Comments, and remove export of checkAmbiguity

diff --git a/compiler/typecheck/TcMType.lhs b/compiler/typecheck/TcMType.lhs
index e85d3b8..3437549 100644 (file)
--- a/compiler/typecheck/TcMType.lhs
+++ b/compiler/typecheck/TcMType.lhs
@@ -47,7 +47,7 @@ module TcMType (
   -- Checking type validity
   Rank, UserTypeCtxt(..), checkValidType, 
   SourceTyCtxt(..), checkValidTheta, checkFreeness,
-  checkValidInstHead, checkValidInstance, checkAmbiguity,
+  checkValidInstHead, checkValidInstance, 
   checkInstTermination, checkValidTypeInst, checkTyFamFreeness,
   validDerivPred, arityErr, 
 
@@ -1030,6 +1030,7 @@ If the list of tv_names is empty, we have a monotype, and then we
 don't need to check for ambiguity either, because the test can't fail
 (see is_ambig).
 
+
 \begin{code}
 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
 checkAmbiguity forall_tyvars theta tau_tyvars
@@ -1038,11 +1039,7 @@ checkAmbiguity forall_tyvars theta tau_tyvars
     complain pred     = addErrTc (ambigErr pred)
     extended_tau_vars = grow theta tau_tyvars
 
-       -- Only a *class* predicate can give rise to ambiguity
-       -- An *implicit parameter* cannot.  For example:
-       --      foo :: (?x :: [a]) => Int
-       --      foo = length ?x
-       -- is fine.  The call site will suppply a particular 'x'
+       -- See Note [Implicit parameters and ambiguity] in TcSimplify
     is_ambig pred     = isClassPred  pred &&
                        any ambig_var (varSetElems (tyVarsOfPred pred))
 

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index f2ad101..be27405 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -411,14 +411,19 @@ 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
+Only a *class* predicate can give rise to ambiguity
+An *implicit parameter* cannot.  For example:
+       foo :: (?x :: [a]) => Int
+       foo = length ?x
+is fine.  The call site will suppply a particular 'x'
+
+Furthermore, the type variables fixed by an implicit parameter
+propagate to the others.  E.g.
+       foo :: (Show a, ?x::[a]) => Int
+       foo = show (?x++?x)
+The type of foo looks ambiguous.  But it isn't, because at a call site
+we might have
+       let ?x = 5::Int in foo
 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'.