Haskell - class vs typeclass - what is the difference

Question

Hello Haskellers and Haskellettes,

I've been fiddling around with Haskell for quite a while but there is this concept of classes I cannot quite grasp. In the following example I have the datatype of ExprTree

data Val a = Num a | Var String deriving (Eq, Ord, Show)
data ExprTree = Leaf {lab::Label, val::(Val a)=> a}
          | Node {lab::Label, fun::Fun, lBranch::ExprTree, rBranch::ExprTree}
          deriving(Eq,Ord)

which leads to

Type constructor `Val' used as a class In the definition
of data constructor `Leaf' In the data type declaration for `ExprTree'

i also tried

data ExprTree' = Leaf {lab::Label, val::Val}
         ...

but randomly changing type signature - neither sounds efficent nor provides enlightenment.

now as far as i know Num a denotes something of class Num but is this is no instance of a datatype - and doesn't let me compile. So what do i have to do in order to make ExprTree well defined.

Thanks in advance for hints and ideas!

---

Edit:

1) Thanks for the fast answers!

2) I changed the val::(Val a)=>a to val::Val a

i had something similar in mind - but then the error: Not in scope type variable a occurs do you have additional advice ??

Answer 1

To answer the question in the title: When talking about Haskell, the word "class" is almost always used to mean "typeclass" as that's the only kind of class there is in Haskell. So there is no difference.

To answer the question in your body:

data ExprTree a =
    Leaf {lab::Label, val::(Val a)}
    | Node {lab::Label, fun::Fun, lBranch::(ExprTree a), rBranch::(ExprTree a)}
    deriving(Eq,Ord)

Writing (Val a)=>a makes no sense because Val isn't a typeclass and you can't just introduce typeclass constraints on the right hand side of a type definition (without extensions anyway - and either way it's not what you want).

Answer 2

The correct type would be

data Val a = Num a | Var String deriving (Eq, Ord, Show)
data ExprTree a = Leaf {lab::Label, val :: Val a}
          | Node {lab::Label, fun::Fun, lBranch::ExprTree a, rBranch::ExprTree a}
          deriving(Eq,Ord)

Since the type Val requires an additional type parameter, you need to provide one whenever you use it^*. I used a type variable, just as in the initial definition; that requires the variable to also be named as a parameter to ExprTree. (The other possibilities would be to use a concrete type such as Int or Maybe String, etc., or to use an existential type; neither makes sense here.)

What you actually used was a typeclass context ("class" is just shorthand for "typeclass"). Val is a type, not a typeclass, so it's not legal there.

^* This isn't quite true; you need a type of kind *. Kinds are the types of types: Int has kind *, Val a has kind *, Val has kind * -> *. That is, by itself Val is a type function which requires a parameter in order to become a full type.

Answer 3

The error points to this part of the type definition:

val::(Val a)=> a

This syntax means, "for any instance a of the typeclass Val, the value of val is of type a." But then (1) Val isn't a typeclass and (2) the type a seems to come out of thin air.

What you may have intended to say is

data Val a = Num a | Var String
           deriving (Eq, Ord, Show)
data ExprTree a = Leaf { val :: Val a }
                | Node { lBranch :: ExprTree a, rBranch :: ExprTree a }
                deriving (Eq, Ord)

Answer 4

Your problem is already solved, but I see you use GHC and sometimes — especially in the beginning — Hugs error messages may be somewhat easier to read, in this case:

Syntax error in data type declaration (unexpected `=>')

This might actually have given you a first idea where to look. Mind you that I'm not advocating Hugs over GHC here, but as a Haskell beginner I think that the former provides more helpful error messages.