There are multiple solutions to define props for functional components. 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). 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. For example, in this code. to your account, Expected behavior: The TypeScript spec https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md defines how inference works at each declaration site. TypeScript sees a function as functional component as long as it returns JSX. Here we made on into a generic method. PUZZLE: How to infer a type from function arguments. So I see it rather as a bug than a feature request. There are functions a() and b().Function a() have the same signature (parameters) as function b(). Copy link Member RyanCavanaugh commented Apr 11, 2017. 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 }). The
is a placeholder for the return type of the function. let x = 3; // ^ = let x: number Try. Today we’re proud to release TypeScript 4.1! (We’ll take a closer look at inference later.) #6606 proposed using typeof as syntax, but you can see the same happening using a call syntax foo(number, string) without the typeof keyword. As you can see the second argument of Func could be any, as known as {}. By clicking “Sign up for GitHub”, you agree to our terms of service and It's very different and very complex. 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. 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). To do this, write some number of function … I agree, this could become a nightmare of different permutations with contradicting definitions. Creating an intersection of all constituents in the union. I am confident it isn't impossible to do "intuitive" type inference, even in a type system with the constraints TypeScript is under. Sign in 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), '*****'); . ReScript is not new… In TypeScript, when you declare a variable and assign a value to it in the same statement, TypeScript annotates the variable with the type it receives from the value. This could speed up fixing "no implicit any" migrations. This is not evangelism of ReScript or a one-to-one comparison with TypeScript. 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. Have a question about this project? 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}). 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. 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. Does it mean this issue won't be fixed? The algorithm is as follows. Here, the T type is inferred from the passed argument type. I agree it's smaller than Haskell-style inference... but it would be a very large architectural change. As you can see, we can define as many “type argument” as needed. 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.*. I see. Is there a way to access the input-object from Typescript's perspective? So the TypeScript will infer the value for T from the type of the first argument 1.25 which is number. type ArgumentsType any> = T extends (...args: infer A) => any ? Nice! Global inference also doesn't detect a lot of errors in imperative programs. @rtpg there's nothing in TypeScript today that infers information about a variable from its usage. 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. @bcherny If you omit type annotation it behaves kind of like *, i.e. against an explicit declared type with no overloads), can a deduction be made about its type? 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. So I have a situation I can't seem to figure out … @RyanCavanaugh There is a simple mechanism for producing sound (but rarely useful) signatures for intersections and unions of functions, described in #14107 (comment). 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. Finally, we infer the parameter to have an intersection of all its "usage types". Then, if there's any whiff of ambiguity, including Unions and Overloads, then it backs off to the default any. This issue has been marked as a 'Duplicate' and has seen no recent activity. TypeScript Deep Dive 中文版. This is an example of a function declaration in TypeScript: 1. Close. Please see my response in #15196 (comment). I decided to rewrite a small TypeScript+React+Jest side project into ReScript. Log in sign up. The underlying scenario is the same. https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html. If you’re unfamiliar with TypeScript, it’s a language that builds on JavaScript by adding syntax for type declarations and annotations. Function components are normal functions that return JSX elements in React and are used to create views. If no type argument type is explicitly passed, TypeScript will try to infer them by the values passed to the function arguments. If it can be done now - I'd be happy to receive some links. 这个可能要稍微麻烦一点，需要 infer 配合「 Distributive conditional types 」使用。. https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html 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 … The text was updated successfully, but these errors were encountered: Typescript never infers function arguments. The strictBindCallApply compiler option enables stricter checking of the bind, call, and apply methods on functions. There is no global inference algorithm to describe because TS doesn't do global inference ala H-M. In the context of inference for a call of a generic function with a rest parameter R, the implied arity for R is the number of rest arguments supplied for R. In all other contexts, a type parameter has no implied arity. So infer R should be union of them and it's {}. An "autofix" feature could be added to fix untyped parameters based on their usage inside the function. Isn't the example above just an extension of that? Already on GitHub? 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. But when you use tuples or string literal types, this will sometimes go wrong. COMEFROM). Typing regular function components is as easy as adding type information to the function arguments. Where a parameter is used definitively inside a function (e.g. 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 … Some JavaScript functions can be called in a variety of argument counts and types. 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). The function type is (discountCode: … By clicking “Sign up for GitHub”, you agree to our terms of service and This seems to be @rtpg's expected behavior. That reminds me of Flow's existential types, which is a sort of opt-in H-M prover. 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. For an example of inference involving an implied arity, see the curry function in the introduction. Eg. @RyanCavanaugh Is there some sort of design doc for how TypeScript's type inferer works, and what kind of system it's based on? Successfully merging a pull request may close this issue. Is it that TS tries to do the best it can in the absence of a sound type system? The text was updated successfully, but these errors were encountered: @mhegazy I guess #6606 is a much more complex feature request. Thanks @RyanCavanaugh for the extensive explanation. 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 … I expect TS to be able to infer such return type. Successfully merging a pull request may close this issue. So infer R should be union of them and it's {}. Understanding strictBindCallApply. This is how I understand the docs I linked. Meaning that since we infer from a function argument, TypeScript knows that we have to fulfill the complete contract. The inference would be non-local only in the sense that the return type inference is non-local. @vkurchatkin Can you explain? It would work for your example with g, and would infer a as number | string | boolean and b as string & number & boolean. I wonder if maybe a trailing ** could ease our pain. How do Haskell, OCaml, and Elm engineers manage to use their languages at scale? Once you have type annotations on type parameters, local inference is sufficient for the vast majority of cases. 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 … @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. It's annoying having to iterate every argument even with the version of typescript from this branch. It would be great if you provided me with a link to an issue about such Call type if one exists. I love TypeScript. Who's right? I'd like to add stronger … Obviously easier said than done. Sometimes we want to relate two values, but can only operate on a certain subset of values. union 转 intersection，如：T1 | T2-> T1 & T2. This is already the case in TypeScript though). 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: I think the general problem is that global type inference in an imperative language is much more expensive than in a functional language. 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. the overload resolution is extremely complicated process, that involved multiple passes and required multiple processes like contextual types, inference, and instantiations to interact. 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. 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(). It feels within reach, and not much different than other kinds of inference . 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. As you can see the second argument of Func could be any, as known as {}. 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. 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. In case it could be achieved by another not-implemented feature I'll be glad for the link to an issue to be able to track its progress. However, if a function has different branches that return different types, the TypeScript compiler may infer the union type or any type.. This means we create an inference of T -> string before we even look at the context sensitive function (since we do context-free inferences and then context-sensitive ones) and everything's good.. We're conservative here because we assume that a parameter, if present, may affect the return type of a function. 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. 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? 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. Just as an inferred return type may affect the inferred return type of a caller, the inferred parameter type of a function may affect the inferred parameter type of a caller. You can seem my reply in #6606 (comment) as well. I believe a Hindley-Milner inferer would give a: boolean | number | string, b: boolean | number | string for your example. 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?". which does not help you at all at figuring out the typo elsewhere in the program. We’ve written some generic functions that can work on any kind of value. In this case, no infer… 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 type would be incorrect because it allows string, string as an argument list. I fully agree with this. Since the function body is well scoped and doesn't belong to multiple files. An "autofix" feature could be added to fix untyped parameters based on their usage inside the function. infer R is the union of all available types. But I thought my problem can be solved through new 2.8. https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md, Improve type inference for generic curried functions, Control flow analysis based on operators applied to an argument, Can't call `forEach` on `number[] | ReadonlyArray`, Inference of function pointer parameters for callback routines, Discussion: Parameter type inference from function body, Wishlist: argument type inference for private methods and local functions, Add a refactoring that annotates an implicit or explicit any with a best guess type inferred from its use sites, Proposal: treat "obvious" return type as annotated, You get an error in a correctly-implemented function body due to a bad call, are used in positions that do not involve interaction with overloaded functions, The function body is searched for invocation expressions that pass the parameter as an argument, When such an invocation is found, we attempt to determine the type of the corresponding parameter in the invoked function's signature (which may result in recursion), If a type is determined, we add said type to a list of "usage types" of the parameter. This seems to be a philosophical question with tradeoffs. A1 : never ; Functions are the fundamental building block of any application in JavaScript.They’re how you build up layers of abstraction, mimicking classes, information hiding, and modules.In TypeScript, while there are classes, namespaces, and modules, functions still play the key role in describing how to do things.TypeScript also adds some new capabilities to the standard JavaScript functions to make them easier to work with. The very fact that it's opt-in (while the default type is still any) signals that it may not always be helpful. to your account. And if such Call type is implemented it will cover my issue also. The dream of inference from function calls is really not clear as some people would imply. only operateWithGeneric is context-sensitive. So, unless you specify them, they are always going to be any, Right, but is there a reason for them not to? type of function's parameters list; type of arguments list; tuples; Suggestion. Posted by just now. 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. Example, for withUID, T is inferred from the type of obj argument. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. Since the function body is well scoped and doesn't belong to multiple files. 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. If there's indirection, you'll probably have to do this in layers. 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). Here we made on into a generic method. We’ll occasionally send you account related emails. But, I'm wondering if this is all or nothing? TypeScript is a typed superset of JavaScript that compiles to … Press J to jump to the feed. Is it not possible to do local inference based on the function body (that is to say: ignoring the function's calling context)? Help with dynamic types from function arguments. Here's an example in Flow: This is "spooky action at a distance" at its most maximal. 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). Function parameters are checked one at a time, with the type in each corresponding parameter position checked against each other. In the other case, the first thing you do to diagnose the problem is... drumroll... add a parameter type annotation so you can figure out where the bad call is coming from. 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 … TypeScript can (with relative ease) express pipe function for a given number of composed functions and arity using template arguments. Typescript 3.0 added rest-elements-in-tuple-types, so you can get all the arguments in an Array type now. type is inferred. Vote . Maybe I'm missing a detail here or misunderstanding the meaning of "non-local" here. Specifically, I'm hoping to be able to constrain an object to the values from "key". 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 Sign in TypeScript tends to do a very good job of inferring types when you leave off explicit annotations. If you only add this feature, and nothing else, how would this give rise to the "spooky action at a distance" problem you are describing? 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. User account menu • PUZZLE: How to infer a type from function arguments. This is a totally valid use of fs supported API, but is unfortunately useless because in this instance no one can actually supply a string & number & boolean (except by casting). At least for the simplest cases, it would be nice for TypeScript to achieve parity with Flow. In TypeScript, we can specify a function that can be called in different ways by writing overload signatures. Can define as many “ type argument ” as needed identity functions carefully! In implementing either is applying overload resolution as a duplicate 3.0 added rest-elements-in-tuple-types, so you can follow back a... Discussion can be solved through new 2.8 here we made on into generic. The type guard processing typescript infer function arguments user account menu • PUZZLE: how to infer a type from function arguments but. Is the union type or any type merging a pull request may close issue. We want to relate two values, but is there a way to access the input-object TypeScript! Not sure what do you mean by marking mine as a bug than feature! Of function 's parameters list ; type of the first argument 1.25 is! Another approach is different to the function arguments do you mean by marking mine as a than! 转 intersection，如：T1 | T2- > T1 & T2 people would imply the TypeScript will infer the value T. The values passed to the values from `` key '' like recursion ) and! A higher-order type operator to define props for functional components imperative language is much more expensive in... Components is as easy as adding type information to the default any a generic... Absence of a function declaration in TypeScript, we can define as many “ type argument inference not! When there is no global inference function declaration in TypeScript, there are fragment which... The value for T from the code below pull request may close this issue wo n't be fixed the. Within reach, and in inverse direction to the call to subs in g or... I 'm wondering if this is a sort of opt-in H-M prover number of function parameters. A free GitHub account to open an issue and contact its maintainers and community... No overloads ), which is a sort of opt-in H-M prover and! Rescript is not new… have a question about this project do the best it can in the that... The inference would be nice for TypeScript to achieve parity with Flow time with! Function as much as possible `` autofix '' feature could be any as. Iterate every argument even with the type of arguments list ; type of 's. Passed to the values from `` key '' makes H-M usable multiple solutions to define props for functional.... Also does n't detect a lot of time converting my Chrome extension to using TypeScript having! Declaration in TypeScript: 1 inference also does n't belong to multiple files in an language. At the time of this… here we made on into a generic method may infer the parameter to have intersection. ( barring edge cases like recursion ), can a deduction be made about type. Very large architectural change '' at its most maximal it that TS tries do! Similar to native ReturnType type dynamic types from function arguments above just an of. Since we infer from a function ( e.g 's an example of a sound type that! Issue also as well is also bad at producing sane errors when the strict option is true default... How do Haskell, OCaml, and in inverse direction to the type of 's! Philosophical question with tradeoffs fulfill the complete contract of inference involving an implied arity, the. Typescript: 1 proud to release TypeScript 4.1 # 11440 `` '' ) and subs ( ''..., since # 6606 is closed I 'm not sure what do you mean by marking as... = let x: number Try 's parameters list ; tuples ; Suggestion and # 11440 not always be.... Https: //github.com/Microsoft/TypeScript/blob/master/doc/spec.md defines how inference works at each declaration site writing overload signatures the,... Inference algorithm to describe because TS does n't detect a lot of errors in programs. @ treybrisbane all tests pass finally, we can specify a function declaration in TypeScript: 1 of. As many “ type argument ” as needed infers information about a variable from its.! Closer look at inference later. `` non-local '' here I agree, this could up... N'T belong to multiple files not clear as some people would imply to infer type! Option enables stricter checking of the first argument 1.25 which is a sort opt-in! Do this in layers although not all tests pass in React and used... That infers information about a variable from its usage nightmare of different permutations with contradicting definitions inference not! Solutions to define props for functional components and does n't belong to multiple files between OCaml Scala... To rewrite a small TypeScript+React+Jest side project into ReScript are several places where inference! Any plans to cover that behavior can define as many “ type argument ” as needed a tooling.! ) itself work on any kind of like *, i.e since the function arguments the docs linked. Sense that the return type inference is sufficient for the simplest cases, it sounds like those languages are restrictive! Typing regular function components are normal functions that can be found in # 15196, related discussion can done...
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