F# / FSharp Generic Types with Comparison for Binary Tree from wikibooks.org

Question

I'm trying to implement a basic Red/Black tree in Fsharp, based public code ( http://en.wikibooks.org/wiki/F_Sharp_Programming/Advanced_Data_Structures )

But I keep running into problems at compile time with the final signature:

type 'a BinaryTree(inner : 'a tree) =
    member this.head = Tree.head inner
    member this.left = BinaryTree(Tree.left inner)
    member this.right = BinaryTree(Tree.right inner)
    member this.exists item = Tree.exists item inner
    member this.insert item = BinaryTree(Tree.insert item inner)
    member this.print() = Tree.print 0 inner
    static member empty = BinaryTree<'a>(E)

Specifically it gives me an error that "A type parameter is missing a constraint 'a when: comparison'" but every thing I've tried to add it hasn't worked. What am I missing?

Answer 1

Try re-write a type declaration as

type BinaryTree<'a when 'a : comparison> (inner : 'a tree) =

I think there is no way to specify constraint on type parameter when you are using 'prefix' generic notation in class declaration