Frederik Eaton wrote:
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
This is planned as part of a template-haskell library allowing automatic lifting of data types to classes. This would most likely be written: $(ttypelift [d| type ShowList = [Show] |]) or for a function $(ttypelift [d| f :: [Show] -> [Show] |] (Here Show is assumed to be defined as a type in the environment) Lifting a list of type [Show] produces an HList where each element is constrained to be an instance of the class Show. If we consider type level naturals we can define a list "[HNat]" where each element is a type level natural number. Keean.