Improve handling of implicit parameters

[ghc-hetmet.git] / compiler / typecheck / TcSimplify.lhs

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 2a3691a..2347d37 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -398,8 +398,8 @@ When m is later unified with [], we can solve both constraints.
                Notes on implicit parameters
        --------------------------------------
 
-Question 1: can we "inherit" implicit parameters
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Note [Inheriting implicit parameters]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Consider this:
 
        f x = (x::Int) + ?y
@@ -424,6 +424,21 @@ BOTTOM LINE: when *inferring types* you *must* quantify
 over implicit parameters. See the predicate isFreeWhenInferring.
 
 
+Note [Implicit parameters and ambiguity] 
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+What type should we infer for this?
+       f x = (show ?y, x::Int)
+Since we must quantify over the ?y, the most plausible type is
+       f :: (Show a, ?y::a) => Int -> (String, Int)
+But notice that the type of the RHS is (String,Int), with no type 
+varibables mentioned at all!  The type of f looks ambiguous.  But
+it isn't, because at a call site we might have
+       let ?y = 5::Int in f 7
+and all is well.  In effect, implicit parameters are, well, parameters,
+so we can take their type variables into account as part of the
+"tau-tvs" stuff.  This is done in the function 'FunDeps.grow'.
+
+
 Question 2: type signatures
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 BUT WATCH OUT: When you supply a type signature, we can't force you
@@ -719,9 +734,9 @@ The net effect of [NO TYVARS]
 \begin{code}
 isFreeWhenInferring :: TyVarSet -> Inst        -> Bool
 isFreeWhenInferring qtvs inst
-  =  isFreeWrtTyVars qtvs inst         -- Constrains no quantified vars
-  && isInheritableInst inst            -- And no implicit parameter involved
-                                       -- (see "Notes on implicit parameters")
+  =  isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
+  && isInheritableInst inst    -- and no implicit parameter involved
+                               --   see Note [Inheriting implicit parameters]
 
 {-     No longer used (with implication constraints)
 isFreeWhenChecking :: TyVarSet -- Quantified tyvars
@@ -2431,7 +2446,8 @@ addTopIPErrs bndrs ips
   where
     (tidy_env, tidy_ips) = tidyInsts ips
     mk_msg ips = vcat [sep [ptext SLIT("Implicit parameters escape from"),
-                           nest 2 (ptext SLIT("the monomorphic top-level binding(s) of")
+                           nest 2 (ptext SLIT("the monomorphic top-level binding") 
+                                           <> plural bndrs <+> ptext SLIT("of")
                                            <+> pprBinders bndrs <> colon)],
                       nest 2 (vcat (map ppr_ip ips)),
                       monomorphism_fix]