[Haskell-cafe] Re: Scraping boilerplate deriving?

[Kevin Jardine]

Thanks Serguey!

The library code compiles, but when I try to use it in client code:

a. I get:

Not in scope: type constructor or class 'A'

and even stranger,

b. GHC cannot find any of my code after the

$(mkNewType "A")

and claims that all the functions I defined there are also not in
scope.

Any ideas?

The CPP solution works but Template Haskell is definitely cooler, so
it would be great to get this to work!

Kevin

On Sep 14, 2:29 pm,  Zefirov <sergu... at gmail.com> wrote:
> 2010/9/14 Kevin Jardine <kevinjard... at gmail.com>:
>
> > I would like to use some macro system (perhaps Template Haskell?) to
> > reduce this to something like
>
> > defObj MyType
>
> > I've read through some Template Haskell documentation and examples,
> > but I find it intimidatingly hard to follow. Does anyone has some code
> > suggestions or pointers to something similar?
>
> The solutions first:
> -------------------------------------------------
> {-# LANGUAGE TemplateHaskell #-}
>
> module T(mkNewType) where
>
> import Language.Haskell.TH
>
> decls = [d|newtype TempDecl = TempDecl Int deriving (Eq,Ord,Show)|]
> decl = do
>         [d] <- decls
>         runIO $ print d -- just to show inetrnals
>         return d
>
> mkNewType :: String -> Q [Dec]
> mkNewType n = do
>         d <- decl
>         let name = mkName n
>         return $ (\x -> [x]) $ case d of
>                 (NewtypeD cxt _ argvars (NormalC _ args) derivings) ->
>                         NewtypeD cxt name argvars (NormalC name args) derivings
> --------------------------------------
> I took perfectly valid declaration, dissected it using case analysis
> and changed relevant parts.
>
> And an example client:
> -------------------------------------
> {-# LANGUAGE TemplateHaskell #-}
>
> import T
>
> $(mkNewType "A")
> -------------------------------------
> It all work together.
>
> I studied how to use Template Haskell that way: I obtained
> declarations of what I need, printed them and looked through
> documentation for relevant data types and constructors. It's not
> harder that any other library in Haskell, actually.
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