Sat Sep 04 2021
ReScript version: rescript@9.1.4
Consider the following polymorphic functions:
let id1: 'a => 'a = x => x
let id2: 'a => 'a = x => x + 1
These compile successfully, but why does id2 compile? Since it now only accepts an int shouldn't it trigger a compiler warning?
Types of the form 'a, 'b, etc. are known as type variables. What's important to note here is that they are actually placeholders for an undetermined type. This means that the type system will fill these types in with more specific types if needed.
However, if we wanted to ensure the implementation actually accepts polymorphic types, then we need to use a slightly different syntax:
let id1: 'a. 'a => 'a = x => x // OK
let id2: 'a. 'a => 'a = x => x + 1 // Error
We can read 'a. as "for all types of 'a".
And here's an example with multiple types:
let fst: 'a 'b. ('a, 'b) => 'a = (a, _) => a