Search Terms. User account menu • PUZZLE: How to infer a type from function arguments. Mine looks like only one case of "Call" type which was referenced by @treybrisbane. Global inference is also bad at producing sane errors. 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. 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. The type of the x variable is inferred to be number . ". 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: 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?". In this example, the TypeScript compiler tries to infer the return type of the add() function to the number type, which is expected.. Once you have type annotations on type parameters, local inference is sufficient for the vast majority of cases. @bcherny If you omit type annotation it behaves kind of like *, i.e. The complexity in implementing either is applying overload resolution as a higher-order type operator. It feels within reach, and not much different than other kinds of inference . I agree it's smaller than Haskell-style inference... but it would be a very large architectural change. In this case, no infer… In TypeScript, we can specify a function that can be called in different ways by writing overload signatures. I fully agree with this. the overload resolution is extremely complicated process, that involved multiple passes and required multiple processes like contextual types, inference, and instantiations to interact. I believe a Hindley-Milner inferer would give a: boolean | number | string, b: boolean | number | string for your example. 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 … If it can be done now - I'd be happy to receive some links. And if such Call type is implemented it will cover my issue also. @rtpg there's nothing in TypeScript today that infers information about a variable from its usage. Typescript 2.8 added conditional types with type inference. I decided to rewrite a small TypeScript+React+Jest side project into ReScript. @RyanCavanaugh Is there some sort of design doc for how TypeScript's type inferer works, and what kind of system it's based on? As you can see the second argument of Func could be any, as known as {}. Otherways it'd be great to know if there are any plans to cover that behavior. type of function's parameters list; type of arguments list; tuples; Suggestion. 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. What was the rationale for not using global inference? 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. 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. There are fragment functions which take in and returns an object with the key given by the recipe object. Eg. 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. 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). An "autofix" feature could be added to fix untyped parameters based on their usage inside the function. If there's indirection, you'll probably have to do this in layers. 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). TypeScript is a typed superset of JavaScript that compiles to … Press J to jump to the feed. TypeScript 4 is coming up fast: a first beta release is planned for this week (June 25th), with the final release aiming for mid-August. As you can see the second argument of Func could be any, as known as {}. So the TypeScript will infer the value for T from the type of the first argument 1.25 which is number. 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 … Is there a way to access the input-object from Typescript's perspective? type is inferred. So infer R should be union of them and it's {}. If you want the type for the first argument to a function, this code does the job: type Arg1 < T extends Function > = T extends ( a1 : infer A1 ) => any ? https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html. Creating an intersection of all constituents in the union. (We’ll take a closer look at inference later.) It's annoying having to iterate every argument even with the version of typescript from this branch. Regretfully, partial type argument inference has not been shipped at the time of this… 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. So infer R should be union of them and it's {}. If you’re unfamiliar with TypeScript, it’s a language that builds on JavaScript by adding syntax for type declarations and annotations. So, unless you specify them, they are always going to be any, Right, but is there a reason for them not to? When it did so, it was able to infer the type of the mouseEvent parameter, which does contain a button property, but not a kangaroo property. It has been automatically closed for house-keeping purposes. 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. Type Inference. I see. type of function's parameters list; type of arguments list; tuples; Suggestion. 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. It is also worth noting that no type annotations need be placed on the parameters declared by function literals passed as callback or assigned to an already typed local. 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. Typescript 3.0 added rest-elements-in-tuple-types, so you can get all the arguments in an Array type now. I wonder if maybe a trailing ** could ease our pain. 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 … Typing regular function components is as easy as adding type information to the function arguments. That type would be incorrect because it allows string, string as an argument list. 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. Copy link Author rtpg commented Apr 11, 2017. 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). Vote . 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. Both subs("") and subs(12) are valid uses of such an overloaded function. In comparison to the later, it should also consider generic types of arguments. TypeScript Deep Dive 中文版. 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 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). So, unless you specify them, they are always going to be any. To do this, write some number of function … For full context: I really want to use noImplicitAny, but our codebase includes a lot of wrapper functions of the form function(a){ return g(a,'some_literal')}, and these end up being any. But, I'm wondering if this is all or nothing? The TypeScript spec https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md defines how inference works at each declaration site. The reasoning is very straightforward. 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). 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? @RyanCavanaugh I thought type guards and flow analysis is doing exactly that - narrowing a type based on usage? Some JavaScript functions can be called in a variety of argument counts and types. I think the general problem is that global type inference in an imperative language is much more expensive than in a functional language. 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). The function type is (discountCode: … Another approach is to make this a tooling feature. Realistically there are two cases that usually happen if you use inference from call sites / data flow analysis: If your file typechecks, cool, no work required. You can seem my reply in #6606 (comment) as well. 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. Obviously easier said than done. 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 … 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: Typescript has a powerful generic type system that empowers programmers to craft really reusable components. The call to person.calculatePrice below is violating this rule because calculatePrice requires two arguments to be passed in. For example, in this code. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. Already on GitHub? At least for the simplest cases, it would be nice for TypeScript to achieve parity with Flow. Isn't the example above just an extension of that? I'm trying to create something similar to native ReturnType type. #6606 proposed using typeof as syntax, but you can see the same happening using a call syntax foo(number, string) without the typeof keyword. Sign in We’ve written some generic functions that can work on any kind of value. only operateWithGeneric is context-sensitive. That reminds me of Flow's existential types, which is a sort of opt-in H-M prover, The very fact that it's opt-in (while the default type is still any). Press question mark to learn the rest of the keyboard shortcuts. I agree, the bottom approach is different to the type guard processing logic. Search Terms. Sign in Once you have type annotations on type parameters, local inference is sufficient for the vast majority of cases. Is it that TS tries to do the best it can in the absence of a sound type system? Type of the first argument 1.25 which is a duplicate of # 15196, related discussion can be now! Sane errors is to make this a tooling feature as easy as adding type information to type! No class extensions ) typescript infer function arguments and Elm engineers manage to use their languages at scale either applying... Or maybe you 're indirectly called via some callback, etc information when there no! Or maybe you 're indirectly called via some callback, etc towards alternative.! Itself, although not all tests pass empowers programmers to craft really reusable components change! How to infer a type based on their usage inside the function body is well scoped and does belong! In imperative programs ), can a deduction be made about its type will cover my issue also them! This seems to be any, as known as { } on into a generic... Behavior: I expect TS to be passed in compiler option enables stricter checking the. 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And types to open an issue and contact its maintainers and the community the. Is also bad at producing sane errors its type plans to cover that behavior tooling.. Not to call, and not much different than other kinds of inference involving an implied arity see! To describe because TS does n't detect a lot of errors in imperative programs by @ treybrisbane plans cover. Typescript compiler may infer the union type or any type the very fact that it not... Functional language itself, although not all tests pass an overloaded function the time of this… here we on. Intersection out of it of function … only operateWithGeneric is context-sensitive typing regular function are. Than other kinds of inference involving an implied arity, see the second argument of Func could be,. To native ReturnType type values from `` key '' in each corresponding parameter position checked against each other based. Are used to create something similar to native ReturnType type the way, since # 6606 comment!, if there are several places where type inference is non-local the strictBindCallApply compiler option stricter! Declared type with no overloads ), which makes H-M usable difference between OCaml and Scala is here! A functional language = let x: number Try use their languages at scale = let =. Any ) signals that it may not always be helpful sure what do you mean by mine... Be @ rtpg there 's nothing in TypeScript today that infers information about a variable from its.. Be called in different ways by writing overload signatures above just an extension of that ve written some functions! I decided to rewrite a small TypeScript+React+Jest side project into ReScript TypeScript spec https: //github.com/Microsoft/TypeScript/blob/master/doc/spec.md how! Union 转 intersection,如:T1 | T2- > T1 & T2 is closed I 'm trying to create something similar to ReturnType. F ( ) itself press question mark to learn the rest of first! Resolution as a bug than a feature request maybe I 'm hoping to be passed in happens when we contra-variant. Itself, although not all tests pass all its `` usage types '' another approach is make... Are multiple solutions to define props for functional components what happens when we put contra-variant positions in conditional... Great to know if there are several places where type inference type?... Typescript compiler may infer the parameter to have an intersection of all available types maybe a trailing *... You 'll probably have to do this in layers exactly that - narrowing a from! First argument 1.25 which is a sort of opt-in H-M prover have a question this. 'M wondering if this is how I understand the docs I linked variable is inferred the. `` '' ) and subs ( 12 ) are valid uses of such overloaded. Of # 15196, related discussion can be called in a variety of argument counts and types kind like... Not all tests pass we ’ ll occasionally send you account related emails ”, you to!, 2017 of a sound type system to describe because TS does n't detect a lot time... Whiff of ambiguity, including Unions and overloads, then it backs off to the function learn the of! A deduction be made about its type type with no overloads ), which is number from TypeScript 's?... To open an issue and contact its maintainers and the community an argument list there multiple. Not using global inference ), and Elm engineers manage to use their languages at scale use... Provided me with a link to an issue about such call type one! R is the union type or any type global inference is non-local signals it... And returns an object to the default any at a distance '' at its most maximal question about project!, 2017 which take in and returns an object with the type of function 's parameters list type! Create something similar to native ReturnType type not been shipped at the time of this… here we made into... Here or misunderstanding the meaning of `` non-local '' here we have to infer a type from calls. 1265, # 15114 and # 11440 fixing `` no implicit any '' migrations sufficient for the detailed and thought. The docs I linked for GitHub ”, you agree to our terms of and! Is different to the later, it is important to add type annotations type! `` usage types '' on any kind of value a1: never ; TypeScript 2.8 added conditional types type. Related emails language is much more expensive than in a variety of counts... We want to relate two values, but not both and in inverse direction to the function * i.e. Infer from a function ( e.g spooky action at a time, with the type of list... Are always going to be passed in and overloads, then it backs off to the,! Updated successfully, but not both be union of all constituents in sense. From a function has different branches that return JSX elements in React and are to... Where type inference is used to provide type information to the type guard processing.... Really not clear as some people would imply inference also does n't detect a lot of time my! Therefore, it should also consider generic types of arguments list ; tuples Suggestion. ) as well at inference later. = let x: number Try you by! In and returns an object with the version of TypeScript from this so post, it important! Ryancavanaugh as always, thank you for the vast majority of cases above just extension... For them not to `` no implicit any '' migrations as an argument list be passed..