content: {-
  (c) The University of Glasgow 2006
  (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
  
  
  Pattern-matching constructors
  -}
  {-# LANGUAGE CPP #-}
  
  module Eta.DeSugar.MatchCon ( matchConFamily, matchPatSyn ) where
  
  import {-# SOURCE #-} Eta.DeSugar.Match     ( match )
  
  import Eta.HsSyn.HsSyn
  import Eta.DeSugar.DsBinds
  import Eta.BasicTypes.ConLike
  import Eta.BasicTypes.DataCon
  import Eta.BasicTypes.PatSyn
  import Eta.TypeCheck.TcType
  import Eta.DeSugar.DsMonad
  import Eta.DeSugar.DsUtils
  import Eta.Core.MkCore   ( mkCoreLets )
  import Eta.Utils.Util
  import Eta.Utils.ListSetOps ( runs )
  import Eta.BasicTypes.Id
  import Eta.BasicTypes.NameEnv
  import Eta.BasicTypes.SrcLoc
  import Eta.Main.DynFlags
  import Eta.Utils.Outputable
  import Control.Monad(liftM)
  
  #include "HsVersions.h"
  
  {-
  We are confronted with the first column of patterns in a set of
  equations, all beginning with constructors from one ``family'' (e.g.,
  @[]@ and @:@ make up the @List@ ``family'').  We want to generate the
  alternatives for a @Case@ expression.  There are several choices:
  \begin{enumerate}
  \item
  Generate an alternative for every constructor in the family, whether
  they are used in this set of equations or not; this is what the Wadler
  chapter does.
  \begin{description}
  \item[Advantages:]
  (a)~Simple.  (b)~It may also be that large sparsely-used constructor
  families are mainly handled by the code for literals.
  \item[Disadvantages:]
  (a)~Not practical for large sparsely-used constructor families, e.g.,
  the ASCII character set.  (b)~Have to look up a list of what
  constructors make up the whole family.
  \end{description}
  
  \item
  Generate an alternative for each constructor used, then add a default
  alternative in case some constructors in the family weren't used.
  \begin{description}
  \item[Advantages:]
  (a)~Alternatives aren't generated for unused constructors.  (b)~The
  STG is quite happy with defaults.  (c)~No lookup in an environment needed.
  \item[Disadvantages:]
  (a)~A spurious default alternative may be generated.
  \end{description}
  
  \item
  ``Do it right:'' generate an alternative for each constructor used,
  and add a default alternative if all constructors in the family
  weren't used.
  \begin{description}
  \item[Advantages:]
  (a)~You will get cases with only one alternative (and no default),
  which should be amenable to optimisation.  Tuples are a common example.
  \item[Disadvantages:]
  (b)~Have to look up constructor families in TDE (as above).
  \end{description}
  \end{enumerate}
  
  We are implementing the ``do-it-right'' option for now.  The arguments
  to @matchConFamily@ are the same as to @match@; the extra @Int@
  returned is the number of constructors in the family.
  
  The function @matchConFamily@ is concerned with this
  have-we-used-all-the-constructors? question; the local function
  @match_cons_used@ does all the real work.
  -}
  
  matchConFamily :: [Id]
                 -> Type
                 -> [[EquationInfo]]
                 -> DsM MatchResult
  -- Each group of eqns is for a single constructor
  matchConFamily (var:vars) ty groups
    = do dflags <- getDynFlags
         alts <- mapM (fmap toRealAlt . matchOneConLike vars ty) groups
         return (mkCoAlgCaseMatchResult dflags var ty alts)
    where
      toRealAlt alt = case alt_pat alt of
          RealDataCon dcon -> alt{ alt_pat = dcon }
          _ -> panic "matchConFamily: not RealDataCon"
  matchConFamily [] _ _ = panic "matchConFamily []"
  
  matchPatSyn :: [Id]
              -> Type
              -> [EquationInfo]
              -> DsM MatchResult
  matchPatSyn (var:vars) ty eqns
    = do alt <- fmap toSynAlt $ matchOneConLike vars ty eqns
         return (mkCoSynCaseMatchResult var ty alt)
    where
      toSynAlt alt = case alt_pat alt of
          PatSynCon psyn -> alt{ alt_pat = psyn }
          _ -> panic "matchPatSyn: not PatSynCon"
  matchPatSyn _ _ _ = panic "matchPatSyn []"
  
  type ConArgPats = HsConDetails (LPat Id) (HsRecFields Id (LPat Id))
  
  matchOneConLike :: [Id]
                  -> Type
                  -> [EquationInfo]
                  -> DsM (CaseAlt ConLike)
  matchOneConLike vars ty (eqn1 : eqns)   -- All eqns for a single constructor
    = do  { arg_vars <- selectConMatchVars val_arg_tys args1
                  -- Use the first equation as a source of
                  -- suggestions for the new variables
  
          -- Divide into sub-groups; see Note [Record patterns]
          ; let groups :: [[(ConArgPats, EquationInfo)]]
                groups = runs compatible_pats [ (pat_args (firstPat eqn), eqn)
                                              | eqn <- eqn1:eqns ]
  
          ; match_results <- mapM (match_group arg_vars) groups
  
          ; return $ MkCaseAlt{ alt_pat = con1,
                                alt_bndrs = tvs1 ++ dicts1 ++ arg_vars,
                                alt_wrapper = wrapper1,
                                alt_result = foldr1 combineMatchResults match_results } }
    where
      ConPatOut { pat_con = L _ con1, pat_arg_tys = arg_tys, pat_wrap = wrapper1,
                  pat_tvs = tvs1, pat_dicts = dicts1, pat_args = args1 }
                = firstPat eqn1
      fields1 = case con1 of
                  RealDataCon dcon1 -> dataConFieldLabels dcon1
                  PatSynCon{}       -> []
  
      val_arg_tys = case con1 of
                      RealDataCon dcon1 -> dataConInstOrigArgTys dcon1 inst_tys
                      PatSynCon psyn1   -> patSynInstArgTys      psyn1 inst_tys
      inst_tys = ASSERT( tvs1 `equalLength` ex_tvs )
                 arg_tys ++ mkTyVarTys tvs1
          -- dataConInstOrigArgTys takes the univ and existential tyvars
          -- and returns the types of the *value* args, which is what we want
  
      ex_tvs = case con1 of
                 RealDataCon dcon1 -> dataConExTyVars dcon1
                 PatSynCon psyn1   -> patSynExTyVars psyn1
  
      match_group :: [Id] -> [(ConArgPats, EquationInfo)] -> DsM MatchResult
      -- All members of the group have compatible ConArgPats
      match_group arg_vars arg_eqn_prs
        = ASSERT( notNull arg_eqn_prs )
          do { (wraps, eqns') <- liftM unzip (mapM shift arg_eqn_prs)
             ; let group_arg_vars = select_arg_vars arg_vars arg_eqn_prs
             ; match_result <- match (group_arg_vars ++ vars) ty eqns'
             ; return (adjustMatchResult (foldr1 (.) wraps) match_result) }
  
      shift (_, eqn@(EqnInfo { eqn_pats = ConPatOut{ pat_tvs = tvs, pat_dicts = ds,
                                                     pat_binds = bind, pat_args = args
                                          } : pats }))
        = do ds_bind <- dsTcEvBinds bind
             return ( wrapBinds (tvs `zip` tvs1)
                    . wrapBinds (ds  `zip` dicts1)
                    . mkCoreLets ds_bind
                    , eqn { eqn_pats = conArgPats val_arg_tys args ++ pats }
                    )
      shift (_, (EqnInfo { eqn_pats = ps })) = pprPanic "matchOneCon/shift" (ppr ps)
  
      -- Choose the right arg_vars in the right order for this group
      -- Note [Record patterns]
      select_arg_vars arg_vars ((arg_pats, _) : _)
        | RecCon flds <- arg_pats
        , let rpats = rec_flds flds
        , not (null rpats)     -- Treated specially; cf conArgPats
        = ASSERT2( length fields1 == length arg_vars,
                   ppr con1 $$ ppr fields1 $$ ppr arg_vars )
          map lookup_fld rpats
        | otherwise
        = arg_vars
        where
          fld_var_env = mkNameEnv $ zipEqual "get_arg_vars" fields1 arg_vars
          lookup_fld (L _ rpat) = lookupNameEnv_NF fld_var_env
                                              (idName (unLoc (hsRecFieldId rpat)))
      select_arg_vars _ [] = panic "matchOneCon/select_arg_vars []"
  matchOneConLike _ _ [] = panic "matchOneCon []"
  
  -----------------
  compatible_pats :: (ConArgPats,a) -> (ConArgPats,a) -> Bool
  -- Two constructors have compatible argument patterns if the number
  -- and order of sub-matches is the same in both cases
  compatible_pats (RecCon flds1, _) (RecCon flds2, _) = same_fields flds1 flds2
  compatible_pats (RecCon flds1, _) _                 = null (rec_flds flds1)
  compatible_pats _                 (RecCon flds2, _) = null (rec_flds flds2)
  compatible_pats _                 _                 = True -- Prefix or infix con
  
  same_fields :: HsRecFields Id (LPat Id) -> HsRecFields Id (LPat Id) -> Bool
  same_fields flds1 flds2
    = all2 (\(L _ f1) (L _ f2)
                            -> unLoc (hsRecFieldId f1) == unLoc (hsRecFieldId f2))
           (rec_flds flds1) (rec_flds flds2)
  
  
  -----------------
  selectConMatchVars :: [Type] -> ConArgPats -> DsM [Id]
  selectConMatchVars arg_tys (RecCon {})      = newSysLocalsDs arg_tys
  selectConMatchVars _       (PrefixCon ps)   = selectMatchVars (map unLoc ps)
  selectConMatchVars _       (InfixCon p1 p2) = selectMatchVars [unLoc p1, unLoc p2]
  
  conArgPats :: [Type]    -- Instantiated argument types
                          -- Used only to fill in the types of WildPats, which
                          -- are probably never looked at anyway
             -> ConArgPats
             -> [Pat Id]
  conArgPats _arg_tys (PrefixCon ps)   = map unLoc ps
  conArgPats _arg_tys (InfixCon p1 p2) = [unLoc p1, unLoc p2]
  conArgPats  arg_tys (RecCon (HsRecFields { rec_flds = rpats }))
    | null rpats = map WildPat arg_tys
          -- Important special case for C {}, which can be used for a
          -- datacon that isn't declared to have fields at all
    | otherwise  = map (unLoc . hsRecFieldArg . unLoc) rpats
  
  {-
  Note [Record patterns]
  ~~~~~~~~~~~~~~~~~~~~~~
  Consider
           data T = T { x,y,z :: Bool }
  
           f (T { y=True, x=False }) = ...
  
  We must match the patterns IN THE ORDER GIVEN, thus for the first
  one we match y=True before x=False.  See Trac #246; or imagine
  matching against (T { y=False, x=undefined }): should fail without
  touching the undefined.
  
  Now consider:
  
           f (T { y=True, x=False }) = ...
           f (T { x=True, y= False}) = ...
  
  In the first we must test y first; in the second we must test x
  first.  So we must divide even the equations for a single constructor
  T into sub-goups, based on whether they match the same field in the
  same order.  That's what the (runs compatible_pats) grouping.
  
  All non-record patterns are "compatible" in this sense, because the
  positional patterns (T a b) and (a `T` b) all match the arguments
  in order.  Also T {} is special because it's equivalent to (T _ _).
  Hence the (null rpats) checks here and there.
  
  
  Note [Existentials in shift_con_pat]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Consider
          data T = forall a. Ord a => T a (a->Int)
  
          f (T x f) True  = ...expr1...
          f (T y g) False = ...expr2..
  
  When we put in the tyvars etc we get
  
          f (T a (d::Ord a) (x::a) (f::a->Int)) True =  ...expr1...
          f (T b (e::Ord b) (y::a) (g::a->Int)) True =  ...expr2...
  
  After desugaring etc we'll get a single case:
  
          f = \t::T b::Bool ->
              case t of
                 T a (d::Ord a) (x::a) (f::a->Int)) ->
              case b of
                  True  -> ...expr1...
                  False -> ...expr2...
  
  *** We have to substitute [a/b, d/e] in expr2! **
  Hence
                  False -> ....((/\b\(e:Ord b).expr2) a d)....
  
  Originally I tried to use
          (\b -> let e = d in expr2) a
  to do this substitution.  While this is "correct" in a way, it fails
  Lint, because e::Ord b but d::Ord a.
  -}
  