Detailed kind errors +k,-k

Detailed kind errors +k,-k

Haskell uses a system of kinds to ensure that type expressions are well-formed: for example, to make sure that each type constructor is applied to the appropriate number of arguments. For example, the following program:

 module Main where
 data Tree a  = Leaf a | Tree a :^: Tree a
 type Example = Tree Int Bool

will cause an error:

 ERROR "Main.hs" (line 3): Illegal type "Tree Int Bool" in
                           constructor application

The problem here is that Tree is a unary constructor of kind * -> *, but the definition of Example uses it as a binary constructor with at least two arguments, and hence expecting a kind of the form (* -> * -> k), for some kind k.

By default, Hugs reports problems like this with a simple message like the one shown above. However, if the +k option is selected, then the interpreter will print a more detailed version of the error message, including details about the kinds of the type expressions that are involved:

 ERROR "Main.hs" (line 3): Kind error in constructor application
 *** expression     : Tree Int Bool
 *** constructor    : Tree
 *** kind           : * -> *
 *** does not match : * -> a -> b

In addition, if the +k option is used, then Hugs will also include information about kinds in the information produced by the :info command:

 Prelude> :info Tree
 -- type constructor with kind * -> *
 data Tree a

 -- constructors:
 Leaf :: a -> Tree a
 (:^:) :: Tree a -> Tree a -> Tree a

 -- instances:
 instance Eval (Tree a)

 Prelude>