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Apply the alternative that allows single element type pack parameter assignment to a type argument

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Added an additional note on type pack element extraction.
This commit is contained in:
Vyacheslav Egorov 2021-10-14 18:12:23 +03:00
parent 7237cd6e98
commit ed70fd019a
1 changed files with 22 additions and 16 deletions
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38
rfcs/syntax-type-alias-type-packs.md
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@ -106,12 +106,13 @@ type F3<S...> = Y<string, S..., (number, S...)> -- T... = (string, S...), U... =
In type parameter list, types inside the parentheses always produce a type pack.
This is in contrast to function return type pack annotation, where `() -> number` is the same as `() -> (number)`.
This is a breaking change.
Users can already have type alias instantiations like these:
However, to preserve backwards-compatibility with optional parenthesis around regular types, type alias instantiation is allowed to assign a non-variadic type pack parameter with a single element to a type argument:
```lua
type X<T> = T?
type A = X<(number)> -- valid right now, typechecking error after this RFC
type X<T, U> = (T) -> U?
type A = X<(number), (string)> -- T = number, U = string
type Y<T...> = (T...) -> ()
type B = Y<(number), (string)> -- error: too many type pack parameters
```
Explicit type pack syntax is not available in other type pack annotation contexts.
@ -129,6 +130,8 @@ type Y<S...> = Car<S...> -- error, not enough regular type argument
type Z = Y<number, string, boolean> -- error, Y doesn't have a valid definition
```
With our immediate instantiation, at the point of `Car<S...>`, we only know that `S...` is a type pack, but contents are not known.
Splitting off a single type is is a common pattern with variadic templates in C++, but we don't allow type alias overloads, so use cases are more limited.
### Type alias can't result in a type pack
@ -150,17 +153,6 @@ Support for variadic types in the middle of a type pack can be found in TypeScri
## Alternatives
### Backwards compatibility for single type in parentheses
It is possible to allow single element type pack parameter assignment to a type argument:
```lua
type X<T> = T?
type A = X<(number)>
```
This is not proposed to keep separation between type and type packs more clear.
If we supported warning generation, we could create a deprecation period, but since our typechecking errors don't block compilation, it is not that critical.
### Function return type syntax for explicit type packs
Another option that was considered is to parse `(T)` as `T`, like we do for return type annotation.
@ -176,3 +168,17 @@ type Y<T..., U...>
--- two items that were enough to satisfy only a single T... in X are enough to satisfy two T..., U... in Y
type E = Y<number, number> -- T... = (number), U... = (number)
```
### Special mark for single type type packs
In the Rust language, there is a special disambiguation syntax for single element tuples and single element type packs using a trailing comma:
```rust
(Type,)
```
In Python, the same idea is used for single element tuple values:
```python
value = (1, )
```
Since our current ruleset no longer has a problem with single element type tuples, I don't think we need syntax-directed disambiguation option like this one.