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,
   -- Checking type validity
   Rank, UserTypeCtxt(..), checkValidType, 
   SourceTyCtxt(..), checkValidTheta, checkFreeness,

-  checkValidInstHead, checkValidInstance, checkAmbiguity,
+  checkValidInstHead, checkValidInstance, 

   checkInstTermination, checkValidTypeInst, checkTyFamFreeness,
   validDerivPred, arityErr, 
 
   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).
 
 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
 \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
 
     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))
 
     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] 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 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'.
 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'.