How to determine a Python variable's type? I think if we can make this syntax nicely handle filtering without having to do all that, that would be great. Type declarations feel like the right place to be manipulating types in this way, properties on interfaces like this, or functions in #6606, feel like a hacky abuse of a language feature. Let's transform the function greet() to use the function overloading: The greet() function has 2 overload signatures and one implementation signature. My daily routine consists of (but not limited to) drinking coffee, coding, writing, coaching, overcoming boredom . Note that the implementation signature is not callable. After all, how much harder is it to write. This is unavoidable due to how the proposal works. What if you'd like to make the greet() function more flexible? So I'm wondering if we couldn't propose a weaker version without these issues, that has equivalent power. You signed in with another tab or window. The existing declare function foo(v: string): bar; declare function foo(v: number): baz; style overloads would get their return types for given inputs calculated with #6606, yeah. I was assuming T - T would be never, which is a type that admits no values (and hence seems like it would be identical to vanish). like a regular function. 2.2 Implementation signature must be general, Covariance and Contravariance in TypeScript, A Simple Explanation of JavaScript Closures, Gentle Explanation of "this" in JavaScript, 5 Differences Between Arrow and Regular Functions, A Simple Explanation of React.useEffect(), 5 Best Practices to Write Quality JavaScript Variables, 4 Best Practices to Write Quality JavaScript Modules, 5 Best Practices to Write Quality Arrow Functions, Important JavaScript concepts explained in simple words, Software design and good coding practices. In the first place because I know how to iterate over tuples types so as to do stuff with them, but don't see a straight-forward way using union types. As an example, making the object type more permissive by allowing strings as well as numbers wouldn't cause an error when the unknown array is explicitly cast to the old type: Instead, the trick I found to make overloaded types behave nicer is to use an arguments array with a union type: That way the TypeScript compiler could detect whether the individual method overloads matched the unified signature. If you want to use an anonymous function rather than a local overloaded function definition, you have to use a type assertion. He's Co-Founder and Random just-woke-up thought regarding filtering, what if we introduced another special-case type like never? I think if we can make this syntax nicely handle filtering without having to do all that, that would be great. Of course the other option is to just leave that alone and wait on some [K in keyof T if ] feature to be added later, and use your more hardcore methods in the meantime. However that's a problem that already needs to be solved for functions, so I don't see that that could be too much of an issue. I could go ahead and make those, I'm not sure if that's neater than keeping that discussion here, or prematurely fractures the discussion. you can also use this with @masaeedu Well, the (admittedly weak) relevance to this issue is that it might play nicely with the type of type switching this enables. I think #12424 is the main proposal that tackles this. There can be only one implementation signature. What drives the appeal and nostalgia of Margaret Thatcher within UK Conservative Party? At the time, even with the silly type Inc = [1, 2, 3]; // thing, that hacky construct to iterate over them just felt like a nicer idea than using 1000-line overloads for inferior functionality (no tuple support) and terrible performance. A single type parameter should be able to handle the rest parameter.

So let's define those: With those overloads defined, now we just need to force TypeScript to treat our Each overload signature describes one way the function can be invoked. But how do I then fulfill this type with an anonymous function? Are we ready for a separate issue for this stuff? Heh, I'd been interpreting things such that the whole type array would be captured into T this way, but yeah whatever, doesn't matter much here. Typescript: Expand existing function definition. Actually though, my assumption is that that might make it less of an issue for this proposal. Why is rapid expansion/compression reversible? this the default export of our macro file, so we'll do all that at once: You can peruse it all together in Even if we assume modules, Module Augmentations would make ordering perplexing. Are we ready for a separate issue for this stuff? variadic generics) + a way to extract params (#14400 / 6606) + some trick to get the proper return types (like 6606 yet not 6606), yeah. I feel that topic is tangentially related to this one as a lot of the discussion relies on type declaration overloading, but it's not exactly a discussion about type declaration overloading. Argument of type 'unknown' is not assignable to parameter of type 'string[]'. For example, returning the opposite type? This overload signature is not compatible with its implementation signature. You aren't missing anything. To learn more, see our tips on writing great answers. Well, this is why I started a type library, so people wouldn't need to reinvent the wheel. user code. Do I have to necessarily also use overloading? I want union iteration myself as well, but don't know of / haven't made proposals so far; not sure what'd be most elegant. You can now choose to sort by Trending, which boosts votes that have happened recently, helping to surface more up-to-date answers. @masaeedu Ahh, I think I probably explained it badly, sorry. It might give a bit of extra complexity for implementation, but it does address the concern. I suppose the argument could be made that we shouldn't add another place where the clashing issue manifests. I'm working on new issues for anonymous type declarations and function types -> anonymous type declarations now. requires some background information: I have a package called Here's how greet() looks after updating the parameter and return types: Updating the function signature directly to support the multiple ways of invocation is the usual and a good approach. It seems your vanish would just be T - T with subtraction types, unless I'm misunderstanding something. So we'll define our types and then make sure that we cast the macro function Such a function would accept a string or an array of strings as an argument, as well as return a string or an array of strings. Maybe even two, one for anonymous type declarations, one for function types -> anonymous type declarations? I help developers understand Frontend technologies. Should a type with no matches map to nothing in a mapped type? Let's see how function overloading works.

If you're suggesting that be added, I think I like that. That still doesn't work in functions, but yeah. Although rest parameters might be a problem. @Aleksey-Bykov Except this thread already contains potential fixes and workarounds for exactly that issue? Director of Developer Experience at Remix! Hmm, that's interesting. Kent's taught hundreds of thousands of For example, let's implement a Greeter class, with an overload method greet(): The Greeter class contains greet() overload method: 2 overload signatures describing how the method can be called, and the implementation signature containing the proper implementation. Or, I suppose, if you think having to work around the existence of overloads could make the merging issue harder to solve. But the specifics of this babel plugin doesn't matter. I seem to have misremembered; I guess we made objects containing keys or never, use keyof to just get the keys without never, then plug the keys back into the original object so as to filter it. babel-plugin-macros which

and practices. I was recently building some helper classes in TypeScript, and those classes could be called in a variety of ways. A function can have multiple overload signatures: corresponding to the different ways you can invoke the function. Show that involves a character cloning his colleagues and making them into videogame characters? to your account. For comparison, the type declaration equivalent of that interface would be: (a: A, b: B, cs: number[]) could be called as a type as <[MyA, MyB, [MyC1, MyC1]]>. I don't want to make that argument because I like this idea, but it does feel a bit wrong to put this in knowing that it's broken. Aside from regular functions, methods in classes can be overload too. What would the ancient Romans have called Hercules' Club? However, there are situations when I'd recommend not to use the function overloading, but rather stick to the function signature. @TheOtherSamP I am actually suggesting that overloaded functions suffer from the same issues. Cannot handle OpenDirect push notification when iOS app is not launched. Compared to #6606, this could be nice as we might not have to wait for every part of this to happen before we can start enjoying some of the benefits. Like { [K in keyof Foo]: If, T[K], never> }, nevers get pruned automatically. Yep! That said, at the end of the day, the consumer's experience is all that matters, But yeah whatever. // @ts-expect-error because when the cb is provided, void is returned so you can't use ".then"! Many functions have different signatures that are compatible, largely through optional parameters. Is there some reason this should be more of a problem than it already is with functions? The only way I can see it being a blocking issue for this is if you think we shouldn't do anything with type declarations at all until it's fixed. If you have a technical question, please write a comment in the corresponding post. All overloads must be directly adjacent so that they have order, The first matching signature is used to resolve the type, Overload signatures may have different lengths, If no signatures match, it's an error just as it is currently. called with the MacroHandler arguments. I was thinking about this a bit last night, I think I'm going to write up a gist with my thoughts on the topic of type filtering because I want to get my thoughts down on (digital) paper, but I don't want to derail this thread too much. But you can overload a methods too! It's called during the compilation process and Open to ideas. @TheOtherSamP There is an issue tracking subtraction types in a different place, I think that is more of a cross-cutting concern in the language. Overload 2 of 2, '(persons: string[]): string[]', gave the following error. That type = {item: T} notation sounds like the anonymous / unapplied types add-on I mentioned. That does fix the generic example. That's a fair point; so far I hadn't considered it should only consider them overloads if next to one another. But some functions can accept a variable number of arguments, arguments of different types, and could even return different types depending on how you invoke the function. If it catches all, that could warrant a "warning, given these equal requirements things could never drop through to the second declaration". To define a function with multiple call signatures, if you're not able to write a single call signature that is assignable to all the call signatures you want, you will have to use either overloading (which has looser rules for compatibility of the call signatures with the implementation signature) or a type assertion. Grep excluding line that ends in 0, but not 10, 100 etc. Have a question about this project? I mentioned it a few times in #16392; best thing I could come up with was proposing some union-to-tuple operator. How would we selectively map over a type with this? A single type parameter should be able to handle the rest parameter. Connect and share knowledge within a single location that is structured and easy to search. Most of the functions accept a fixed set of arguments. implementation is actually a babel macro, so it looks nothing like the way that Source #13470 (comment). Specify return type in TypeScript arrow function. Reminds me a bit of a recent quote from gcanti/typelevel-ts#8 (comment): A very large part of the useful abstractions in TypeScript end up being hacky mapped types. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. During method overloading, both the overload signatures and implementation signature are now a part of the class. However, as far as TypeScript is Not sure about nice workarounds there. I was primarily suggesting them to allow for the function type conversion, but it would be nice if they could also solve the inlining problem like this too. babel-plugin-codegen src/macro.ts file. macro file like the codegen function we've defined. How to convert a string to number in TypeScript? I think you should only need a single type parameter for the rest parameter. The overload signature defines the parameter and return types of the function, and doesn't have a body. What I like about this, is that it can actually be done nicely in parts. I guess you could do it given this #17636 + those anynomous / unapplied types + 5453 (bonus if incl. I haven't really thought this through at all yet, just throwing it out really. // anonymous types, syntax made up on the spot, not necessarily overload-friendly. Regarding anonymous types (which might need their own issue soon), what about something like this? No overload matches this call. Ok, so keep in mind that the macro function is not actually ever called by For example, don't use the function overloading for optional parameters: Using the optional parameters in the function signature should be enough: For more details check Function overloading Do's and Don'ts. This idea was spawned in #17325, credit to @SimonMeskens for wanting to discuss it properly and pushing me to post it. I am, however, falling asleep and probably embarrassing myself by talking nonsense at this point, so who knows. Can climbing up a tree prevent a creature from being targeted with Magic Missile? I'm thinking that it might be a way to avoid introducing the new syntax/concepts that #6606 is waiting on, and that overloaded type declarations should be capable of describing everything that function signatures can. Initially I'm thinking vanish | T -> vanish, vanish & T -> T, so the opposite of never. Adding some anonymous type declaration syntax after that would then allow this to be a little neater as you'd gain the ability to inline things. There's a desire to have some form of type switching in the type system. Pretty much, yeah, you can fake variadic generics by cramming stuff into tuples. Doesn't seem particularly relevant to this issue. I don't want to put up yet another issue just yet (I feel I've spammed them a little recently) but I may put that up for discussion at some point. Yeah, the mainstream approach is just Partial. vanish would completely remove the property. This function will be called by babel-plugin-macros, and it'll be What does matter is that So Foo and Bar are identical here. In the case of rest parameters typeof ( (args: T) => T[] ), I was thinking the type argument would just still be T, so the produced type would be type = T[]. The text was updated successfully, but these errors were encountered: Potential problems: Does this clash with declaration merging somewhere? I was imagining adjacency would be required in the same way it is for function overloads. Let's consider a function that returns a welcome message to a particular person: The function above accepts 1 argument of type string: the name of the person. I do think it demonstrates some of the power of TDO though. Well, I certainly hadn't thought of that. I think so. Just a thought though, while this gets us type switching, we don't quite have type filtering here just yet. @masaeedu never isn't identical to vanish. babel-plugin-codegen My first instinct is that's a little uncomfortable because it's a different use for the same syntax, but I'm not sure. The only question I'd have is whether you'd ever legitimately want to produce an unpruned never, which this would seemingly prevent. Apparently not though. Allow union types to pattern match type constraints, Allow object types to have property-like associated types, Mapped type filtering - thoughts and discussion. It's a bit weird I know, but a property of type vanish just wouldn't exist. I'm still not quite getting how this would work, as the function isn't being called so we don't have the arguments. So there's a little awkwardness here in introducing syntax that needs to be in a certain place (all have to be adjacent or similar concerns). I may be being thick. He lives with his wife and four kids in Utah. The implementation signature function greet(person: unknown): unknown { } contains the proper logic how the function works. privacy statement. I'd just reason (args: any[]) -> (args: T) -> T must be some array. For the helper classes I was writing, the different signatures weren't compatible, and required some branching logic in order to support the different scenarios. For example: It's not immediately apparent to me how the same thing could nicely be implemented with subtraction types. That doesn't actually eliminate the merging problem though, as with functions those still become overloads. Could some operator (perhaps typeof) be applied to function types to generate matching (anonymous) type declarations? I wasn't aware of that, and a quick test failed to recreate it. Once you have anonymous type declarations, adding a conversion from function types to anonymous type declarations would get you all the features of. Full time educator making our world better, Subscribe to the newsletter to stay up to date with articles,