What's the closest thing to Haskell GADTs and typeclasses in F#?

Question

F# is an ML with OOP. What\'s the closest it comes to Haskell generalized algebraic data types and typeclasses?

Answer 1

In F#, you often use interfaces and inheritance for these purposes.

For examples' sake, here is a simple typeclass using interfaces and object expressions:

/// Typeclass
type MathOps<'T> =
    abstract member Add : 'T -> 'T -> 'T
    abstract member Mul : 'T -> 'T -> 'T

/// An instance for int
let mathInt = 
    { new MathOps with
       member __.Add x y = x + y
       member __.Mul x y = x * y }

/// An instance for float
let mathFloat = 
    { new MathOps with
       member __.Add x y = x + y
       member __.Mul x y = x * y }

let XtimesYplusZ (ops: MathOps<'T>) x y z =
    ops.Add (ops.Mul x y) z

printfn "%d" (XtimesYplusZ mathInt 3 4 1)
printfn "%f" (XtimesYplusZ mathFloat 3.0 4.0 1.0)

It may not look very beautiful, but it's F#-ish way to do it. For a more Haskell-like solution which uses a dictionary-of-operations, you can have a look at this nice answer.