3 ms·

Could you elaborate on this point for someone who isn't familiar with typescript (or javascript, for that matter). I'm no stranger to strongly typed languages b

by melq 6y ago

Could you elaborate on this point for someone who isn't familiar with typescript (or javascript, for that matter). I'm no stranger to strongly typed languages but that function signature seems pretty complex to me. By elaborate I mean explain what's going on in that signature, and what the critical information it supplies is?

flatline 6y ago
I would say, there is some complexity in reading (and writing) the thing. But there is probably not as much complexity in using it. And you are explicitly codifying the complexity in one place, that would presumably still exist in a dynamically typed language, just implicitly, and likely spread out across the code. This looks pretty nice to me at a glance, just like anything once you write a few of them and come across a few in the wild and take the time to pick them apart they no longer seem so scary.
jcheng 6y ago
It's from the TypeScript 4.0 beta blog post[0], which describes it as: > partialCall takes a function along with the initial few arguments that that function expects. It then returns a new function that takes any other arguments the function needs, and calls them together. The type signature looks like: type Arr = readonly unknown[]; function partialCall<T extends Arr, U extends Arr, R>(f: (...args: [...T, ...U]) => R, ...headArgs: T) { First of all, know that in TypeScript, colon separates a variable or parameter name from its type. So in C or Java you'd say "Date lastModified", in TypeScript it's "lastModified: Date". Now, looking at it piece by piece: function partialCall declares a function named partialCall. <T extends Arr, U extends Arr, R> says that this is a generic function with type parameters T, U, and R (the first two of which must extend Arr, that is, a readonly array of objects whose types are unknown). This function's first parameter is: f: (...args: [...T, ...U]) => R The name of the parameter is `f`, and the type of this parameter is `(...args: [...T, ...U]) => R`. This means that `f` must be a function whose return type is R, and its parameters must match `...args: [...T, ...U]`. The `...` in `...args` makes it a rest parameter[1] (variadic parameter in other languages). The type of `...args` is `[...T, ...U]`, which you can think of as the concatenation of the T and U arrays. (It's a bit strange to see `...` in a type specifier, I can't recall having needed this before.) ...headArgs: T says that the `partialCall` function itself is also variadic, and its arguments are gathered up into headArgs. The type of headArgs is T. I don't know if I'd call this normal or clear compared to the signatures I encounter daily, but it's pretty elegant for a higher-order function that takes a function with any signature, and some arguments that match the given function's parameters, and returns a function with the matched parameters removed. And the implementation of partialCall is just this one line! return (...b: U) => f(...headArgs, ...b) [0]: https://devblogs.microsoft.com/typescript/announcing-typescript-4-0-beta/ https://devblogs.microsoft.com/typescript/announcing-typescr... [1]: https://www.typescriptlang.org/docs/handbook/functions.html#rest-parameters https://www.typescriptlang.org/docs/handbook/functions.html#...
- melq 6y ago
  Thank you for taking the time to explain that, it is much appreciated.

by melq 6y ago

flatline 6y ago

I would say, there is some complexity in reading (and writing) the thing. But there is probably not as much complexity in using it. And you are explicitly codifying the complexity in one place, that would presumably still exist in a dynamically typed language, just implicitly, and likely spread out across the code. This looks pretty nice to me at a glance, just like anything once you write a few of them and come across a few in the wild and take the time to pick them apart they no longer seem so scary.

jcheng 6y ago

It's from the TypeScript 4.0 beta blog post[0], which describes it as: > partialCall takes a function along with the initial few arguments that that function expects. It then returns a new function that takes any other arguments the function needs, and calls them together. The type signature looks like: type Arr = readonly unknown[]; function partialCall<T extends Arr, U extends Arr, R>(f: (...args: [...T, ...U]) => R, ...headArgs: T) { First of all, know that in TypeScript, colon separates a variable or parameter name from its type. So in C or Java you'd say "Date lastModified", in TypeScript it's "lastModified: Date". Now, looking at it piece by piece: function partialCall declares a function named partialCall. <T extends Arr, U extends Arr, R> says that this is a generic function with type parameters T, U, and R (the first two of which must extend Arr, that is, a readonly array of objects whose types are unknown). This function's first parameter is: f: (...args: [...T, ...U]) => R The name of the parameter is `f`, and the type of this parameter is `(...args: [...T, ...U]) => R`. This means that `f` must be a function whose return type is R, and its parameters must match `...args: [...T, ...U]`. The `...` in `...args` makes it a rest parameter[1] (variadic parameter in other languages). The type of `...args` is `[...T, ...U]`, which you can think of as the concatenation of the T and U arrays. (It's a bit strange to see `...` in a type specifier, I can't recall having needed this before.) ...headArgs: T says that the `partialCall` function itself is also variadic, and its arguments are gathered up into headArgs. The type of headArgs is T. I don't know if I'd call this normal or clear compared to the signatures I encounter daily, but it's pretty elegant for a higher-order function that takes a function with any signature, and some arguments that match the given function's parameters, and returns a function with the matched parameters removed. And the implementation of partialCall is just this one line! return (...b: U) => f(...headArgs, ...b) [0]: https://devblogs.microsoft.com/typescript/announcing-typescript-4-0-beta/ https://devblogs.microsoft.com/typescript/announcing-typescr... [1]: https://www.typescriptlang.org/docs/handbook/functions.html#rest-parameters https://www.typescriptlang.org/docs/handbook/functions.html#...

melq 6y ago
Thank you for taking the time to explain that, it is much appreciated.