Commit f6f3819f authored by simonpj's avatar simonpj
Browse files

[project @ 2004-12-06 10:51:36 by simonpj]

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	Bug in loop detection in TcSimplify
	------------------------------------

The type-class context simplifier has been able to 
build recursive dictionaries for some time: co-induction.
That is, you can build a proof for constraint C by assuming
that C holds when proving the preconditions of C.

You need to be in -fallow-undecidable-instances land to 
make use of this: see comments with [RECURSIVE DICTIONARIES]
in TcSimplify.lhs.

Anyway, this is all fine, but I'd implemented it wrong!  You need
to be very careful with superclasses, or you can make a bogus
loop by mistake.  This commit fixes it; tests LoopOfTheDay{1,2,3}
will test it (thanks Ralf Laemmel).
parent 23843129
......@@ -1754,7 +1754,7 @@ addAvailAndSCs avails inst avail
avails1 = addToFM avails inst avail
is_loop inst = any (`tcEqType` idType (instToId inst)) dep_tys
-- Note: this compares by *type*, not by Unique
deps = findAllDeps emptyVarSet avail
deps = findAllDeps (unitVarSet (instToId inst)) avail
dep_tys = map idType (varSetElems deps)
findAllDeps :: IdSet -> Avail -> IdSet
......@@ -1762,12 +1762,17 @@ addAvailAndSCs avails inst avail
-- See Note [SUPERCLASS-LOOP]
-- Watch out, though. Since the avails may contain loops
-- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
findAllDeps so_far (Rhs _ kids)
= foldl findAllDeps
(extendVarSetList so_far (map instToId kids)) -- Add the kids to so_far
[a | Just a <- map (lookupFM avails) kids] -- Find the kids' Avail
findAllDeps so_far other = so_far
findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids
findAllDeps so_far other = so_far
find_all :: IdSet -> Inst -> IdSet
find_all so_far kid
| kid_id `elemVarSet` so_far = so_far
| Just avail <- lookupFM avails kid = findAllDeps so_far' avail
| otherwise = so_far'
where
so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far
kid_id = instToId kid
addSCs :: (Inst -> Bool) -> Avails -> Inst -> TcM Avails
-- Add all the superclasses of the Inst to Avails
......
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