{- Language/Haskell/TH/Desugar.hs

(c) Richard Eisenberg 2013
rae@cs.brynmawr.edu
-}

{-# LANGUAGE CPP, MultiParamTypeClasses, FunctionalDependencies,
             TypeSynonymInstances, FlexibleInstances, LambdaCase #-}

-----------------------------------------------------------------------------
-- |
-- Module      :  Language.Haskell.TH.Desugar
-- Copyright   :  (C) 2014 Richard Eisenberg
-- License     :  BSD-style (see LICENSE)
-- Maintainer  :  Richard Eisenberg (rae@cs.brynmawr.edu)
-- Stability   :  experimental
-- Portability :  non-portable
--
-- Desugars full Template Haskell syntax into a smaller core syntax for further
-- processing. The desugared types and constructors are prefixed with a D.
--
----------------------------------------------------------------------------

module Language.Haskell.TH.Desugar (
  -- * Desugared data types
  DExp(..), DLetDec(..), DPat(..), DType(..), DKind, DCxt, DPred(..),
  DTyVarBndr(..), DMatch(..), DClause(..), DDec(..),
  DDerivClause(..), DerivStrategy(..), DPatSynDir(..), DPatSynType,
  Overlap(..), PatSynArgs(..), NewOrData(..),
  DTypeFamilyHead(..), DFamilyResultSig(..), InjectivityAnn(..),
  DCon(..), DConFields(..), DBangType, DVarBangType,
  Bang(..), SourceUnpackedness(..), SourceStrictness(..),
  DForeign(..),
  DPragma(..), DRuleBndr(..), DTySynEqn(..), DInfo(..), DInstanceDec,
  Role(..), AnnTarget(..),

  -- * The 'Desugar' class
  Desugar(..),

  -- * Main desugaring functions
  dsExp, dsDecs, dsType, dsInfo,
  dsPatOverExp, dsPatsOverExp, dsPatX,
  dsLetDecs, dsTvb, dsCxt,
  dsCon, dsForeign, dsPragma, dsRuleBndr,

  -- ** Secondary desugaring functions
  PatM, dsPred, dsPat, dsDec, dsDerivClause, dsLetDec,
  dsMatches, dsBody, dsGuards, dsDoStmts, dsComp, dsClauses,
  dsBangType, dsVarBangType,
#if __GLASGOW_HASKELL__ > 710
  dsTypeFamilyHead, dsFamilyResultSig,
#endif
#if __GLASGOW_HASKELL__ >= 801
  dsPatSynDir,
#endif

  -- * Converting desugared AST back to TH AST
  module Language.Haskell.TH.Desugar.Sweeten,

  -- * Expanding type synonyms
  expand, expandType,

  -- * Reification
  reifyWithWarning,

  -- | The following definitions allow you to register a list of
  -- @Dec@s to be used in reification queries.
  withLocalDeclarations, dsReify, reifyWithLocals_maybe, reifyWithLocals,
  DsMonad(..), DsM,

  -- * Nested pattern flattening
  scExp, scLetDec,

  -- * Utility functions
  applyDExp, applyDType,
  dPatToDExp, removeWilds,
  getDataD, dataConNameToDataName, dataConNameToCon,
  nameOccursIn, allNamesIn, flattenDValD, getRecordSelectors,
  mkTypeName, mkDataName, newUniqueName,
  mkTupleDExp, mkTupleDPat, maybeDLetE, maybeDCaseE,
  substTy,
  tupleDegree_maybe, tupleNameDegree_maybe,
  unboxedSumDegree_maybe, unboxedSumNameDegree_maybe,
  unboxedTupleDegree_maybe, unboxedTupleNameDegree_maybe,
  strictToBang,

  -- ** Extracting bound names
  extractBoundNamesStmt, extractBoundNamesDec, extractBoundNamesPat
  ) where

import Language.Haskell.TH.Desugar.Core
import Language.Haskell.TH.Desugar.Util
import Language.Haskell.TH.Desugar.Sweeten
import Language.Haskell.TH.Syntax
import Language.Haskell.TH.Desugar.Reify
import Language.Haskell.TH.Desugar.Expand
import Language.Haskell.TH.Desugar.Match

import qualified Data.Map as M
import qualified Data.Set as S
#if __GLASGOW_HASKELL__ < 709
import Data.Foldable ( foldMap )
#endif
import Prelude hiding ( exp )

-- | This class relates a TH type with its th-desugar type and allows
-- conversions back and forth. The functional dependency goes only one
-- way because `Type` and `Kind` are type synonyms, but they desugar
-- to different types.
class Desugar th ds | ds -> th where
  desugar :: DsMonad q => th -> q ds
  sweeten :: ds -> th

instance Desugar Exp DExp where
  desugar = dsExp
  sweeten = expToTH

instance Desugar Type DType where
  desugar = dsType
  sweeten = typeToTH

instance Desugar Cxt DCxt where
  desugar = dsCxt
  sweeten = cxtToTH

instance Desugar TyVarBndr DTyVarBndr where
  desugar = dsTvb
  sweeten = tvbToTH

instance Desugar [Dec] [DDec] where
  desugar = dsDecs
  sweeten = decsToTH

instance Desugar [Con] [DCon] where
  desugar = concatMapM dsCon
  sweeten = map conToTH

-- | If the declaration passed in is a 'DValD', creates new, equivalent
-- declarations such that the 'DPat' in all 'DValD's is just a plain
-- 'DVarPa'. Other declarations are passed through unchanged.
-- Note that the declarations that come out of this function are rather
-- less efficient than those that come in: they have many more pattern
-- matches.
flattenDValD :: Quasi q => DLetDec -> q [DLetDec]
flattenDValD dec@(DValD (DVarPa _) _) = return [dec]
flattenDValD (DValD pat exp) = do
  x <- newUniqueName "x" -- must use newUniqueName here because we might be top-level
  let top_val_d = DValD (DVarPa x) exp
      bound_names = S.elems $ extractBoundNamesDPat pat
  other_val_ds <- mapM (mk_val_d x) bound_names
  return $ top_val_d : other_val_ds
  where
    mk_val_d x name = do
      y <- newUniqueName "y"
      let pat'  = wildify name y pat
          match = DMatch pat' (DVarE y)
          cas   = DCaseE (DVarE x) [match]
      return $ DValD (DVarPa name) cas

    wildify name y p =
      case p of
        DLitPa lit -> DLitPa lit
        DVarPa n
          | n == name -> DVarPa y
          | otherwise -> DWildPa
        DConPa con ps -> DConPa con (map (wildify name y) ps)
        DTildePa pa -> DTildePa (wildify name y pa)
        DBangPa pa -> DBangPa (wildify name y pa)
        DSigPa pa ty -> DSigPa (wildify name y pa) ty
        DWildPa -> DWildPa

flattenDValD other_dec = return [other_dec]

fvDType :: DType -> S.Set Name
fvDType = go
  where
    go (DForallT tvbs _cxt ty) = go ty `S.difference` (foldMap dtvbName tvbs)
    go (DAppT ty1 ty2)         = go ty1 `S.union` go ty2
    go (DSigT ty ki)           = go ty `S.union` fvDType ki
    go (DVarT n)               = S.singleton n
    go (DConT _)               = S.empty
    go DArrowT                 = S.empty
    go (DLitT {})              = S.empty
    go DWildCardT              = S.empty
    go DStarT                  = S.empty

dtvbName :: DTyVarBndr -> S.Set Name
dtvbName (DPlainTV n)    = S.singleton n
dtvbName (DKindedTV n _) = S.singleton n

-- | Produces 'DLetDec's representing the record selector functions from
-- the provided 'DCon's.
--
-- Note that if the same record selector appears in multiple constructors,
-- 'getRecordSelectors' will return only one binding for that selector.
-- For example, if you had:
--
-- @
-- data X = X1 {y :: Symbol} | X2 {y :: Symbol}
-- @
--
-- Then calling 'getRecordSelectors' on @[X1, X2]@ will return:
--
-- @
-- [ DSigD y (DAppT (DAppT DArrowT (DConT X)) (DConT Symbol))
-- , DFunD y [ DClause [DConPa X1 [DVarPa field]] (DVarE field)
--           , DClause [DConPa X2 [DVarPa field]] (DVarE field) ] ]
-- @
--
-- instead of returning one binding for @X1@ and another binding for @X2@.

-- See https://github.com/goldfirere/singletons/issues/180 for an example where
-- the latter behavior can bite you.
getRecordSelectors :: Quasi q
                   => DType        -- ^ the type of the argument
                   -> [DCon]
                   -> q [DLetDec]
getRecordSelectors arg_ty cons = merge_let_decs `fmap` concatMapM get_record_sels cons
  where
    get_record_sels (DCon _ _ con_name con _) = case con of
      DRecC fields -> go fields
      _ -> return []
      where
        go fields = do
          varName <- qNewName "field"
          let tvbs     = fvDType arg_ty
              forall'  = DForallT (map DPlainTV $ S.toList tvbs) []
              num_pats = length fields
          return $ concat
            [ [ DSigD name (forall' $ DArrowT `DAppT` arg_ty `DAppT` res_ty)
              , DFunD name [DClause [DConPa con_name (mk_field_pats n num_pats varName)]
                                    (DVarE varName)] ]
            | ((name, _strict, res_ty), n) <- zip fields [0..]
            , fvDType res_ty `S.isSubsetOf` tvbs   -- exclude "naughty" selectors
            ]

    mk_field_pats :: Int -> Int -> Name -> [DPat]
    mk_field_pats 0 total name = DVarPa name : (replicate (total-1) DWildPa)
    mk_field_pats n total name = DWildPa : mk_field_pats (n-1) (total-1) name

    merge_let_decs :: [DLetDec] -> [DLetDec]
    merge_let_decs decs =
      let (name_clause_map, decs') = gather_decs M.empty S.empty decs
       in augment_clauses name_clause_map decs'
        -- First, for each record selector-related declarations, do the following:
        --
        -- 1. If it's a DFunD...
        --   a. If we haven't encountered it before, add a mapping from its Name
        --      to its associated DClauses, and continue.
        --   b. If we have encountered it before, augment the existing Name's
        --      mapping with the new clauses. Then remove the DFunD from the list
        --      and continue.
        -- 2. If it's a DSigD...
        --   a. If we haven't encountered it before, remember its Name and continue.
        --   b. If we have encountered it before, remove the DSigD from the list
        --      and continue.
        -- 3. Otherwise, continue.
        --
        -- After this, scan over the resulting list once more with the mapping
        -- that we accumulated. For every DFunD, replace its DClauses with the
        -- ones corresponding to its Name in the mapping.
        --
        -- Note that this algorithm combines all of the DClauses for each unique
        -- Name, while preserving the order in which the DFunDs were originally
        -- found. Moreover, it removes duplicate DSigD entries. Using Maps and
        -- Sets avoid quadratic blowup for data types with many record selectors.
      where
        gather_decs :: M.Map Name [DClause] -> S.Set Name -> [DLetDec]
                    -> (M.Map Name [DClause], [DLetDec])
        gather_decs name_clause_map _ [] = (name_clause_map, [])
        gather_decs name_clause_map type_sig_names (x:xs)
          -- 1.
          | DFunD n clauses <- x
          = let name_clause_map' = M.insertWith (\new old -> old ++ new)
                                                n clauses name_clause_map
             in if n `M.member` name_clause_map
                then gather_decs name_clause_map' type_sig_names xs
                else let (map', decs') = gather_decs name_clause_map'
                                           type_sig_names xs
                      in (map', x:decs')

          -- 2.
          | DSigD n _ <- x
          = if n `S.member` type_sig_names
            then gather_decs name_clause_map type_sig_names xs
            else let (map', decs') = gather_decs name_clause_map
                                       (n `S.insert` type_sig_names) xs
                  in (map', x:decs')

          -- 3.
          | otherwise =
              let (map', decs') = gather_decs name_clause_map type_sig_names xs
               in (map', x:decs')

        augment_clauses :: M.Map Name [DClause] -> [DLetDec] -> [DLetDec]
        augment_clauses _ [] = []
        augment_clauses name_clause_map (x:xs)
          | DFunD n _ <- x, Just merged_clauses <- n `M.lookup` name_clause_map
          = DFunD n merged_clauses:augment_clauses name_clause_map xs
          | otherwise = x:augment_clauses name_clause_map xs