src/Distribution/Backpack/ModSubst.hs

    {-# LANGUAGE FlexibleInstances #-}
    {-# LANGUAGE PatternGuards #-}

    -- | A type class 'ModSubst' for objects which can have 'ModuleSubst'
    -- applied to them.
    --
    -- See also <https://github.com/ezyang/ghc-proposals/blob/backpack/proposals/0000-backpack.rst>

    module Distribution.Backpack.ModSubst (
        ModSubst(..),
    ) where

    import Prelude ()
    import Distribution.Compat.Prelude hiding (mod)

    import Distribution.Backpack
    import Distribution.ModuleName

    import qualified Data.Map as Map
    import qualified Data.Set as Set

    -- | Applying module substitutions to semantic objects.
    class ModSubst a where
        -- In notation, substitution is postfix, which implies
        -- putting it on the right hand side, but for partial
        -- application it's more convenient to have it on the left
        -- hand side.
        modSubst :: OpenModuleSubst -> a -> a

    instance ModSubst OpenModule where
        modSubst subst (OpenModule cid mod_name) = OpenModule (modSubst subst cid) mod_name
        modSubst subst mod@(OpenModuleVar mod_name)
            | Just mod' <- Map.lookup mod_name subst = mod'
            | otherwise = mod

    instance ModSubst OpenUnitId where
        modSubst subst (IndefFullUnitId cid insts) = IndefFullUnitId cid (modSubst subst insts)
        modSubst _subst uid = uid

    instance ModSubst (Set ModuleName) where
        modSubst subst reqs
            = Set.union (Set.difference reqs (Map.keysSet subst))
                        (openModuleSubstFreeHoles subst)

    -- Substitutions are functorial.  NB: this means that
    -- there is an @instance 'ModSubst' 'ModuleSubst'@!
    instance ModSubst a => ModSubst (Map k a) where
        modSubst subst = fmap (modSubst subst)
    instance ModSubst a => ModSubst [a] where
        modSubst subst = fmap (modSubst subst)
    instance ModSubst a => ModSubst (k, a) where
        modSubst subst (x,y) = (x, modSubst subst y)