was the rationale for not using global inference? It's very different and very complex. Purescript (and its record types) has some functionality close to this, so inference works pretty well (though you tend to lose type aliases. This is an example of a function declaration in TypeScript: 1. https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html. type of function's parameters list; type of arguments list; tuples; Suggestion. That is, I've got a function. If we have to explicitly declare the type signatures on all of these it adds a lot of busy work and makes the "strict type" pitch that much harder. Huh? This is already the case in TypeScript though). TypeScript sees a function as functional component as long as it returns JSX. You'll end up with a file full of parameter type annotations, which is good since you'll need them anyway for cross-file typechecks. I agree it's smaller than Haskell-style inference... but it would be a very large architectural change. We’ve written some generic functions that can work on any kind of value. The function type is (discountCode: … Expected behavior: The promised function signature of the third object property should be inferred in the same respect as the previous two promised functions.. Actual behavior: Typescript is able to infer the promised function signature when it's provided through => Promise.resolve or => new Promise(... but unable to do so when supplied as a result of an async function. Copy link Author rtpg commented Apr 11, 2017. @rtpg there's nothing in TypeScript today that infers information about a variable from its usage. https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html This post explores using identity functions with carefully constructed type signatures to guide inference towards alternative types. Thanks @RyanCavanaugh for the extensive explanation. So basically any sane programmer is going to put type annotations on parameters anyway (since you must at module boundaries anyway, and you must if you want sane errors). And it's still nonlocal since presumably code like this would be expected to produce inferences. The strictBindCallApply compiler option enables stricter checking of the bind, call, and apply methods on functions. The TypeScript spec https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md defines how inference works at each declaration site. Here's an example in Flow: This is "spooky action at a distance" at its most maximal. I love TypeScript. The underlying scenario is the same. You can seem my reply in #6606 (comment) as well. I expect TS to be able to infer such return type. When a user calls with the string "firstNameChanged', TypeScript will try to infer the right type for K.To do that, it will match K against the content prior to "Changed" and infer the string "firstName".Once TypeScript figures that out, the on method can fetch the type of firstName on the original object, which is string in … COMEFROM). Example, for withUID, T is inferred from the type of obj argument. Parameters: If the compiler option --noImplicitAny is on (which it is if --strict is on), then the type of each parameter must be either inferrable or explicitly specified. Both subs("") and subs(12) are valid uses of such an overloaded function. 1 TypeScript Tutorial - 'infer' keyword 2 TypeScript Tutorial - conditional types 3 TypeScript Tutorial - return based on args 4 TypeScript Tutorial - Use "as const" to export colors! Conditional types in typescript allow you to introduce type variables into the expression in a rather dynamic way. Posted by just now. How do Haskell, OCaml, and Elm engineers manage to use their languages at scale? Since the function body is well scoped and doesn't belong to multiple files. infer R is the union of all available types. You also mention OCaml which while it has much stronger type inference than TypeScript or Scala, exhibits some very counterintuitive behavior with respect to inference of function parameter types For example, in this code. Vote . TypeScript can (with relative ease) express pipe function for a given number of composed functions and arity using template arguments. The text was updated successfully, but these errors were encountered: Typescript never infers function arguments. I created a function that converts Chrome built-in functions from using a callback to being asynchronous, and it's been working well for years. 1-2 errors is restrictive because it requires me to explicitly declare a type with 2 optional members before my program will compile, but as you said, the benefit is errors are local, which makes debugging fast and pleasant. While this is a generic function, the neat thing is that TypeScript can infer this type from the type of the arguments that are passed to it: if you pass it a string, it knows that it will return a string. Eg. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. Already on GitHub? Is there a way to access the input-object from Typescript's perspective? This could speed up fixing "no implicit any" migrations. Search Terms. Typescript 2.8 added conditional types with type inference. You can have the call to subs in g, or the call in the s initializer, but not both. no class extensions), which makes H-M usable. There are multiple solutions to define props for functional components. In this example, the TypeScript compiler tries to infer the return type of the add() function to the number type, which is expected.. Infer return type of a function with provided argument types, // typeof one === 1 # everything is great, // typeof two === 2 # everything is great, // ReturnTypeByArg is inspired by `ReturnType` from TS 2.8.*. Isn't the example above just an extension of that? to your account. I fully agree with this. If you’re unfamiliar with TypeScript, it’s a language that builds on JavaScript by adding syntax for type declarations and annotations. By clicking “Sign up for GitHub”, you agree to our terms of service and TypeScript already has the control flow analysis in place to keep track of when x:string|number ends up actually being string or number, so it feels like you could do some of this work in reverse? Meaning that since we infer from a function argument, TypeScript knows that we have to fulfill the complete contract. I see the argument for not having Haskell-style square inference, but what I'm talking about feels like it's much smaller and more well-defined in scope (see usages of parameters in the function body only, infer from that). I've spent a lot of time converting my Chrome extension to using TypeScript. This is because typescript compiler only allows 2 patterns regarding a generic function call: either provide all type arguments explicitly or delegate inferring task entirely to the compiler. only operateWithGeneric is context-sensitive. which does not help you at all at figuring out the typo elsewhere in the program. Once you have type annotations on type parameters, local inference is sufficient for the vast majority of cases. @RyanCavanaugh For the following example: The ask is for subs to be inferred as having the overloaded type ((s: string) => number) & ((s: number) => string), based on the usage of s within the body of subs. TypeScript is a typed superset of JavaScript that compiles to … Press J to jump to the feed. You signed in with another tab or window. And if such Call type is implemented it will cover my issue also. Please see my response in #15196 (comment). Already on GitHub? In TypeScript, we can specify a function that can be called in different ways by writing overload signatures. There are functions a() and b().Function a() have the same signature (parameters) as function b(). The compiler can build itself, although not all tests pass. But, I'm wondering if this is all or nothing? From this SO post, it sounds like those languages are more restrictive than JS in what they can express (eg. It would work for your example with g, and would infer a as number | string | boolean and b as string & number & boolean. Note that in this example, TypeScript could infer both the type of the E type parameter (from the given string array), as well as the type O based on the return value of the function expression.. To provide the TypeScript compiler some information about expected argument types and return type of the function when arguments are provided in that specific order, we declare functions … There's a reason Flow limits its "global" inference to file boundaries - it's not practical to scale up global inference to an entire large JS program. Constraints. This seems to be @rtpg's expected behavior. Trying to do inference from call sites looks neat in simple examples but can make it very hard to trace errors to their actual sources, and is unscalable to large programs. Successfully merging a pull request may close this issue. An "autofix" feature could be added to fix untyped parameters based on their usage inside the function. Creating an intersection of all constituents in the union. I think a reasonable alternative would be "try to deduce argument types from usage within the function, and opt for any if all collected possibilities in the function body have any inconsistencies (for example number and string, but not {x:string, y:string} and {x:string}, which ends up with {x:string, y:string}). union 转 intersection，如：T1 | T2-> T1 & T2. As you can see the second argument of Func could be any, as known as {}. So infer R should be union of them and it's {}. Enabling this, at least in the current compiler implementation, for higher order types is tantamount to rewriting the core of the compiler; something we are not looking forward to. I decided to rewrite a small TypeScript+React+Jest side project into ReScript. Eg. ". For example, you might write a function to produce a Date that takes either a timestamp (one argument) or a month/day/year specification (three arguments). Successfully merging a pull request may close this issue. There is no global inference algorithm to describe because TS doesn't do global inference ala H-M. Is it that TS tries to do the best it can in the absence of a sound type system? The complexity in implementing either is applying overload resolution as a higher-order type operator. [From an earlier comment] Trying to do inference from call sites looks neat in simple examples but can make it very hard to trace errors to their actual sources, and is unscalable to large programs. Sign in ReScript is not new… So, unless you specify them, they are always going to be any, Right, but is there a reason for them not to? Type Inference. Both of those are static one-pass things that you can follow back as a human. https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html, Allow binding generic functions to a given type, Add type operators that can answer "given function type T, what is the type of its return value when arguments P are passed in? The text was updated successfully, but these errors were encountered: @mhegazy I guess #6606 is a much more complex feature request. Sign in For a given infer type variable V, if any candidates were inferred from co-variant positions, the type inferred for V is a union of those candidates. For an example of inference involving an implied arity, see the curry function in the introduction. 0 errors is more correct (the union type should be handled downstream, and if it's not, the compiler can warn about an unexhaustive match or possibly missing member), but the drawback is nonlocal errors when you're wondering "why is the compiler saying this a union type?". Copy link Member RyanCavanaugh commented Apr 11, 2017. You signed in with another tab or window. We’ll occasionally send you account related emails. Once you have type annotations on type parameters, local inference is sufficient for the vast majority of cases. Close • Posted by 6 minutes ago. Regretfully, partial type argument inference has not been shipped at the time of this… I see. TypeScript 4.0 is supposed to be released in August 2020, ... Function arguments are tuples # Every function head can be described in a tuple type. That reminds me of Flow's existential types, which is a sort of opt-in H-M prover. It has been automatically closed for house-keeping purposes. Global inference also doesn't detect a lot of errors in imperative programs. to your account, Expected behavior: This is what happens when we put contra-variant positions in a conditional type: TypeScript creates an intersection out of it. The very fact that it's opt-in (while the default type is still any) signals that it may not always be helpful. type of function's parameters list; type of arguments list; tuples; Suggestion. So it's great for a single-file playground demo but for "real" software development, you'll end up with approximately the same number of parameter type annotations as if you used a more local inference algorithm. To do this, write some number of function … It's annoying having to iterate every argument even with the version of typescript from this branch. The type of the x variable is inferred to be number . So the TypeScript will infer the value for T from the type of the first argument 1.25 which is number. One of TypeScript’s core principles is that type checking focuses on the shape that values have.This is sometimes called “duck typing” or “structural subtyping”.In TypeScript, interfaces fill the role of naming these types, and are a powerful way of defining contracts within your code as well as contracts with code outside of your project. I may have missed something in the docs, but I can't find any way in typescript to get the types of the parameters in a function. This is very different from a function call - you can be in a function on line 1500 and the first call might be on line 4000. Close. Log in sign up. When you consider that any of those calls might have union types or overloaded signatures in their parameter types, it's not even clear what should happen for a single call (let alone multiple calls): Current inference is very straightforward: types almost always come from initializers or contextual types. TypeScript tends to do a very good job of inferring types when you leave off explicit annotations. For a given infer type variable V, if any candidates were inferred from co-variant positions, the type inferred for V is a union of those candidates. So we would have to infer the same signature as f() itself. In reality g should be inferred as having an identical overloaded signature to f, but it requires some thought to figure out some sensible rules from which this behavior emerges. TypeScript Version: 3.1.0-dev.20180821 Search Terms: function argument infer overload return type rest tuple Code Considering the simplicity of the example, I imagine I might be missing an important detail in the type system that makes this the proper inference result, but I haven't figured it out yet. I am glad that you referenced Scala because it also has essentially the same requirements for where type annotations must be placed as TypeScript does under --noImplicitAny, they are only required on the parameters of functions that are not callbacks and not a value assigned to a typed local. An "autofix" feature could be added to fix untyped parameters based on their usage inside the function. There are fragment functions which take in and returns an object with the key given by the recipe object. This would allow for small anonymous functions to get the "obvious" typing while still requiring to be explicit in cases where there's no good answer. Hello, this is starting the article for advanced TypeScript tutorial series. It feels within reach, and not much different than other kinds of inference. A1 : never ; If no type argument type is explicitly passed, TypeScript will try to infer them by the values passed to the function arguments. As you can see, we can define as many “type argument” as needed. Where a parameter is used definitively inside a function (e.g. Does it mean this issue won't be fixed? @RyanCavanaugh As always, thank you for the detailed and well thought out responses. PUZZLE: How to infer a type from function arguments. Typescript has a powerful generic type system that empowers programmers to craft really reusable components. Instead of manually copying list of parameters with their types from b() to a(), there should be a way to assign types of parameters to a() from b(). Maybe I'm missing a detail here or misunderstanding the meaning of "non-local" here. This is a duplicate of #15196, related discussion can be found in #1265, #15114 and #11440. Search Terms. Or maybe you're indirectly called via some callback, etc.. So infer R should be union of them and it's {}. So, unless you specify them, they are always going to be any. Then, if there's any whiff of ambiguity, including Unions and Overloads, then it backs off to the default any. The reasoning is very straightforward. Here we made on into a generic method. It's important to note that TypeScript does not follow semver, so 4.0 is not as big a deal as it sounds!There can be (and often are) breaking changes between any minor TypeScript versions, and major version bumps like this happen primarily for … Work out all the inevitable hairy edge cases, none of which I've thought about, and bail out in some sensible and graceful way (usually just return undefined, which results in the parameter being, Deal with cycles properly (the algorithm is identical to the stack based one for determining the function's return type, just need to work it into the currently naive implementation), Deal with invocation expressions involving overloaded functions by inferring an overloaded signature for the current function with respect to the used parameter. In comparison to the later, it should also consider generic types of arguments. I would expect the inferred type of g to be (x:any)=>any, and the infered type of h to be (x:number) => number (due to the restrictions placed by the call of f), When compiling with noImplicitAny, I would only expect an error on g, not on h. Actual behavior: But when you use tuples or string literal types, this will sometimes go wrong. type MyComponentProps = {name: string; age: number;}; function MyComponent ({name, age }: … However, if a function has different branches that return different types, the TypeScript compiler may infer the union type or any type.. Sometimes we want to relate two values, but can only operate on a certain subset of values. Right, but is there a reason for them not to? I believe a Hindley-Milner inferer would give a: boolean | number | string, b: boolean | number | string for your example. There are functions a() and b().Function a() have the same signature (parameters) as function b(). (We’ll take a closer look at inference later.) Function components are normal functions that return JSX elements in React and are used to create views. I think the general problem is that global type inference in an imperative language is much more expensive than in a functional language. privacy statement. That type would be incorrect because it allows string, string as an argument list. 这个可能要稍微麻烦一点，需要 infer 配合「 Distributive conditional types 」使用。. Specifically, I'm hoping to be able to constrain an object to the values from "key". If there's indirection, you'll probably have to do this in layers. @RyanCavanaugh relative to your initial example, where you call f (with 3 different type definitions): I feel like in that case it would be reasonable to have this generate 3 different types for g. Maybe this leads to combinatorial explosion of types in some cases, but I imagine in most cases it won't because in real world examples function wrappers exist to make more specific calls to the inner functions. For all parameters that cannot otherwise be typed, an additional step is tried before bailing out and inferring any: Obviously this is nowhere near complete, and the implementation does not even fit with the codebase style, but it is something concrete to play with. Initially, they are stateless components, but with the arrival of React hooks, they can be made stateful and smart/ Defining a React function component takes the React.FunctionComponent shape: I imagine it's a lot of work to build this if it's just some optional feature, though (as opposed to feature everyone would use). Who's right? Sign up for a free GitHub account to open an issue and contact its maintainers and the community. Is it not possible to do local inference based on the function body (that is to say: ignoring the function's calling context)? function test(a: string, b: number) { console.log(a); console.log(b) } I want access to the types string and number, likely as a tuple. let x = 3; // ^ = let x: number Try. (Chapter 3 of Effective TypeScript is devoted to this topic.) Have a question about this project? I agree, this could become a nightmare of different permutations with contradicting definitions. I see the argument for not having Haskell-style square inference, but what I'm talking about feels like it's much smaller and more well-defined in scope (see usages of parameters in the function body only, infer from that). Therefore, it is important to add type annotations to a function as much as possible. Defining statically typed functions # Function declarations # This is an example of a function declaration in TypeScript: function repeat1 (str: string, times: number): string { // (A) return str.repeat(times); } assert.equal( repeat1('*', 5), '*****'); . I created a function that converts Chrome built-in functions from using a callback to being asynchronous, and it's been working well for years. And why does the wrapper function change the behavior of this at all (replacing subs with swapNumberString makes the errors go away), if it's all inferential magic? Typescript never infers function arguments. The difference between OCaml and Scala is instructive here. A : never; type Func = (a: number, b: string) => boolean; type Args = ArgumentsType
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