Hi, to support deriving NT instances properly, TcDeriv needs to be modified. But there are some structural obstacles: So far, all classes which had to be had the “type of interest” as their only type argument. Therefore, DerivSpec, the data structure in that file, carries information about what instances to derive, in the field dc_tc :: TyCon.
But with NT it is different: * It has two parameters, * the type of interest is likely not the last parameter (e.g. NT Age Int) * and it can happen that there is a type variable in the last parameter (e.g. NT Int a => NT Age a). All that is difficult to squeeze into the existing data structure. The data type also has the ds_tys :: [Type] parameter, which contains all type parameters to the class (e.g. [Int, a]). So at first I thought about simply removing dc_tc and use the data from (last ds_tys) whenever needed. But in the case of type families, ds_tc is not just the type constructor of (last ds_tys), but rather what it resolves to. So before I rewrite lots of code there, I’d like to get some advice: Should I extend the DerivSpec to support multi-parameter-typeclasses (and figure out how to support data families here), or rather bypass the whole lot and handle NT instance generation separately? Or is there maybe another, more elegant way? Thanks, Joachim -- Joachim “nomeata” Breitner m...@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nome...@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nome...@debian.org
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