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ajeffrey@roblox.com 2021-05-04 14:16:49 -05:00
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rfcs/generic-functions.md
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@ -6,7 +6,7 @@ Extend the syntax and semantics of functions to support explicit generic functio
## Motivation
Currently Luau allows generic functions to be inferred but not given expliit type annotations. For example
Currently Luau allows generic functions to be inferred but not given explicit type annotations. For example
```
function id(x) return x end
@ -79,7 +79,7 @@ We propose supporting type parameters which can be instantiated with any type (j
This is a breaking change, in that examples like the unsound program above will no longer typecheck.
Types become more complex, so harder for programmers to reason about, and adding to their space usage. This is particularly noticable anywhere the typechecker has exponential blowup, since small increases in type size can result in large increases in space or time usage.
Types become more complex, so harder for programmers to reason about, and adding to their space usage. This is particularly noticeable anywhere the typechecker has exponential blowup, since small increases in type size can result in large increases in space or time usage.
Not having higher-kinded types stops some examples which are parameterized on container types, for example:
@ -92,7 +92,7 @@ Not having higher-kinded types stops some examples which are parameterized on co
Not having bounded types stops some examples like giving a type to the function that sums an non-empty array:
```
functionm sum(xs)
function sum(xs)
local result = x[0]
for i=1,#xs
result += x[i]
@ -103,8 +103,8 @@ 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 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.
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.
Rather than using Rank-N types, we could use SML-style polymorphism, but this would need something like the (http://users.cis.fiu.edu/~smithg/cop4555/valrestr.html)[value restriction] to be sound.
Rather than using Rank-N types, we could use SML-style polymorphism, but this would need something like the [value restriction](http://users.cis.fiu.edu/~smithg/cop4555/valrestr.html) to be sound.