a : -> b 在 Haskell 中是什么意思？

Question

I am trying to implement a dictionary in Haskell and I see that the data type must be

data Rel a b = a :-> b

but I don't see what it means, I think it's the same thing as key-value or similar

Answer 1

Looks like :-> is just a fancy constructor name:

Prelude> data Rel a b = a :-> b
Prelude> :t (5 :-> 6)
(5 :-> 6) :: (Num a, Num b) => Rel a b

In this code, 5 :-> 6 produces a Rel value.

One could've used data Rel ab = a :-% b instead, for example, where :-% would be the constructor name.

Answer 2

This declaration defines what :-> means: it's an infix data constructor.

> :t 3 :-> 5
3 :-> 5 :: (Num a, Num b) => Rel a b

The definition could also have been written

data Rel a b = (:->) a b

in the usual prefix notation.

It's a lot like an ordinary infix operator, except an infix data constructor must start with a : . You may have seen the pseudo-Haskell definition of the list type:

data [] a = [] | a : [a]

which defines (:) as the infix data constructor for non-empty lists.