Language.Haskell.RoleAnnots.Check

This module uses Template Haskell to check whether a declared type has the desired roles. (In versions of GHC before roles, these checks always succeed.)

Documentation

checkRoles :: Name -> [Role] -> Q [Dec]

This function ensures that a declared type has a desired set of roles. Call it in a top-level Template Haskell splice, like this:

 {-# LANGUAGE TemplateHaskell #-}
 module MyMap where

 import Language.Haskell.RoleAnnots
 import Language.Haskell.RoleAnnots.Check

 data MyMap k v = (Nominal k, Representational v) => ...

 $(checkRoles ''MyMap [NominalR, RepresentationalR])

If the roles are not as desired, the checkRoles will cause a compile- time error.

The two quote marks there are Template Haskell syntax to quote an in-scope name. Also, due to the way Template Haskell works, the declaration you are checking must come before the call to checkRoles.

checkRoles may be called in a separate module from where the datatype of interest is defined. It might be useful, for example, in a testsuite.

checkRolesB :: Name -> [Role] -> Q Exp

This function is like checkRoles, but it can be used in a context expecting a Bool value, like this:

 rolesAreCorrect :: Bool
 rolesAreCorrect = $(checkRolesB ''MyMap [NominalR, RepresentationalR])

checkRolesB never produces a compile-time error.

Role is re-exported from Template Haskell for convenience.

data Role

This declaration mirrors the declaration within Template Haskell, for use in earlier versions of GHC.

Constructors

NominalR

RepresentationalR

PhantomR

InferR

Instances

Eq Role

Data Role

Show Role

Typeable Role