[Haskell-cafe] Scraping boilerplate deriving?

Sean Leather leather at cs.uu.nl
Tue Sep 14 04:58:22 EDT 2010

On Tue, Sep 14, 2010 at 10:24, Kevin Jardine wrote:

> I have a set of wrapper newtypes that are always of the same format:
> newtype MyType = MyType Obj deriving (A,B,C,D)
> where Obj, A, B, C, and D are always the same. Only MyType varies.
> A, B, C, and D are automagically derived by GHC using the
> {-# LANGUAGE GeneralizedNewtypeDeriving #-}
> feature.
> 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?

This works in TH:

> [d|newtype Blah = Blah Int deriving (Num,Show,Eq)|]

But the parameterized variations on this theme do not:

> derive1 name = [d|newtype $name = Blah Int deriving (Num,Show,Eq)|]
Malformed head of type or class declaration

> derive2 name = [d|newtype Blah = $name Int deriving (Num,Show,Eq)|]
parse error in data/newtype declaration

I think it has something to do with the type of the splice. Perhaps you can
look into further:

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