Too many type binders [GHC-05989]

Language extension: StandaloneKindSignatures

This error means that a type constructor is declared with more arguments than are specified in a corresponding standalone kind signature.

Examples

Mismatched data declaration

Error Message

Main.hs:5:1: error: [GHC-05989]
    • Not a function kind: *
      but extra binders found: y
    • In the data type declaration for ‘A’
  |
5 | data A x y
  | ^^^^^^^^^^

Explanation

This example declares A to have kind * -> *, meaning it is a type constructor that takes one argument. However, the data declaration for A has two arguments (x and y), so it does not match the kind signature.

This can be fixed by either updating the data declaration to match the kind signature (by removing the argument y) or updating the kind signature to match the data declaration (by adding an argument of kind *).

Main1.hs

Before

module Main1 where

type A :: * -> *

data A x y

After

module Main1 where

type A :: * -> *

data A x = A x

Main2.hs

Before

module Main2 where

type A :: * -> *

data A x y

After

module Main2 where

type A :: * -> * -> *

data A x y = A x y