typescript infer function arguments

TypeScript sees a function as functional component as long as it returns JSX. 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 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. 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. infer R is the union of all available types. What was the rationale for not using global inference? Is it that TS tries to do the best it can in the absence of a sound type system? 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! 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. TypeScript Version: 3.1.0-dev.20180821 Search Terms: function argument infer overload return type rest tuple Code Where a parameter is used definitively inside a function (e.g. At least for the simplest cases, it would be nice for TypeScript to achieve parity with Flow. 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. 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?". I've made a rudimentary attempt at this in master...masaeedu:master that makes the case in the OP (and some others I am interested in) work. This option is true by default when the strict option is true.. The inference is in unidirectional (barring edge cases like recursion), and in inverse direction to the call graph. An "autofix" feature could be added to fix untyped parameters based on their usage inside the function. This is an example of a function declaration in TypeScript: 1. Finally, we infer the parameter to have an intersection of all its "usage types". 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. @vkurchatkin Can you explain? 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). Have a question about this project? That reminds me of Flow's existential types, which is a sort of opt-in H-M prover. Understanding strictBindCallApply. I've spent a lot of time converting my Chrome extension to using TypeScript. I expect TS to be able to infer such return type. Function components. TypeScript tends to do a very good job of inferring types when you leave off explicit annotations. Close • Posted by 6 minutes ago. Is there a way to access the input-object from Typescript's perspective? 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 3.0 added rest-elements-in-tuple-types, so you can get all the arguments in an Array type now. Function parameters are checked one at a time, with the type in each corresponding parameter position checked against each other. Creating an intersection of all constituents in the union. 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. 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). type of function's parameters list; type of arguments list; tuples; Suggestion. Expected behavior: 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). Global inference also doesn't detect a lot of errors in imperative programs. 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: In TypeScript, there are several places where type inference is used to provide type information when there is no explicit type annotation. The difference between OCaml and Scala is instructive here. 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: Or maybe you're indirectly called via some callback, etc.. Another approach is to make this a tooling feature. The reasoning is very straightforward. 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. Isn't the example above just an extension of that? @RyanCavanaugh I thought type guards and flow analysis is doing exactly that - narrowing a type based on usage? I guess the idea will be clear from the code below. PUZZLE: How to infer a type from function arguments. Thanks @RyanCavanaugh for the extensive explanation. no class extensions), which makes H-M usable. privacy statement. This is already the case in TypeScript though). But when you use tuples or string literal types, this will sometimes go wrong. The algorithm is as follows. 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. That type would be incorrect because it allows string, string as an argument list. Therefore, it is important to add type annotations to a function as much as possible. We’ll occasionally send you account related emails. So we would have to infer the same signature as f() itself. You signed in with another tab or window. 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 … Log in sign up. Have a question about this project? Function components are normal functions that return JSX elements in React and are used to create views. The call to person.calculatePrice below is violating this rule because calculatePrice requires two arguments to be passed in. Specifically, I'm hoping to be able to constrain an object to the values from "key". The strictBindCallApply compiler option enables stricter checking of the bind, call, and apply methods on functions. But, I'm wondering if this is all or nothing? Already on GitHub? The function type is (discountCode: … Typing regular function components is as easy as adding type information to the function arguments. An "autofix" feature could be added to fix untyped parameters based on their usage inside the function. Sign in It's very different and very … As you can see the second argument of Func could be any, as known as {}. The compiler can build itself, although not all tests pass. ". 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. This is a duplicate of #15196, related discussion can be found in #1265, #15114 and #11440. And if such Call type is implemented it will cover my issue also. @joewood type guards / flow analysis are straightforward (so to speak...) because they're "top-down" - given a statement in a function, it's relatively easy to determine which control paths it's reachable from, because JavaScript doesn't have control flow structures that can go "up" (e.g. In this example, the TypeScript compiler tries to infer the return type of the add() function to the number type, which is expected.. Consider this OCaml: @aluanhaddad Nit: your example is an Ocamlism, and is not a statement about H-M in general. Please see my response in #15196 (comment). Sign up for a free GitHub account to open an issue and contact its maintainers and the community. This could speed up fixing "no implicit any" migrations. Typescript has a powerful generic type system that empowers programmers to craft really reusable components. 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. 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. Notice the infer keyword. So infer R should be union of them and it's {}. But I thought my problem can be solved through new 2.8. A : never; type Func = (a: number, b: string) => boolean; type Args = ArgumentsType // type Args = [number, … 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. #6606 proposed using typeof as syntax, but you can see the same happening using a call syntax foo(number, string) without the typeof keyword. This is what happens when we put contra-variant positions in a conditional type: TypeScript creates an intersection out of it. For an example of inference involving an implied arity, see the curry function in the introduction. Here's an example in Flow: This is "spooky action at a distance" at its most maximal. The inference would be non-local only in the sense that the return type inference is non-local. 这个可能要稍微麻烦一点，需要 infer 配合「 Distributive conditional types 」使用。. TypeScript can (with relative ease) express pipe function for a given number of composed functions and arity using template arguments. COMEFROM). I'd like to add stronger … Search Terms. To do this, write some number of function … 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. 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 . 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(). 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. By clicking “Sign up for GitHub”, you agree to our terms of service and By clicking “Sign up for GitHub”, you agree to our terms of service and Sign up for a free GitHub account to open an issue and contact its maintainers and the community. (Chapter 3 of Effective TypeScript is devoted to this topic.) Sign in It feels within reach, and not much different than other kinds of inference . 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 Since the function body is well scoped and doesn't belong to multiple files. There are functions a() and b().Function a() have the same signature (parameters) as function b(). 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 … 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), '*****'); . 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. which does not help you at all at figuring out the typo elsewhere in the program. privacy statement. I agree, the bottom approach is different to the type guard processing logic. 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.
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