Why writeinfer explicitly? Could we do like destructuring?<, string> instead of<infer, string>

By default, it would infer.

@alfaproject I wrote my rationale down in#26242

Would this PR enable this scenario? I didn't see a test quite like it. Basically extracting an inferred type parameter from a specified type parameter.

typeBox<T>={value:T};typeHasBoxedNumber=Box<number>;declarefunctionfoo<TextendsBox<S>,S>(arg:T):S;declareconsthbn:HasBoxedNumber;foo<HasBoxedNumber,infer>(hbn).toFixed();

Based on the design meeting feedback, this has been swapped to variant 2 from the proposal - using the* sigil as a placeholder for inference. We'll need updates to our tmlanguage to get syntax highlighting right (although we already parsed* in type positions for jsdoc, so we probably should have already).

Would this PR enable this scenario? I didn't see a test quite like it. Basically extracting an inferred type parameter from a specified type parameter.

As is, no. Other type parameters (supplied or no) are not currently inference sites for a type parameter. We could enable it here (just by performing some extrainferType calls between the supplied types and their parameters' constraints), probably, but... should we?@ahejlsberg you have an opinion here?

@ahejlsberg you have an opinion here?

I don't think we want constraints to be inference sites, at least not without some explicit indication. At some point we might consider allowinginfer declarations in type parameter lists just as we do in conditional types:

typeUnbox<TextendsBox<inferU>>=U;

Though you can get pretty much the same effect with conditional types:

typeUnbox<TextendsBox<any>>=TextendsBox<inferU> ?U :never;

Alright, I'll leave this as is then and just mention that it'savailable as a branch if we ever change our minds in the future.

@weswigham It seems inconsistent (and kinda strange) to use the* sigil for this when we already use theinfer keyword to denote explicit type inference...

typeTagged<Oextendsobject,T>=O&{__tag:T};// "Infer a type, and make it available under the alias 'T'"declarefunctiongetTag<OextendsTagged<any,any>>(object:O):OextendsTagged<any, inferT> ?T :never;// "Infer a type, and make it available to 'getTag' under the alias at the first type position"getTag<infer>({foo:string,__tag:'bar'})// => 'bar'

This seems like an obvious syntactic duality to me... What was the reason you instead decided to go with*?

The existinginfer T keyword produces a new binding forT; this wouldn't be available in argument positions (e.g. you can't writegetFoo<infer T, T>()). Having theinfer keyword have arity 1 in conditional types and arity 0 in type argument positions seems like a decrease in overall consistency rather than an increase.

It would probably be nice to be able to declare infer on the functions, ex:function foo<A, B = infer>(b: B, c: SomeComplexType<A,B>): SomeOtherComplexType<A,B>

@RyanCavanaugh

Having the infer keyword have arity 1 in conditional types and arity 0 in type argument positions seems like a decrease in overall consistency rather than an increase.

Thanks for the response. :)

Fair enough, but I'd argue that this decrease in consistency is far less than that of introducing an entirely new sigil for this purpose. Is there really a benefit to users in using such a radically different syntax for something whose only difference toinfer T is the arity?

Something else to consider is that TypeScript supports JSDoc, and* in JSDoc meansany. I'm not sure it's a good idea to reuse a symbol that meansany in one context for something that means "please infer this type for me" in another context.

If we're concerned about making operators/keywords context-sensitive, then again it seems like makinginfer context-sensitive is far less of an evil than doing the same for*.

I don't mind* as it jives with flow. Users of typescript can just avoid* in jsdoc and always useany for the purpose easily enough?

I'd also like to see this:

constinstance=newBlah<T,**>(1,'b',false,newDate())

I have a class that bundles many string literal types and I have to enumerate them all at every callsite even when I'm using the code from this branch. Everytime I add a new string literal I have to update every single callsite which is a massive drag ;)

Consider:

typeLiteralMap<S1extendsstring,S2extendsstring,S3extendsstring>={item1:S1,item2:S2,item3:S3}

With this feature at every definition using this type I have to use:

functionuser(map:LiteralMap<*,*,*>){}

Now if I need to add a new literal to my map I have to update this to:

typeLiteralMap<S1extendsstring,S2extendsstring,S3extendsstring,S4extendsstring>={item1:S1,item2:S2,item3:S3,item4:S4,}

which is no big deal, but now I also have to update every single use of this to:

functionuser(map:LiteralMap<*,*,*,*>){}

WithLiteralMap<**> I can just update the definition without affecting every area it is used.

Or it could follow the tuple system

typeLiteralMap<S1?,S2?,S3?>={item1:S1,item2:S2,item3:S3}functionuser(map:LiteralMap){}// infer, infer, inferfunctionuser(map:LiteralMap<boolean>){}// boolean, infer, inferfunctionuser(map:LiteralMap<_,boolean>){}// infer, boolean, infertypeLiteralMap<S1,S2,S3?>={item1:S1,item2:S2,item3:S3}functionuser(map:LiteralMap){}// not allowed, S1 and S2 missingfunctionuser(map:LiteralMap<boolean>){}// not allowed, S2 missingfunctionuser(map:LiteralMap<_,boolean>){}// infer, boolean, infer

alternatively it could use the default assignation (which I guess makes more sense, since if you want it to infer the default type makes no sense?)

typeLiteralMap<S1=_,S2=_,S3=_>={item1:S1,item2:S2,item3:S3}functionuser(map:LiteralMap){}// infer, infer, inferfunctionuser(map:LiteralMap<boolean>){}// boolean, infer, inferfunctionuser(map:LiteralMap<*,boolean>){}// infer, boolean, infertypeLiteralMap<S1,S2,S3=_>={item1:S1,item2:S2,item3:S3}functionuser(map:LiteralMap){}// not allowed, S1 and S2 missingfunctionuser(map:LiteralMap<boolean>){}// not allowed, S2 missingfunctionuser(map:LiteralMap<_,boolean>){}// infer, boolean, infer

Much better than#23696. Great stuff 👍 Thanks!

@RyanCavanaugh What the problem with this PR? Is this too complicated? I wait this feature more than 2 years. I really need this one in a lot of places. Currently I need to use dirty hacks like

functionx<A>(){return<B>(prop:{a:A,b:B})=>void}x<number>()({a:1,b:'b'})

I've throught quite a bit about partial type inference and I agree that, while this is a good step forward, I have a need for definition-site inference much more often (I suspect the divergence of opinions we've seen upthread is likely splitting across library users vs. library authors - the latter of whom are concerned that requiring the caller to opt into inference doesn't really help in making safe/ergonomic/hard-to-misuse APIs). So I'd also like to see at least some thought toward whether/how this syntax fits in with a hypothetical future definition-site syntax.

FWIW, I put together a gist a while back exploring this (see here, feedback appreciated), and concluded thatinfer T, while a better analog with the conditional type syntax, doesn't make as much sense because it really is about providing a default. Additionally, for the case mentioned several times upthread where the usershould not ever specify this type directly, I proposed aprivate T = infer (orprivate T = _, if that placeholder sticks). There's a lot of synergy to consideringinfer andprivate together: while each has value on its own, the combination is greater than the sum of its parts.

Can we split generic types by some symbol, like ";".
Before the splitter, generic types that can be provided from outside.
After the splitter, they are never be provided from outside.

declarefunctionfoo<// available outsideOextendsRecord<string,any>;// privateVextendsstring>(value:V):O&Record<'value',V>consttest=foo<{test:true}>('value')// test: { test: true } & { value: 'value' }

Can we split generic types by some symbol, like ";". Before the splitter, generic types that can be provided from outside. After the splitter, they are never be provided from outside.

declarefunctionfoo<// available outsideOextendsRecord<string,any>;// privateVextendsstring>(value:V):O&Record<'value',V>consttest=foo<{test:true}>('value')// test: { test: true } & { value: 'value' }

Considering that EcmaScript has introduced and TypeScript has adopted an ergonomic# to specify "private" fields in classes, I think it would be intuitive enough to use the same ergonomic approach to specify private type parameters:

declarefunctionfoo<OextendsRecord<string,any>,  #Vextends string>(value: #V):O&Record<'value', #V>consttest=foo<{test:true}>('value')// test: { test: true } & { value: 'value' }

I'd prefer if TypeScript didn't add non-standard features that look like private class fields, as it may increase usage of private class fields, which don't support certain libraries that rely on Proxies (including Vue).

Alternatively, we could support both# and theprivate keyword, like TypeScript already does in class declarations.

Can we split generic types by some symbol, like ";". Before the splitter, generic types that can be provided from outside. After the splitter, they are never be provided from outside.

declarefunctionfoo<// available outsideOextendsRecord<string,any>;// privateVextendsstring>(value:V):O&Record<'value',V>consttest=foo<{test:true}>('value')// test: { test: true } & { value: 'value' }

Considering that EcmaScript has introduced and TypeScript has adopted an ergonomic# to specify "private" fields in classes, I think it would be intuitive enough to use the same ergonomic approach to specify private type parameters:

declarefunctionfoo<OextendsRecord<string,any>,  #Vextends string>(value: #V):O&Record<'value', #V>consttest=foo<{test:true}>('value')// test: { test: true } & { value: 'value' }

Looks qualified to me. I suggested ";" like in "for i" loop syntax. Any way, there are no cases to mix private and public types.

I'm not sure about

declarefunctionfoo<  #Vextends string,OextendsRecord<string, #V>>(value: #V):O&Record<'value', #V>consttest=foo<{test:true}>('value')// test: { test: true } & { value: 'value' }

This discussion of private generic parameters is getting a bit off-topic to this issue (though as I asserted above, it's tangentially relevant insofar as the syntax should work well together). I am very interested in continuing the private types discussion, but I think it's more appropriate on#42388 so I've moved the discussion there.

As far as this specific issue goes, I'm definitely in favor, don't particularly care about_ vsinfer, and would love to see the follow-up of definition-site partial inference happen sooner rather than later.

any updates here? would love the ability to partially supply generics while the remainder continue to infer

Coming from this issue:#19205 . It would be nice if the inference wouldn't be turned off.

Any news on this topic? Following the issue for years. It would be great to have insight on whether this is approved or not. Seeing the comments here this feature seems quite demanded. How it's done is another thing.

@ChibiBlasphem Please readhttps://github.com/microsoft/TypeScript/wiki/FAQ#any-updates.

All the recent activity on this PR has been design discussion. I think we should close this and go back to design to get something workable there.