Questions on data/type

[Matthew Wong]

Hi,

I have the following data defined.

data TypeCon a = ValConA a | ValConB [a] | ValConC [[a]]

So I can use functions with type like  (a->a->a) -> TypeCon a -> TypeCon a -> TypeCon a
for all 3 value types, and I think is easier to define one single typeclass for (+), (*) etc.

If I want to express the following idea (the following won't compiler):

data TypeCon a = ValConA a | ValConB [ValConA a] | ValConC [ValConB a]

Is this even a good idea?  If so how could I proceed?  The closest thing I can get to compiler is like this:

data TypeCon a = ValConA a | ValConB [TypeCon a]

Which is a nightmare when I try to manipulate anything in this structure.  The alternative is use 3 different type constructors,

data TypeConA a = ValConA a
data TypeConB a = ValConB a
data TypeConC a = ValConC a

but then I can't use one signal typeclass for (+) etc.

thx,

//pip

[Jason Dusek]

2009/11/15 Matthew Wong <[hidden email]>:
> If I want to express the following idea (the following won't compiler):
>
> data TypeCon a = ValConA a | ValConB [ValConA a] | ValConC [ValConB a]

  In this definition, you are confusing constructors with types.
  You can't have a list of `ValConB a` -- the type is actually
  `TypeCon a`.

> The alternative is use 3 different type constructors,
>
> data TypeConA a = ValConA a
> data TypeConB a = ValConB a
> data TypeConC a = ValConC a

  This is more promising. I'd need to see the typeclass you're
  trying to define to say more.

--
Jason Dusek