Thanks, that does help. Very clear description.
Any good ideas about how to tweak my example to do what was intended ;-)?
The desired goal was that everywhere I construct a value using the Assign
constructor, that the resulting value's type to be tainted by the AssignCap
constraint.
Actually... to go a bit further, I thought there was some way to arrange
this so that, upon GHC type-checking the case statement, it would realize
that certain cases are forbidden based on the type, and would consider the
pattern match complete without them (or issue an error if they are present).
Thanks,
-Ryan
On Tue, Jul 12, 2011 at 11:17 AM, Dimitrios Vytiniotis <
dimit...@microsoft.com> wrote:
> 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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