Data family instance binding illegally to built in syntax: ()

Question

I'm trying to define a data type family, one of whose parameters results in the type of the empty tuple (), but it doesn't compile. Here is a minimal working example:

{-# LANGUAGE TypeFamilies #-}

data family F a
data instance F Int = ()

The compiler error thrown says "Illegal binding of built-in syntax: ()". Why am I getting this error, even though I'm not trying to change the definition of (), but rather set it as the output of some computation (evaluation of a type family)?

For what it's worth, the program compiled when () is changed to Bool instead.

Answer 1

With data families, you're supposed to provide an ADT or GADT definition on the right side of the equation. () is not a valid definition of a constructor. data instance F Int = Bool declares a single constructor with the name Bool, which works, but doesn't have anything to do with the type Bool. It's just that Bool is available as a constructor name.

What you're trying to do can be realized with type families instead:

type family F a
type instance F Int = ()

-- or in closed form
type family F a where
    F Int = ()

Or you can give a right hand side for the data instance which's equivalent to ():

data instance F Int = FUnit