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-bounces@haskell.org [mailto:haskell-cafe-bounces@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