{- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 Pattern-matching constructors -} {-# LANGUAGE CPP #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE ViewPatterns #-} module MatchCon ( matchConFamily, matchPatSyn ) where #include "HsVersions.h" import GhcPrelude import {-# SOURCE #-} Match ( match ) import HsSyn import DsBinds import ConLike import BasicTypes ( Origin(..) ) import TcType import DsMonad import DsUtils import MkCore ( mkCoreLets ) import Util import Id import NameEnv import FieldLabel ( flSelector ) import SrcLoc import Outputable import Control.Monad(liftM) import Data.List (groupBy) {- 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 alts <- mapM (fmap toRealAlt . matchOneConLike vars ty) groups return (mkCoAlgCaseMatchResult 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 GhcTc) (HsRecFields GhcTc (LPat GhcTc)) matchOneConLike :: [Id] -> Type -> [EquationInfo] -> DsM (CaseAlt ConLike) matchOneConLike vars ty (eqn1 : eqns) -- All eqns for a single constructor = do { let inst_tys = ASSERT( all tcIsTcTyVar ex_tvs ) -- ex_tvs can only be tyvars as data types in source -- Haskell cannot mention covar yet (Aug 2018). 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_orig = Generated , 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 = groupBy 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 = (dL->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 = conLikeExTyCoVars 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( fields1 `equalLength` 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 (dL->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 GhcTc (LPat GhcTc) -> HsRecFields GhcTc (LPat GhcTc) -> Bool same_fields flds1 flds2 = all2 (\(dL->L _ f1) (dL->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 GhcTc] 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 (groupBy 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. -}