KindsDoNotUnify Error

Example

> import Type.Proxy
> x = Proxy :: Proxy Array
Error found:
in module $PSCI
at line 1, column 5 - line 1, column 24

  Could not match kind

    Type

  with kind

    Type -> Type


while checking the kind of Proxy Array

Cause

This error occurs when the compiler requires two kinds to be equal, but they are not equal.

In the example above, the type constructor Proxy takes an argument of kind Type, but Array has kind Type -> Type, hence the error message.

Fix

• Look at the information in the error message to find the type with the offending kind.

Notes

Order of Error Message

In the Proxy Array example above it is unclear if Type refers to the expected kind and Type -> Type refers to the actual kind found on line 1 or vice-versa. The current implementation of Purescript sometimes presents the expected value first and other times presents the actual value first. In this sense the presentation of this error is inconsistent, so please don't read any order into the error message.

Additional/Missing Type Arguments

This error can occur when a type argument is missing, or an additional type argument is provided:

instance functorArray :: Functor (Array a) where
  ...

This results in a kind error, due to the additional type argument a passed to Array (in Functor (Array a)). The solution is to use Functor Array instead.