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"
11 import {-# SOURCE #-} Match ( match )
13 import HsSyn ( Pat(..), HsConDetails(..), isEmptyLHsBinds )
14 import DsBinds ( dsHsNestedBinds )
15 import DataCon ( isVanillaDataCon, dataConTyVars, dataConOrigArgTys )
16 import TcType ( tcTyConAppArgs )
17 import Type ( substTys, zipTopTvSubst, mkTyVarTys )
24 import ListSetOps ( equivClassesByUniq )
25 import SrcLoc ( unLoc, Located(..) )
26 import Unique ( Uniquable(..) )
30 We are confronted with the first column of patterns in a set of
31 equations, all beginning with constructors from one ``family'' (e.g.,
32 @[]@ and @:@ make up the @List@ ``family''). We want to generate the
33 alternatives for a @Case@ expression. There are several choices:
36 Generate an alternative for every constructor in the family, whether
37 they are used in this set of equations or not; this is what the Wadler
41 (a)~Simple. (b)~It may also be that large sparsely-used constructor
42 families are mainly handled by the code for literals.
44 (a)~Not practical for large sparsely-used constructor families, e.g.,
45 the ASCII character set. (b)~Have to look up a list of what
46 constructors make up the whole family.
50 Generate an alternative for each constructor used, then add a default
51 alternative in case some constructors in the family weren't used.
54 (a)~Alternatives aren't generated for unused constructors. (b)~The
55 STG is quite happy with defaults. (c)~No lookup in an environment needed.
57 (a)~A spurious default alternative may be generated.
61 ``Do it right:'' generate an alternative for each constructor used,
62 and add a default alternative if all constructors in the family
66 (a)~You will get cases with only one alternative (and no default),
67 which should be amenable to optimisation. Tuples are a common example.
69 (b)~Have to look up constructor families in TDE (as above).
73 We are implementing the ``do-it-right'' option for now. The arguments
74 to @matchConFamily@ are the same as to @match@; the extra @Int@
75 returned is the number of constructors in the family.
77 The function @matchConFamily@ is concerned with this
78 have-we-used-all-the-constructors? question; the local function
79 @match_cons_used@ does all the real work.
81 matchConFamily :: [Id]
85 matchConFamily (var:vars) ty eqns_info
87 -- Sort into equivalence classes by the unique on the constructor
88 -- All the EqnInfos should start with a ConPat
89 eqn_groups = equivClassesByUniq get_uniq eqns_info
90 get_uniq (EqnInfo { eqn_pats = ConPatOut (L _ data_con) _ _ _ _ _ : _}) = getUnique data_con
92 -- Now make a case alternative out of each group
93 mappM (match_con vars ty) eqn_groups `thenDs` \ alts ->
94 returnDs (mkCoAlgCaseMatchResult var ty alts)
97 And here is the local function that does all the work. It is
98 more-or-less the @matchCon@/@matchClause@ functions on page~94 in
99 Wadler's chapter in SLPJ. The function @shift_con_pats@ does what the
100 list comprehension in @matchClause@ (SLPJ, p.~94) does, except things
101 are trickier in real life. Works for @ConPats@, and we want it to
102 fail catastrophically for anything else (which a list comprehension
103 wouldn't). Cf.~@shift_lit_pats@ in @MatchLits@.
106 match_con vars ty eqns
107 = do { -- Make new vars for the con arguments; avoid new locals where possible
108 arg_vars <- selectMatchVars (map unLoc arg_pats1) arg_tys
109 ; eqns' <- mapM shift eqns
110 ; match_result <- match (arg_vars ++ vars) ty eqns'
111 ; return (con, tvs1 ++ dicts1 ++ arg_vars, match_result) }
113 ConPatOut (L _ con) tvs1 dicts1 _ (PrefixCon arg_pats1) pat_ty = firstPat (head eqns)
115 shift eqn@(EqnInfo { eqn_wrap = wrap,
116 eqn_pats = ConPatOut _ tvs ds bind (PrefixCon arg_pats) _ : pats })
117 = do { prs <- dsHsNestedBinds bind
118 ; return (eqn { eqn_wrap = wrap . wrapBinds (tvs `zip` tvs1)
119 . wrapBinds (ds `zip` dicts1)
121 eqn_pats = map unLoc arg_pats ++ pats }) }
123 -- Get the arg types, which we use to type the new vars
124 -- to match on, from the "outside"; the types of pats1 may
125 -- be more refined, and hence won't do
126 arg_tys = substTys (zipTopTvSubst (dataConTyVars con) inst_tys)
127 (dataConOrigArgTys con)
128 inst_tys | isVanillaDataCon con = tcTyConAppArgs pat_ty -- Newtypes opaque!
129 | otherwise = mkTyVarTys tvs1
132 Note [Existentials in shift_con_pat]
133 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
135 data T = forall a. Ord a => T a (a->Int)
137 f (T x f) True = ...expr1...
138 f (T y g) False = ...expr2..
140 When we put in the tyvars etc we get
142 f (T a (d::Ord a) (x::a) (f::a->Int)) True = ...expr1...
143 f (T b (e::Ord b) (y::a) (g::a->Int)) True = ...expr2...
145 After desugaring etc we'll get a single case:
149 T a (d::Ord a) (x::a) (f::a->Int)) ->
154 *** We have to substitute [a/b, d/e] in expr2! **
156 False -> ....((/\b\(e:Ord b).expr2) a d)....
158 Originally I tried to use
159 (\b -> let e = d in expr2) a
160 to do this substitution. While this is "correct" in a way, it fails
161 Lint, because e::Ord b but d::Ord a.