Complete -fmono-pat-binds patch

When adding the experimental -fmono-pat-binds, I forgot to check
for type signatures of the now-monomorphic patterns.  This patch
completes the job.

I documented the design too:
   http://haskell.galois.com/cgi-bin/haskell-prime/trac.cgi/wiki/MonomorphicPatternBindings

diff --git a/compiler/typecheck/TcBinds.lhs b/compiler/typecheck/TcBinds.lhs
index e189a98..076de00 100644 (file)
--- a/compiler/typecheck/TcBinds.lhs
+++ b/compiler/typecheck/TcBinds.lhs
@@ -643,9 +643,21 @@ tcLhs sig_fn (FunBind { fun_id = L nm_loc name, fun_infix = inf, fun_matches = m
 
 tcLhs sig_fn bind@(PatBind { pat_lhs = pat, pat_rhs = grhss })
   = do { mb_sigs <- mapM (tcInstSig_maybe sig_fn) names
+       ; mono_pat_binds <- doptM Opt_MonoPatBinds
+               -- With -fmono-pat-binds, we do no generalisation of pattern bindings
+               -- But the signature can still be polymoprhic!
+               --      data T = MkT (forall a. a->a)
+               --      x :: forall a. a->a
+               --      MkT x = <rhs>
+               -- The function get_sig_ty decides whether the pattern-bound variables
+               -- should have exactly the type in the type signature (-fmono-pat-binds), 
+               -- or the instantiated version (-fmono-pat-binds)
 
        ; let nm_sig_prs  = names `zip` mb_sigs
-             tau_sig_env = mkNameEnv [ (name, sig_tau sig) | (name, Just sig) <- nm_sig_prs]
+             get_sig_ty | mono_pat_binds = idType . sig_id
+                        | otherwise      = sig_tau
+             tau_sig_env = mkNameEnv [ (name, get_sig_ty sig) 
+                                     | (name, Just sig) <- nm_sig_prs]
              sig_tau_fn  = lookupNameEnv tau_sig_env
 
              tc_pat exp_ty = tcPat (LetPat sig_tau_fn) pat exp_ty unitTy $ \ _ ->