Commit f30f90b5 authored by dimitris's avatar dimitris

More informative commentary as to why we allow family interactions with inerts...

More informative commentary as to why we allow family interactions with inerts when simplEqsOnly is on.
parent e74fbf0c
......@@ -756,9 +756,7 @@ interactWithInertsStage :: WorkItem -> TcS StopOrContinue
interactWithInertsStage wi
= do { ctxt <- getTcSContext
; if simplEqsOnly ctxt && not (isCFunEqCan wi) then
-- Why not just "simplEqsOnly"? Well our inert sets can't tolerate two family
-- equations with the /same/ head so we have to enable some reactions. The
-- example that breaks otherwise is indexed_types/should_compile/T2291.hs
-- Why not just "simplEqsOnly"? See Note [SimplEqsOnly and InteractWithInerts]
return (ContinueWith wi)
else
do { traceTcS "interactWithInerts" $ text "workitem = " <+> ppr wi
......@@ -790,7 +788,37 @@ interactWithInertsStage wi
-> do { updInertSetTcS atomic_inert
; return (ContinueWith wi) }
}
\end{code}
Note [SimplEqsOnly and InteractWithInerts]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It may be possible when we are simplifying a RULE that we have two wanted constraints
of the form:
[W] c1 : F Int ~ Bool
[W] c2 : F Int ~ alpha
When we simplify RULES we only do equality reactions (simplEqsOnly). So the question is:
are we allowed to do type family interactions? We definitely do not want to apply top-level
family and dictionary instances but what should we do with the constraint set above?
Suppose that c1 gets processed first and enters the inert. Remember that he will enter a
CtFamHead map with (F Int) as the index. Now c2 comes along, we can't add him to the inert
set since it has exactly the same key, so we'd better react him with the inert c1. In fact
one might think that we should react him anyway to learn that (alpha ~ Bool). This is why
we allow CFunEqCan's to perform reactions with the inerts.
If we don't allow this, we will try to add both elements to the inert set and will panic!
The relevant example that fails when we don't allow such family reactions is:
indexed_types/should_compile/T2291.hs
NB: In previous versions of TcInteract the extra guard (not (isCFunEqCan wi)) was not there
but family reactions were actually happening earlier, during canonicalization. So the behaviour
has not changed -- previously this tricky point was completely lost and worked by accident.
\begin{code}
--------------------------------------------
doInteractWithInert :: Ct -> Ct -> TcS InteractResult
......
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