Commit 2a7f4de3 authored by Simon Peyton Jones's avatar Simon Peyton Jones
Browse files

Further wibbbling to type error message reporting

* We now never report derived-constraint type errors, even
  in the "insolubles".  See Note [Insoluble derived constraints]
  in TcRnTypes.

* The cec_suppress mechanism in TcErrors is refactored a bit so that:
   - We suppress *all* errors in unreachable code (they can be jolly
     confusing)
   - We no longer suppress *all* non-insoluble errors if there are *any
     insolubles anywhere.  Instead we are a bit more refined.
  See Note [Suppressing error messages] in TcErrors
parent 6ebab3df
......@@ -92,11 +92,6 @@ in TcErrors. TcErrors.reportTidyWanteds does not print the errors
and does not fail if -fdefer-type-errors is on, so that we can continue
compilation. The errors are turned into warnings in `reportUnsolved`.
Note [Suppressing error messages]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If there are any insolubles, like (Int~Bool), then we suppress all less-drastic
errors (like (Eq a)). Often the latter are a knock-on effect of the former.
\begin{code}
reportUnsolved :: WantedConstraints -> TcM (Bag EvBind)
reportUnsolved wanted
......@@ -131,8 +126,7 @@ report_unsolved mb_binds_var defer wanted
err_ctxt = CEC { cec_encl = []
, cec_tidy = tidy_env
, cec_defer = defer
, cec_suppress = insolubleWC wanted
-- See Note [Suppressing error messages]
, cec_suppress = False -- See Note [Suppressing error messages]
, cec_binds = mb_binds_var }
; traceTc "reportUnsolved (after unflattening):" $
......@@ -161,8 +155,23 @@ data ReportErrCtxt
, cec_suppress :: Bool -- True <=> More important errors have occurred,
-- so create bindings if need be, but
-- don't issue any more errors/warnings
-- See Note [Suppressing error messages]
}
\end{code}
Note [Suppressing error messages]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The cec_suppress flag says "don't report any errors. Instead, just create
evidence bindings (as usual). It's used when more important errors have occurred.
Specifically (see reportWanteds)
* If there are insoluble Givens, then we are in unreachable code and all bets
are off. So don't report any further errors.
* If there are any insolubles (eg Int~Bool), here or in a nested implication,
then suppress errors from the flat constraints here. Sometimes the
flat-constraint errors are a knock-on effect of the insolubles.
\begin{code}
reportImplic :: ReportErrCtxt -> Implication -> TcM ()
reportImplic ctxt implic@(Implic { ic_skols = tvs, ic_given = given
, ic_wanted = wanted, ic_binds = evb
......@@ -188,17 +197,29 @@ reportImplic ctxt implic@(Implic { ic_skols = tvs, ic_given = given
Just {} -> Just evb }
reportWanteds :: ReportErrCtxt -> WantedConstraints -> TcM ()
reportWanteds ctxt (WC { wc_flat = flats, wc_insol = insols, wc_impl = implics })
= do { reportFlats (ctxt { cec_suppress = False }) (mapBag (tidyCt env) insols)
; reportFlats ctxt (mapBag (tidyCt env) flats)
reportWanteds ctxt wanted@(WC { wc_flat = flats, wc_insol = insols, wc_impl = implics })
= do { reportFlats ctxt (mapBag (tidyCt env) insol_given)
; reportFlats ctxt1 (mapBag (tidyCt env) insol_wanted)
; reportFlats ctxt2 (mapBag (tidyCt env) flats)
-- All the Derived ones have been filtered out of flats
-- by the constraint solver. This is ok; we don't want
-- to report unsolved Derived goals as errors
-- See Note [Do not report derived but soluble errors]
; mapBagM_ (reportImplic ctxt) implics }
where
; mapBagM_ (reportImplic ctxt1) implics }
-- NB ctxt1: don't suppress inner insolubles if there's only a
-- wanted insoluble here; but do suppress inner insolubles
-- if there's a given insoluble here (= inaccessible code)
where
(insol_given, insol_wanted) = partitionBag isGivenCt insols
env = cec_tidy ctxt
-- See Note [Suppressing error messages]
suppress0 = cec_suppress ctxt
suppress1 = suppress0 || not (isEmptyBag insol_given)
suppress2 = suppress0 || insolubleWC wanted
ctxt1 = ctxt { cec_suppress = suppress1 }
ctxt2 = ctxt { cec_suppress = suppress2 }
reportFlats :: ReportErrCtxt -> Cts -> TcM ()
reportFlats ctxt flats -- Here 'flats' includes insolble goals
= traceTc "reportFlats" (ppr flats) >>
......
......@@ -970,13 +970,33 @@ ctPred :: Ct -> PredType
ctPred ct = ctEvPred (cc_ev ct)
dropDerivedWC :: WantedConstraints -> WantedConstraints
dropDerivedWC wc@(WC { wc_flat = flats })
= wc { wc_flat = filterBag isWantedCt flats }
-- Don't filter the insolubles, because derived
-- insolubles should stay so that we report them.
-- See Note [Insoluble derived constraints]
dropDerivedWC wc@(WC { wc_flat = flats, wc_insol = insols })
= wc { wc_flat = filterBag isWantedCt flats
, wc_insol = filterBag (not . isDerivedCt) insols }
-- Keep Givens from insols because they indicate unreachable code
-- The implications are (recursively) already filtered
\end{code}
Note [Insoluble derived constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In general we discard derived constraints at the end of constraint solving;
see dropDerivedWC. For example,
* If we have an unsolved (Ord a), we don't want to complain about
an unsolved (Eq a) as well.
* If we have kind-incompatible (a::* ~ Int#::#) equality, we
don't want to complain about the kind error twice.
Arguably, for *some* derived contraints we might want to report errors.
Notably, functional dependencies. If we have
class C a b | a -> b
and we have
[W] C a b, [W] C a c
where a,b,c are all signature variables. Then we could reasonably
report an error unifying (b ~ c). But it's probably not worth it;
after all, we also get an error because we can't discharge the constraint.
%************************************************************************
%* *
......
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