Question

I want to implement a heap data structure and want to apply the code to any type that supports comparison, i.e. < = > <= >= operations.

How to do this in F# as it is statically typed.

Answer 1

It does, but you have to annotate as follows:

type Thing<'a when 'a:comparison> =
    | Pair of ('a*'a)
    with 
      member m.InOrder() = 
        match m with
        | Pair (a,b) when a<=b -> true
        | _ -> false
      member m.Equal() =
        match m with 
        | Pair (a,b) when a=b -> true
        | _ -> false

Pair(1,2).InOrder() //true
Pair(3,2).InOrder() //false
Pair(42,42).Equal() //true

Try replacing Thing<'a when 'a:comparison> by Thing<'a when 'a:equality> to watch the InOrder() method fail, while Equal() still works. Replace Thing<'a when 'a:comparison> by Thing<'a> and both methods won’t work.