[Haskell-cafe] tuple and HList

[Frederik Eaton]

That's a neat technique. Since it's so general it would be nice if
there were a way to make it more automatic, could one use template
haskell? It seems one should be able to write

HListConstraint $(mkConstraint Show) l

to generate the declarations automatically.

Frederik

On Sun, Mar 20, 2005 at 11:05:06PM +0000, Keean Schupke wrote:
> Frederik Eaton wrote:
> 
> >Another thing which I don't think is mentioned in the paper, which is
> >convenient, is that you can define HLists all of whose elements are
> >members of a given class:
> >
> >class HListShow l
> >instance HListShow HNil
> >instance (Show a, HListShow l) => HListShow (a :* l)
> > 
> >
> You can avoid the need to declare a new class for each constrained list
> by using the following:
> 
> >class Constraint c a
> >
> >data SHOW
> >instance Show a => Constraint SHOW a
> >
> >class HListConstraint c l
> >instance HListConstraint c HNil
> >instance (Constraint c a,HListConstraint c l) => HListConstraint c 
> (HCons a l)
> 
> You can now constrain a list as follows:
> 
> >assertShow :: HListConstraint SHOW l => l -> l
> >assertShow = id
> 
> The type parameter can be made first class using:
> 
> >showConstraint :: SHOW
> >showConstraint = undefined
> 
> So we can now pass this as a parameter:
> 
> >assertConstraintOnHList :: HListConstraint c l => c -> l -> l
> >assertConstraintOnHList _ = id
> 
>    Keean.
> 
> 
> 
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