Prune.hs

{-# LANGUAGE UndecidableInstances, TemplateHaskell, FlexibleInstances #-}

module Hyper.Type.Prune
    ( Prune(..), W_Prune(..), _Pruned, _Unpruned
    ) where

import qualified Control.Lens as Lens
import           Hyper
import           Hyper.Class.Traversable
import           Hyper.Class.Unify (UnifyGen)
import           Hyper.Combinator.Compose (HComposeConstraint1)
import           Hyper.Infer
import           Hyper.Infer.Blame (Blame(..))
import           Hyper.Unify.New (newUnbound)

import           Hyper.Internal.Prelude

data Prune h =
    Pruned | Unpruned (h :# Prune)
    deriving Generic

makeCommonInstances [''Prune]
makePrisms ''Prune
makeHTraversableAndBases ''Prune
makeZipMatch ''Prune
makeHContext ''Prune

-- `HPointed` and `HApplicative` instances in the spirit of `Maybe`

instance HPointed Prune where
    hpure f = Unpruned (f (HWitness W_Prune_Prune))

instance HApply Prune where
    hzip Pruned _ = Pruned
    hzip _ Pruned = Pruned
    hzip (Unpruned x) (Unpruned y) = x :*: y & Unpruned

instance RNodes Prune
instance c Prune => Recursively c Prune
instance RTraversable Prune

type instance InferOf (HCompose Prune t) = InferOf t

instance
    ( Infer m t
    , HPointed (InferOf t)
    , HTraversable (InferOf t)
    , HNodesConstraint t (HComposeConstraint1 (Infer m) Prune)
    ) =>
    Infer m (HCompose Prune t) where
    inferBody (HCompose Pruned) =
        withDict (inferContext (Proxy @m) (Proxy @t)) $
        hpure (Proxy @(UnifyGen m) #> MkContainedH newUnbound)
        & hsequence
        <&> (_HCompose # Pruned, )
    inferBody (HCompose (Unpruned (HCompose x))) =
        hmap
        ( \_ (HCompose (InferChild i)) ->
            i <&> (\(InferredChild r t) -> InferredChild (_HCompose # r) t)
            & InferChild
        ) x
        & inferBody
        <&> Lens._1 %~ (hcomposed _Unpruned #)
    inferContext m _ = withDict (inferContext m (Proxy @t)) Dict

instance
    ( Blame m t
    , HNodesConstraint t (HComposeConstraint1 (Infer m) Prune)
    , HNodesConstraint t (HComposeConstraint1 (Blame m) Prune)
    , HNodesConstraint t (HComposeConstraint1 RNodes Prune)
    , HNodesConstraint t (HComposeConstraint1 (Recursively HFunctor) Prune)
    , HNodesConstraint t (HComposeConstraint1 (Recursively HFoldable) Prune)
    , HNodesConstraint t (HComposeConstraint1 RTraversable Prune)
    ) =>
    Blame m (HCompose Prune t) where
    inferOfUnify _ = inferOfUnify (Proxy @t)
    inferOfMatches _ = inferOfMatches (Proxy @t)