Fix #1662: do not simplify constraints for vanilla pattern matches

See Note [Arrows and patterns] in TcPat.

This fixes Trac 1662.   Test is arrows/should_compile/arrowpat.hs

Please merge

diff --git a/compiler/typecheck/TcPat.lhs b/compiler/typecheck/TcPat.lhs
index ecca249..26bfdd4 100644 (file)
--- a/compiler/typecheck/TcPat.lhs
+++ b/compiler/typecheck/TcPat.lhs
@@ -601,21 +601,42 @@ tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
         -> TcM (Pat TcId, [TcTyVar], a)
 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
   = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _) = dataConFullSig data_con
-             skol_info = PatSkol data_con
-             origin    = SigOrigin skol_info
+             skol_info  = PatSkol data_con
+             origin     = SigOrigin skol_info
+             full_theta = eq_theta ++ dict_theta
 
          -- Instantiate the constructor type variables [a->ty]
+         -- This may involve doing a family-instance coercion, and building a wrapper
        ; (ctxt_res_tys, coi) <- boxySplitTyConAppWithFamily tycon pat_ty
+       ; let pat_ty' = mkTyConApp tycon ctxt_res_tys
+                                     -- pat_ty /= pat_ty iff coi /= IdCo
+              wrap_res_pat res_pat
+               = mkCoPatCoI coi (unwrapFamInstScrutinee tycon ctxt_res_tys res_pat) pat_ty
+
+         -- Add the stupid theta
+       ; addDataConStupidTheta data_con ctxt_res_tys
+
        ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs  -- Get location from monad,
                                                        -- not from ex_tvs
        ; let tenv     = zipTopTvSubst (univ_tvs ++ ex_tvs)
                                       (ctxt_res_tys ++ mkTyVarTys ex_tvs')
-             eq_spec' = substEqSpec tenv eq_spec
-             theta'   = substTheta  tenv (eq_theta ++ dict_theta)
-             arg_tys' = substTys    tenv arg_tys
+             arg_tys' = substTys tenv arg_tys
+
+       ; if null ex_tvs && null eq_spec && null full_theta
+         then do {     -- The common case; no class bindings etc (see Note [Arrows and patterns])
+                   (arg_pats', inner_tvs, res) <- tcConArgs data_con arg_tys' 
+                                                              arg_pats pstate thing_inside
+                 ; let res_pat = ConPatOut { pat_con = L con_span data_con, 
+                                             pat_tvs = [], pat_dicts = [], pat_binds = emptyLHsBinds,
+                                             pat_args = arg_pats', pat_ty = pat_ty' }
 
+                   ; return (wrap_res_pat res_pat, inner_tvs, res) }
+
+         else do       -- The general case, with existential, and local equality constraints
+       { let eq_spec' = substEqSpec tenv eq_spec
+             theta'   = substTheta  tenv full_theta
        ; co_vars <- newCoVars eq_spec' -- Make coercion variables
-        ; traceTc (text "tcConPat: refineAlt")
+       ; traceTc (text "tcConPat: refineAlt")
        ; pstate' <- refineAlt data_con pstate ex_tvs' co_vars pat_ty
         ; traceTc (text "tcConPat: refineAlt done!")
        
@@ -625,22 +646,15 @@ tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
        ; loc <- getInstLoc origin
        ; dicts <- newDictBndrs loc theta'
        ; dict_binds <- tcSimplifyCheckPat loc co_vars (pat_reft pstate') 
-                                          ex_tvs' dicts lie_req
-
-       ; addDataConStupidTheta data_con ctxt_res_tys
+                          ex_tvs' dicts lie_req
 
-        ; let pat_ty' = mkTyConApp tycon ctxt_res_tys
-                                     -- pat_ty /= pat_ty iff coi /= IdCo
-              res_pat = ConPatOut { pat_con = L con_span data_con, 
+        ; let res_pat = ConPatOut { pat_con = L con_span data_con, 
                                    pat_tvs = ex_tvs' ++ co_vars,
                                    pat_dicts = map instToVar dicts, 
                                    pat_binds = dict_binds,
                                    pat_args = arg_pats', pat_ty = pat_ty' }
-       ; return 
-           (mkCoPatCoI coi
-               (unwrapFamInstScrutinee tycon ctxt_res_tys res_pat) pat_ty,
-            ex_tvs' ++ inner_tvs, res)
-       }
+       ; return (wrap_res_pat res_pat, ex_tvs' ++ inner_tvs, res)
+       } }
   where
     -- Split against the family tycon if the pattern constructor 
     -- belongs to a family instance tycon.
@@ -767,6 +781,30 @@ addDataConStupidTheta data_con inst_tys
     inst_theta = substTheta tenv stupid_theta
 \end{code}
 
+Note [Arrows and patterns]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+(Oct 07) Arrow noation has the odd property that it involves "holes in the scope". 
+For example:
+  expr :: Arrow a => a () Int
+  expr = proc (y,z) -> do
+          x <- term -< y
+          expr' -< x
+
+Here the 'proc (y,z)' binding scopes over the arrow tails but not the
+arrow body (e.g 'term').  As things stand (bogusly) all the
+constraints from the proc body are gathered together, so constraints
+from 'term' will be seen by the tcPat for (y,z).  But we must *not*
+bind constraints from 'term' here, becuase the desugarer will not make
+these bindings scope over 'term'.
+
+The Right Thing is not to confuse these constraints together. But for
+now the Easy Thing is to ensure that we do not have existential or
+GADT constraints in a 'proc', and to short-cut the constraint
+simplification for such vanilla patterns so that it binds no
+constraints. Hence the 'fast path' in tcConPat; but it's also a good
+plan for ordinary vanilla patterns to bypass the constraint
+simplification step.
+
 
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