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!

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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

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.