（Purescript）如何在代数数据类型上模式匹配“空类型”

Question

I'm working with the following algebraic data type in PureScript...

data Extended a = Infinite | Finite a

v1 = Finite 11
v2 = Infinite

I'm having trouble figuring out how to pattern match the "Infinite" case, since it appears that v2 has type forall t140. Extended t140 forall t140. Extended t140 . I'm assuming t140 is some sort of placeholder the compiler automatically fills in. The type of v1 is Extended Int . So if I setup an instance of Eq to compare values of Extended, the Infinite case does not match...

instance extendedEq :: (Eq a) => Eq (Extended a) where
  eq (Finite a) (Finite b) = eq a b
  eq Infinite Infinite = true
  eq Infinite _ = false
  eq _ Infinite = false

So when I try and run v2 == v2 I get the error...

No type class instance was found for Prelude.Eq (Extended _0)

Which makes sense, since I imagine it's trying to find an Eq instance for t140.

So my question is, how can I pattern matching on the Infinite type?

Answer 1

The problem is not about pattern matching or your instance implementation. Your ADT has the same structure as Maybe , and if I try

main = print (Nothing == Nothing)

I get the error code: https://github.com/purescript/purescript/wiki/Error-Code-NoInstanceFound

Your type parameter t140 could be anything in the Eq type class, so the compiler is not able to select an instance. You need to add a type annotation to at least one of the operands of == :

v2 = Infinite :: Extended Int

But I admit it would be more satisfactory if the compiler could figure out that Infinite == Infinite for any (same) type parameter...