Arrowcase1

[Simon Peyton-Jones]

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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