2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[MatchCon]{Pattern-matching constructors}
7 module MatchCon ( matchConFamily ) where
9 #include "HsVersions.h"
13 import {-# SOURCE #-} Match ( match )
15 import HsSyn ( Pat(..), HsConDetails(..) )
16 import DsBinds ( dsLHsBinds )
17 import DataCon ( isVanillaDataCon, dataConInstOrigArgTys )
18 import TcType ( tcTyConAppArgs )
19 import Type ( mkTyVarTys )
26 import ListSetOps ( equivClassesByUniq )
27 import SrcLoc ( unLoc, Located(..) )
28 import Unique ( Uniquable(..) )
32 We are confronted with the first column of patterns in a set of
33 equations, all beginning with constructors from one ``family'' (e.g.,
34 @[]@ and @:@ make up the @List@ ``family''). We want to generate the
35 alternatives for a @Case@ expression. There are several choices:
38 Generate an alternative for every constructor in the family, whether
39 they are used in this set of equations or not; this is what the Wadler
43 (a)~Simple. (b)~It may also be that large sparsely-used constructor
44 families are mainly handled by the code for literals.
46 (a)~Not practical for large sparsely-used constructor families, e.g.,
47 the ASCII character set. (b)~Have to look up a list of what
48 constructors make up the whole family.
52 Generate an alternative for each constructor used, then add a default
53 alternative in case some constructors in the family weren't used.
56 (a)~Alternatives aren't generated for unused constructors. (b)~The
57 STG is quite happy with defaults. (c)~No lookup in an environment needed.
59 (a)~A spurious default alternative may be generated.
63 ``Do it right:'' generate an alternative for each constructor used,
64 and add a default alternative if all constructors in the family
68 (a)~You will get cases with only one alternative (and no default),
69 which should be amenable to optimisation. Tuples are a common example.
71 (b)~Have to look up constructor families in TDE (as above).
75 We are implementing the ``do-it-right'' option for now. The arguments
76 to @matchConFamily@ are the same as to @match@; the extra @Int@
77 returned is the number of constructors in the family.
79 The function @matchConFamily@ is concerned with this
80 have-we-used-all-the-constructors? question; the local function
81 @match_cons_used@ does all the real work.
83 matchConFamily :: [Id]
87 matchConFamily (var:vars) ty eqns_info
89 -- Sort into equivalence classes by the unique on the constructor
90 -- All the EqnInfos should start with a ConPat
91 groups = equivClassesByUniq get_uniq eqns_info
92 get_uniq (EqnInfo { eqn_pats = ConPatOut (L _ data_con) _ _ _ _ _ : _}) = getUnique data_con
94 -- Get the wrapper from the head of each group. We're going to
95 -- use it as the pattern in this case expression, so we need to
96 -- ensure that any type variables it mentions in the pattern are
97 -- in scope. So we put its wrappers outside the case, and
98 -- zap the wrapper for it.
99 wraps :: [CoreExpr -> CoreExpr]
100 wraps = map (eqn_wrap . head) groups
102 groups' = [ eqn { eqn_wrap = idWrapper } : eqns | eqn:eqns <- groups ]
104 -- Now make a case alternative out of each group
105 mappM (match_con vars ty) groups' `thenDs` \ alts ->
106 returnDs (adjustMatchResult (foldr (.) idWrapper wraps) $
107 mkCoAlgCaseMatchResult var ty alts)
110 And here is the local function that does all the work. It is
111 more-or-less the @matchCon@/@matchClause@ functions on page~94 in
112 Wadler's chapter in SLPJ. The function @shift_con_pats@ does what the
113 list comprehension in @matchClause@ (SLPJ, p.~94) does, except things
114 are trickier in real life. Works for @ConPats@, and we want it to
115 fail catastrophically for anything else (which a list comprehension
116 wouldn't). Cf.~@shift_lit_pats@ in @MatchLits@.
119 match_con vars ty eqns
120 = do { -- Make new vars for the con arguments; avoid new locals where possible
121 arg_vars <- selectMatchVars (map unLoc arg_pats1) arg_tys
122 ; eqns' <- mapM shift eqns
123 ; match_result <- match (arg_vars ++ vars) ty eqns'
124 ; return (con, tvs1 ++ dicts1 ++ arg_vars, match_result) }
126 ConPatOut (L _ con) tvs1 dicts1 _ (PrefixCon arg_pats1) pat_ty = firstPat (head eqns)
128 shift eqn@(EqnInfo { eqn_wrap = wrap,
129 eqn_pats = ConPatOut _ tvs ds bind (PrefixCon arg_pats) _ : pats })
130 = do { prs <- dsLHsBinds bind
131 ; return (eqn { eqn_wrap = wrap . wrapBinds (tvs `zip` tvs1)
132 . wrapBinds (ds `zip` dicts1)
134 eqn_pats = map unLoc arg_pats ++ pats }) }
136 -- Get the arg types, which we use to type the new vars
137 -- to match on, from the "outside"; the types of pats1 may
138 -- be more refined, and hence won't do
139 arg_tys = dataConInstOrigArgTys con inst_tys
140 inst_tys | isVanillaDataCon con = tcTyConAppArgs pat_ty -- Newtypes opaque!
141 | otherwise = mkTyVarTys tvs1
144 Note [Existentials in shift_con_pat]
145 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
147 data T = forall a. Ord a => T a (a->Int)
149 f (T x f) True = ...expr1...
150 f (T y g) False = ...expr2..
152 When we put in the tyvars etc we get
154 f (T a (d::Ord a) (x::a) (f::a->Int)) True = ...expr1...
155 f (T b (e::Ord b) (y::a) (g::a->Int)) True = ...expr2...
157 After desugaring etc we'll get a single case:
161 T a (d::Ord a) (x::a) (f::a->Int)) ->
166 *** We have to substitute [a/b, d/e] in expr2! **
168 False -> ....((/\b\(e:Ord b).expr2) a d)....
170 Originally I tried to use
171 (\b -> let e = d in expr2) a
172 to do this substitution. While this is "correct" in a way, it fails
173 Lint, because e::Ord b but d::Ord a.