[Haskell-cafe] Type checking oddity -- maybe my own confusion

[Steffen Schuldenzucker]

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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