TH.hs

{-# LANGUAGE ExplicitNamespaces, CPP #-}

-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Singletons.TH
-- Copyright   :  (C) 2013 Richard Eisenberg
-- License     :  BSD-style (see LICENSE)
-- Maintainer  :  Richard Eisenberg (eir@cis.upenn.edu)
-- Stability   :  experimental
-- Portability :  non-portable
--
-- This module contains everything you need to derive your own singletons via
-- Template Haskell.
--
-- TURN ON @-XScopedTypeVariables@ IN YOUR MODULE IF YOU WANT THIS TO WORK.
--
----------------------------------------------------------------------------

module Data.Singletons.TH (
  -- * Primary Template Haskell generation functions
  singletons, singletonsOnly, genSingletons,
  promote, promoteOnly, genDefunSymbols, genPromotions,

  -- ** Functions to generate equality instances
  promoteEqInstances, promoteEqInstance,
  singEqInstances, singEqInstance,
  singEqInstancesOnly, singEqInstanceOnly,
  singDecideInstances, singDecideInstance,

  -- ** Functions to generate Ord instances
  promoteOrdInstances, promoteOrdInstance,

  -- ** Functions to generate Ord instances
  promoteBoundedInstances, promoteBoundedInstance,

  -- ** Utility function
  cases,

  -- * Basic singleton definitions
  Sing(SFalse, STrue, STuple0, STuple2, STuple3, STuple4, STuple5, STuple6, STuple7),
  module Data.Singletons,

  -- * Auxiliary definitions
  -- | These definitions might be mentioned in code generated by Template Haskell,
  -- so they must be in scope.

  PEq(..), If, sIf, (:&&), SEq(..),
  POrd(..),
  Any,
  SDecide(..), (:~:)(..), Void, Refuted, Decision(..),
  Proxy(..), KProxy(..), SomeSing(..),

  Error, ErrorSym0,
  TrueSym0, FalseSym0,
  LTSym0, EQSym0, GTSym0,
  Tuple0Sym0,
  Tuple2Sym0, Tuple2Sym1, Tuple2Sym2,
  Tuple3Sym0, Tuple3Sym1, Tuple3Sym2, Tuple3Sym3,
  Tuple4Sym0, Tuple4Sym1, Tuple4Sym2, Tuple4Sym3, Tuple4Sym4,
  Tuple5Sym0, Tuple5Sym1, Tuple5Sym2, Tuple5Sym3, Tuple5Sym4, Tuple5Sym5,
  Tuple6Sym0, Tuple6Sym1, Tuple6Sym2, Tuple6Sym3, Tuple6Sym4, Tuple6Sym5, Tuple6Sym6,
  Tuple7Sym0, Tuple7Sym1, Tuple7Sym2, Tuple7Sym3, Tuple7Sym4, Tuple7Sym5, Tuple7Sym6, Tuple7Sym7,

  SuppressUnusedWarnings(..)

 ) where

import Data.Singletons
import Data.Singletons.Single
import Data.Singletons.Promote
import Data.Singletons.Prelude.Instances
import Data.Singletons.Prelude.Bool
import Data.Singletons.Prelude.Eq
import Data.Singletons.Prelude.Ord
import Data.Singletons.Types
import Data.Singletons.Void
import Data.Singletons.Decide
import Data.Singletons.TypeLits
import Data.Singletons.SuppressUnusedWarnings
import Language.Haskell.TH.Desugar

import GHC.Exts
import Language.Haskell.TH
import Data.Singletons.Util
import Control.Applicative

-- | The function 'cases' generates a case expression where each right-hand side
-- is identical. This may be useful if the type-checker requires knowledge of which
-- constructor is used to satisfy equality or type-class constraints, but where
-- each constructor is treated the same.
cases :: DsMonad q
      => Name        -- ^ The head of the type of the scrutinee. (Like @''Maybe@ or @''Bool@.)
      -> q Exp       -- ^ The scrutinee, in a Template Haskell quote
      -> q Exp       -- ^ The body, in a Template Haskell quote
      -> q Exp
cases tyName expq bodyq = do
  info <- reifyWithLocals tyName
  dinfo <- dsInfo info
  case dinfo of
    DTyConI (DDataD _ _ _ _ ctors _) _ -> fmap expToTH $ buildCases ctors
    _ -> fail $ "Using <<cases>> with something other than a type constructor: "
                ++ (show tyName)
  where buildCases ctors =
          DCaseE <$> (dsExp =<< expq) <*>
                     mapM (\con -> DMatch (conToPat con) <$> (dsExp =<< bodyq)) ctors

        conToPat :: DCon -> DPat
        conToPat (DCon _ _ name fields) =
          DConPa name (map (const DWildPa) $ tysOfConFields fields)