check_class_pred_tys dflags ctxt tys
= case ctxt of
TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
- InstThetaCtxt -> gla_exts || all tcIsTyVarTy tys
+ InstThetaCtxt -> gla_exts || undecidable_ok || all tcIsTyVarTy tys
-- Further checks on head and theta in
-- checkInstTermination
other -> gla_exts || all tyvar_head tys
where
- gla_exts = dopt Opt_GlasgowExts dflags
+ gla_exts = dopt Opt_GlasgowExts dflags
+ undecidable_ok = dopt Opt_AllowUndecidableInstances dflags
-------------------------
tyvar_head ty -- Haskell 98 allows predicates of form
ptext SLIT("While checking") <+> pprSourceTyCtxt ctxt ]
badSourceTyErr sty = ptext SLIT("Illegal constraint") <+> pprPred sty
-predTyVarErr pred = sep [ptext SLIT("Non-type variable argument"),
+predTyVarErr pred = sep [ptext SLIT("Non type-variable argument"),
nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
dupPredWarn dups = ptext SLIT("Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
-- Check that instance inference will terminate (if we care)
-- For Haskell 98, checkValidTheta has already done that
; when (gla_exts && not undecidable_ok) $
- checkInstTermination theta inst_tys
+ checkInstTermination theta inst_tys
-- The Coverage Condition
; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
(instTypeErr (pprClassPred clas inst_tys) msg)
}
where
- msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
+ msg = parens (ptext SLIT("the Coverage Condition fails for one of the functional dependencies"))
\end{code}
Termination test: each assertion in the context satisfies
(2) the assertion has fewer constructors and variables (taken together
and counting repetitions) than the head.
This is only needed with -fglasgow-exts, as Haskell 98 restrictions
-(which have already been checked) guarantee termination.
+(which have already been checked) guarantee termination.
+
+The underlying idea is that
+
+ for any ground substitution, each assertion in the
+ context has fewer type constructors than the head.
+
\begin{code}
checkInstTermination :: ThetaType -> [TcType] -> TcM ()
checkInstTermination theta tys
- = do
- mappM_ (check_nomore (fvTypes tys)) theta
- mappM_ (check_smaller (sizeTypes tys)) theta
+ = do { mappM_ (check_nomore (fvTypes tys)) theta
+ ; mappM_ (check_smaller (sizeTypes tys)) theta }
check_nomore :: [TyVar] -> PredType -> TcM ()
check_nomore fvs pred