Kind Annotations

Brief explanation

In Haskell 98, each type constructor and variable has a kind. Though the Report uses a notation for kinds, they do not appear in Haskell source.

GHC permits types and new type constructors and variables to be annotated with a kind signature, e.g.

type T (f :: * -> *) = f Int

References

• Kinds in the Haskell 98 report
• Explicitly-kinded quantification in the GHC User's Guide

Tickets

Pros

• Allows more machine-checked documentation.
• Haskell 98 defaults kinds to *, which is a nuisance if you want a higher kind. Kind annotations remove the need for awkward workarounds like dummy data constructors. See KindInference for alternative solutions to this problem.

Cons

• con

Proposal

To adopt explicit kind annotations as part of the language, more or less exactly as GHC permits.