[Haskell-cafe] GADT pattern match in non-rigid context

Brandon S. Allbery KF8NH allbery at ece.cmu.edu
Mon Sep 28 17:59:34 EDT 2009

On Sep 28, 2009, at 17:56 , João Paulo wrote:
> GADT pattern match in non-rigid context for `LeftCons'
>      Solution: add a type signature
> In the pattern: LeftCons l' h_down                    [marked with #  
> in the code]
> The suggestion is quite clear :D (in fact, I think this compiler  
> suggestion is the result of previous interactions in this mailing  
> list)
> The thing is that, in order to add a type signature to the suggested  
> pattern, I believe I have to use the type variables in the signature  
> of function 'move_down' (which I don't think is possible);

You want the ScopedTypeVariables extension.  Read the documentation  
before using; you need to explicitly forall the type variables you  
want to use later.

brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH

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