[Haskell-cafe] Re: Type classes: Missing language feature?

[apfelmus]

DavidA wrote:
>
> newtype Lex = Lex Monomial deriving (Eq)
> newtype Glex = Glex Monomial deriving (Eq)
>
> Now, what I'd like to do is have Lex and Glex, and any further monomial 
> orderings I define later, automatically derive Show and Num instances from 
> Monomial (because it seems like boilerplate to have to define Show and Num 
> instances by hand).

Good news: it's already implemented and called newtype deriving :)

http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#newtype-deriving

In short, you just write

 newtype Lex = Lex Monomial deriving (Eq, Show, Num)

I guess that the Show instance will add the constructor Lex , though.

Regards,
apfelmus