deSugar/MatchCon.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    Pattern-matching constructors
    -}

    {-# LANGUAGE CPP #-}

    module MatchCon ( matchConFamily, matchPatSyn ) where

    #include "HsVersions.h"

    import {-# SOURCE #-} Match     ( match )

    import HsSyn
    import DsBinds
    import ConLike
    import TcType
    import DsMonad
    import DsUtils
    import MkCore   ( mkCoreLets )
    import Util
    import ListSetOps ( runs )
    import Id
    import NameEnv
    import FieldLabel ( flSelector )
    import SrcLoc
    import DynFlags
    import Outputable
    import Control.Monad(liftM)

    {-
    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  { let inst_tys = ASSERT( tvs1 `equalLength` ex_tvs )
                             arg_tys ++ mkTyVarTys tvs1

                  val_arg_tys = conLikeInstOrigArgTys con1 inst_tys
            -- dataConInstOrigArgTys takes the univ and existential tyvars
            -- and returns the types of the *value* args, which is what we want

                  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)

            ; 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 = map flSelector (conLikeFieldLabels con1)

        ex_tvs = conLikeExTyVars con1

        -- Choose the right arg_vars in the right order for this group
        -- Note [Record patterns]
        select_arg_vars :: [Id] -> [(ConArgPats, EquationInfo)] -> [Id]
        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 {})      = newSysLocalsDsNoLP 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.

    -}