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ajeffrey@roblox.com 2021-05-06 09:30:59 -05:00
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19
rfcs/generic-functions.md
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@ -30,11 +30,19 @@ Functions may also take generic type pack arguments for varargs, for instance:
function compose<a...>(... : a...) -> (a...) return ... end
```
This change is *not* only syntax, as explicit type parameters need to be part of the semantics of types. For example:
This change is *not* only syntax, as explicit type parameters need to be part of the semantics of types. For example, we can define a generic identity function
```
local function id(x) return x end
local x: string = id("hi")
local y: number = id(37)
type Id = typeof(id)
```
and two functions
```
function f()
function id(x) return x end
return id
end
function g()
@ -57,7 +65,7 @@ The types of these functions are
so this is okay:
```
local i = f()
local i: Id = f()
local x: string = i("hi")
local y: number = i(37)
```
@ -65,7 +73,8 @@ so this is okay:
but this is not:
```
local i = g()
-- This assignment shouldn't typecheck!
local i: Id = g()
local x: string = i("hi")
-- This is unsound, since it assigns a string to a variable of type number
local y: number = i(37)
@ -103,7 +112,7 @@ Not having bounded types stops some examples like giving a type to the function
## Alternatives
We did originally consider Rank-1 types, but the problem is that's not backward-compatible, as DataBrain pointed out in the [Dev Forum](https://devforum.roblox.com/t/luau-recap-march-2021/1141387/29), since `typeof` allows users to construct generic types even without syntax for them.
We did originally consider Rank-1 types, but the problem is that's not backward-compatible, as DataBrain pointed out in the [Dev Forum](https://devforum.roblox.com/t/luau-recap-march-2021/1141387/29), since `typeof` allows users to construct generic types even without syntax for them. Rank-1 types give a false positive type error in this case, which comes from deployed code.
We could introduce syntax for generic types without changing the semantics, but then there'd be a gap between the syntax (where the types `() -> <a>(a) -> a` and `<a>() -> (a) -> a` are different) and the semantics (where they are not). As noted above, this isn't sound.