I assume you've seen http://hackage.haskell.org/trac/ghc/ticket/4222 There are non-obvious design choices here
Simon | -----Original Message----- | From: haskell-cafe-boun...@haskell.org [mailto:haskell-cafe-boun...@haskell.org] On | Behalf Of Serguey Zefirov | Sent: 28 July 2010 11:07 | To: Jonas Almström Duregård | Cc: Ivan Lazar Miljenovic; haskell | Subject: Re: [Haskell-cafe] Template Haskell sees into abstract data types | | 2010/7/28 Jonas Almström Duregård <jonas.dureg...@gmail.com>: | > Hi, | > | >> I cannot write classes that see into internal structure. For example, | >> I cannot write my own (de)serialization without using from/toAscList. | > | > Actually I don't believe you can do this with TH either. TH splices | > code into the module where you use it. The generated code is then type | > checked in this module. If constructors that are not exported are used | > in the generated code, I believe you will get an error. | > | > This could still be an issue because your TH code won't know if the | > constructors are exported or not, but i doubt you can actually do | > things with TH that you can't do with plain H. | | I doubt that doubt first. ;) | | >> At least, it looks like I can, I didn't tried, actually. | > Neither have I. | | So I did. And succeed: TH sees into data types. | | (ghc 6.12.1) | | Module A.hs, contains definition of abstract data type A, class Class | and some primitive instance generator for that Class. Instance | generator takes a data declaration name, takes first constructor | (which should be argumentless) and makes it a value for definition of | "c" function. | ---------------------------------------------------------- | {-# LANGUAGE TemplateHaskell #-} | | module A(A,Class(..),mkSimpleClass) where | | import Language.Haskell.TH | | data A = A1 | A2 | deriving Show | | class Class a where | c :: a | | mkSimpleClass :: Name -> Q [Dec] | mkSimpleClass name = do | TyConI (DataD [] dname [] cs _) <- reify name | ((NormalC conname []):_) <- return cs | ClassI (ClassD [] cname [_] [] [SigD mname _]) <- reify ''Class | return [InstanceD [] (AppT (ConT cname) (ConT dname)) [FunD mname | [Clause [] (NormalB (ConE conname)) []]]] | ---------------------------------------------------------- | | Module B.hs, imports A.hs, uses mkSimpleClass on A.A name: | ---------------------------------------------------------- | {-# LANGUAGE TemplateHaskell #-} | | module B where | | import A | | $(mkSimpleClass ''A) | ---------------------------------------------------------- | | I successfully loaded B.hs into ghci, Expression "c :: A" successfully | evaluates to A1. | | My view on that problem is that we can add TyConIAbs for incompletely | exported and abstract data types. | | When someone get TyConIAbs after reification, he will know that he | doesn't know everything about that type. | | So, empty data declaration like "data Z" will return TyConI with empty | list of constructors, TyConIAbs will have empty list of constructors | for abstract data type. | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
