箭头（ - >）作为haskell数据构造函数

Question

I'm confused about the arrow and what it actually means in the data constructor. I'm surprised it even compiled but I have no idea how to use it.

If I try to use it as the name of a data constructor it doesn't parse, and I'm not sure how else to interpret it. If I interpret it as a function type then the data constructor has no name which also does not make sense to me.

data Type
  = TBool
  | TInt
  | Arrow Type Type
  | (->) Type Type

test :: Type 
test = Arrow TBool TInt -- Ok

test' :: Type
test' = (->) TBool TInt -- Parse Error on input '->'

Answer 1

It does look like a GHC bug (thanks to Kevin Buhr for reporting it) in the use case you provided.

Note

This does fail to parse with GADTs :

data Type where
  TBool :: Type
  TInt :: Type
  Arrow :: Type -> Type -> Type
  (->) :: Type -> Type -> Type

Answer 2

As @leftaroundabout commented, and According to this you must add a : in order to create infix constructors: And according to this question :

Unlike data constructors, infix type constructors are not allowed (other than (->)).

So by example this won't work:

data Type
  = TBool
  | TInt
  | Arrow Type Type
  | (-**) Type Type

test :: Type 
test = Arrow TBool TInt -- Ok

test' :: Type
test' = (-**) TBool TInt

but this will:

data Type
  = TBool
  | TInt
  | Arrow Type Type
  | (:-**) Type Type

test :: Type 
test = Arrow TBool TInt -- Ok

test' :: Type
test' = (:-**) TBool TInt

and this:

data Type
  = TBool
  | TInt
  | Arrow Type Type
  | (:-*) Type Type

test :: Type 
test = Arrow TBool TInt -- Ok

test' :: Type
test' = (:-*) TBool TInt

and in your case you will want something like:

data Type
  = TBool
  | TInt
  | Arrow Type Type
  | (:->) Type Type

test :: Type 
test = Arrow TBool TInt -- Ok

test' :: Type
test' = (:->) TBool TInt