Sum.hs

{-# LANGUAGE UndecidableInstances #-} -- due to type hell
{-# LANGUAGE AllowAmbiguousTypes  #-} -- due to generic typeclass design

module Generic.Data.Function.Traverse.Sum where

import GHC.Generics
import Generic.Data.Function.Common.Generic
import Generic.Data.Function.Common.Generic.Meta
import Generic.Data.Function.Traverse.Constructor

import Data.Text ( Text )
import Control.Applicative qualified as Applicative
import Control.Applicative ( Alternative((<|>)) )

-- | Sum type monads that can be generically 'traverse'd.
class GenericTraverse tag => GenericTraverseSum tag where
    -- | Try to parse a prefix tag of type 'pt'.
    --
    -- Relevant metadata is provided as arguments.
    genericTraverseSumPfxTagAction
        :: GenericTraverseC tag pt
        => String   -- ^ data type name
        -> GenericTraverseF tag pt

    -- | Parse error due to no constructor matching the parsed prefix tag.
    --
    -- Relevant metadata is provided as arguments.
    genericTraverseSumNoMatchingCstrAction
        :: String   -- ^ data type name
        -> [String] -- ^ non-matching constructor names
        -> Text     -- ^ prefix tag, prettified
        -> GenericTraverseF tag a

-- | How to use a type as a prefix tag in a generic sum type parser.
data PfxTagCfg a = PfxTagCfg
  { pfxTagCfgFromCstr :: String -> a
  -- ^ How to turn a constructor name into a prefix tag.

  , pfxTagCfgEq :: a -> a -> Bool
  -- ^ How to compare prefix tags for equality.
  --
  -- By shoving this into our generic derivation config, we can avoid adding an
  -- insidious 'Eq' constraint. In general, you will want to set this to '(==)'.

  , pfxTagCfgShow :: a -> Text
  -- ^ Make a prefix tag human-readable. 'show' is often appropriate.
  }

class GTraverseSum tag gf where
    -- have to stuff pt constraint in here because we can't access pt externally
    gTraverseSum
        :: GenericTraverseC tag pt
        => PfxTagCfg pt -> GenericTraverseF tag (gf p)

instance GenericTraverse tag => GTraverseSum tag V1 where
    gTraverseSum _ = genericTraverseV1 @tag

-- | Test all constructors of the given non-void data type; if they all fail,
--   run a failure action and pass it all the constructors names in the type.
instance
  ( Alternative (GenericTraverseF tag)
  , Monad (GenericTraverseF tag)
  , GenericTraverseSum tag, GTraverseCSum tag cd gf
  , Datatype cd
  , KnownSymbols (CstrNames gf)
  ) => GTraverseSum tag (D1 cd gf) where
    gTraverseSum ptc = do
        pt <- genericTraverseSumPfxTagAction @tag cd
        M1 <$> (gTraverseCSum @tag @cd ptc pt <|> parseErrorNoMatch pt)
      where
        cd = datatypeName' @cd
        parseErrorNoMatch pt =
            genericTraverseSumNoMatchingCstrAction @tag cd testedCstrs
                ((pfxTagCfgShow ptc) pt)
        testedCstrs = symbolVals @(CstrNames gf)

class GTraverseCSum tag cd gf where
    gTraverseCSum :: PfxTagCfg pt -> pt -> GenericTraverseF tag (gf p)

-- | Combine constructor options with '(<|>)' ("or").
instance
  ( Alternative (GenericTraverseF tag)
  , GTraverseCSum tag cd l
  , GTraverseCSum tag cd r
  ) => GTraverseCSum tag cd (l :+: r) where
    gTraverseCSum ptc pt = l <|> r
      where
        l = L1 <$> gTraverseCSum @tag @cd ptc pt
        r = R1 <$> gTraverseCSum @tag @cd ptc pt

-- | If the constructor matches the expected prefix tag, then return the action
--   handling that constructor's contents, else return the empty action.
instance
  ( Alternative (GenericTraverseF tag)
  , GTraverseC tag cd cc 0 gf, Constructor cc
  ) => GTraverseCSum tag cd (C1 cc gf) where
    gTraverseCSum ptc pt = do
        if   (pfxTagCfgEq ptc) pt ptCstr
        then M1 <$> gTraverseC @tag @cd @cc @0
        else Applicative.empty
      where
        ptCstr = (pfxTagCfgFromCstr ptc) (conName' @cc)