On Tuesday 23 April 2013, 15:05:05, gs wrote:
In http://code.accursoft.com/binding/src/c13ccbbec0ba8e326369ff2252863f20a891ef 76/binding-core/src/Data/Binding/Simple.hs there's a data constructor data Source v a = Variable v => Source {bindings :: v [Binding a], var
:: v a}
Source is used many times in this file and in http://code.accursoft.com/binding/src/c13ccbbec0ba8e326369ff2252863f20a891ef 76/binding-core/src/Data/Binding/List.hs, and each time, I have to repeat the context Variable v. Is there any way to avoid this redundancy?
Use a GADT, {-# LANGUAGE GADTs #-} data Source x y where Source :: Variable v => { bindings :: v [Binding a], var :: v a } -> Source v a The `Variable v` context becomes available by pattern-matching on the constructor `Source` (but not by using the field names to deconstruct a value of type `Source v a`!). With the original datatype context, that wasn't so (thus datatype contexts were mostly unhelpful; they have been removed from the language, and now require the DatatypeContexts extension in GHC). If you can't change the definition of the datatype, you can't avoid the redundancy, you have to mention the `Variable v` context everywhere Source is used (unless you omit type signatures altogether).