Hi Ryan, Think of AssignCap as an extra argument packaged up with the Assign constructor. When you pattern match against Assign you make the AssignCap constraint *available* for use in the RHS of the pattern; so there's no need for quantification, you already have the constraint you want packaged inside your argument. (Back in the old times when GHC did not implement implication constraints maybe you'd get the type you say). Does that help?
Thanks d- From: haskell-cafe-boun...@haskell.org [mailto:haskell-cafe-boun...@haskell.org] On Behalf Of Ryan Newton Sent: 12 July 2011 16:02 To: Haskell Cafe Subject: [Haskell-cafe] Type checking oddity -- maybe my own confusion 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". 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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