[Haskell-cafe] Morphing ASTs and scrapping boilerplate code

[Twan van Laarhoven]

Joel Reymont wrote:
> I have a lot of boilerplate code like this and wonder how I can  scrape it.
> 
> instance Morpher Type C.Type where
>     morph TyInt = return C.TyInt
>     morph TyFloat = return C.TyFloat
>     morph TyStr = return C.TyStr
>     morph TyBool = return C.TyBool
>     morph TyColor = return C.TyColor
>     morph TyStyle = return C.TyStyle
>     morph (TyList ty) = liftM C.TyList (morph ty)
>     morph (TyArray ty) = liftM C.TyArray (morph ty)
>     morph (TySeries ty) = liftM C.TySeries (morph ty)
>     morph (TyInput ty) = liftM C.TyProp (morph ty)
>     morph (TyRef ty) = liftM C.TyRef (morph ty)
>     morph TyUnit = return C.TyUnit
>     morph TyPrintDest = return C.TyPrintDest

One option is to change your data types. I would suggest something like 
this:
 > data Type   = TyPrim   TyPrim | TyCon   TyCon Type
 > data C.Type = C.TyPrim TyPrim | C.TyCon TyCon C.Type

where TyPrim and TyCon (primitves and constructors) can be shared:
 > data TyPrim = TyInt | TyString | TyFloat | TyBool | etc.
 > data TyCon  = TyList | TyArray | TySeries | TyInput | TyRef

Now the class instance can be:
 > instance Morpher Type C.Type where
 >     morph (TyPrim  p) = return (C.TyPrim p)
 >     morph (TyCon c t) = C.TyCon c <$> morph t
 >      -- or liftM for people not so in love with Applicative :)

Twan