Data constructor in template haskell

I'm trying to create the ring Z/n (like normal arithmetic, but modulo some integer). An example instance is Z4:

instance Additive.C Z4 where
  zero = Z4 开发者_如何学Go0
  (Z4 x) + (Z4 y) = Z4 $ (x + y) `mod` 4

And so on for the ring. I'd like to be able to quickly generate these things, and I think the way to do it is with template haskell. Ideally I'd like to just go $(makeZ 4) and have it spit out the code for Z4 like I defined above.

I'm having a lot of trouble with this though. When I do genData n = [d| data $n = $n Integer] I get "parse error in data/newtype declaration". It does work if I don't use variables though: [d| data Z5 = Z5 Integer |], which must mean that I'm doing something weird with the variables. I'm not sure what though; I tried constructing them via newName and that didn't seem to work either.

Can anyone help me with what's going on here?

The Template Haskell documentation lists the things you are allowed to splice.

A splice can occur in place of

• an expression; the spliced expression must have type Q Exp
• an type; the spliced expression must have type Q Typ
• a list of top-level declarations; the spliced expression must have type Q [Dec]

In both occurrences of $n, however, you're trying to splice a name.

This means you can't do this using quotations and splices. You'll have to build declaration using the various combinators available in the Language.Haskell.TH module.

I think this should be equivalent to what you're trying to do.

genData :: Name -> Q [Dec]
genData n = fmap (:[]) $ dataD (cxt []) n []
                           [normalC n [strictType notStrict [t| Integer |]]] []

Yep, it's a bit ugly, but there you go. To use this, call it with a fresh name, e.g.

$(genData (mkName "Z5"))