On 07/12/2011 05:01 PM, Ryan Newton wrote:
Hi all,
Is there something wrong with the code below? My anticipation was that the type of "test" would include the class constraint, because it uses the Assign constructor. But if you load this code in GHCI you can see that the inferred type was "test :: E m -> E m".
When I complete the pattern match in 'test', it might look like this: test x = case x of Assign v e1 e2 -> x Varref v -> x (which is just id :: E m -> E m). Of course, we want to be able to write
test (Varref v)
for any v :: V, and match the second case. But as 'Varref' does not add an AssignCap constraint, 'test' must not either. Hope that helps. Steffen
Thanks, -Ryan
{-# LANGUAGE GADTs #-}
class AssignCap m data PureT data IOT instance AssignCap IOT
data E m where Assign :: AssignCap m => V -> E m -> E m -> E m Varref :: V -> E m -- ...
type V = String
-- I expected the following type but am not getting it: -- test :: AssignCap m => E m -> E m test x = case x of Assign v e1 e2 -> Assign v e1 e2 -- And this is the same: Assign v e1 e2 -> x
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