#2748: Fundep-laden code compiled and ran fine in 6.8, fails in 6.10
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Reporter: sedillard | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.8.3 | Severity: normal
Keywords: | Testcase:
Architecture: Unknown/Multiple | Os: Unknown/Multiple
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I was kindly alerted by Ian Lynagh that some code of mine on hackage was
causing 6.10-rc1 to fail to terminate. This code compiles and runs find on
6.8. Instead of diverging, 6.10.1 raises a type error. I've made a test
case, and I'm creating this ticket to figure out what's going in. Is it,
A) The code is incorrect and 6.8 was erroneously accepting it, and only by
luck producing a valid program
B) The code is correct and 6.10.1's typechecker is being overly
conservative.
At the core is overlapping instances. I've spent considerable effort
removing them from the library, because I can't just look at the code and
say for certain "this will never overlap with that" because its too
complicated. Overlap errors become the users problem, I myself having been
bitten by them many times as a user of my own library. The solution is to
do without overlapping instances. The types of my instances then go from
something nice like
{{{
instance (A t) => C t
instance (B t) => C t --overlap
}}}
to something nasty like
{{{
instance (A (T (T x))) => C (T (T x))
instance (B (T (T (T x)))) => C (T (T (T x))) --no overlap
}}}
It's uglier for me to read and write but ultimately better for the user
because I've eliminated a category of errors. If the compiler will let
me, I try to abbreviate 'T (T (T x)))' to 't' in the instance head, but
I'm not always able to because 't' is more general than 'T (T (T x)))'.
I'm not entirely sure how that factors into things. To see what I'm
talking about, compile the attached file and look for the error that says
{{{Couldn't match expected type `vmt' against inferred type `vv :. (b' :.
v)' `vmt' is a rigid type variable...}}}. It has something to do with
functional dependencies. The fundep involved demands that 'vmt' be of the
type 'x :. y :. z :. etc' but in this particular case it's so verbose I
just call it 'vmt'. 6.8 lets me do that. 6.10 doesn't. Who's right?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2748>
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