Ross
The test arrows/should_compile/arrowcase1 is failing Lint. The bug is in the
desugarer.
Here's a smaller test case:
h :: ArrowChoice a => Int -> a (Int,Int) Int
h x = proc (y,z) -> case compare x y of
GT -> returnA -< z+x
The type checker turns the case into
case compare x y of
GT { p77 = plusInt } -> returnA -< p77 z x
Here p77 is a local binding for the (+) operation. In general, patterns can
contain bindings (used to discharge constraints that are bound by the pattern).
In this case the binding isn't strictly necessary, but in general it is -
consider existentials. It's equivalent to adding a 'let' around the RHS, but
since the patters are perhaps nested, and one pattern might use a constraint
that is bound by another, the pattern is the right place to attach the binding.
This has come up because GHC is binding things a little earlier than before,
but an existential would have exposed it before.
The trouble is that the suspicious-looking replaceLeaves code in DsArrows (line
528 or so) doesn't know about these bindings.
I don't understand DsArrows at all. Indeed the whole Arrows code feels smelly
to me. Maybe we tried to share too much code?
Anyway, could you look at it? Thanks. It's Trac #1080
Simon
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