Implementing custom comparison with CustomComparison and CustomEquality in F# tuple

Question

I\'m here to ask a specific topic - I really found few info about this on the web. I\'m implementing a F# version of Minimax algorithm. The problem I\'m having now is that I wan

Answer 1

First of all, you're getting the exception because the compare function calls the CompareTo method of the values you're comparing (that is x.ComaperTo(y)). The values you're comparing using compare in the custom implementation of CompareTo are the values that the you are asked to compare (by the runtime), so this causes the stack overflow.

The usual way to implement CompareTo or Equals is to compare only some values that you store in your type. For example, you could write something like this:

EDIT: You can write a helper function mycopare to do the comparison (or you could simply change the CompareTo implementation). However, if you want to use a function, you need to move it inside the type declaration (so that it knows about the type - note that in F#, the order of declaration matters!)

One way of writing it is this:

[<CustomEquality; CustomComparison >] 
type TreeOfPosition = 
  | LeafP   of Position * int 
  | BranchP of Position * TreeOfPosition list 

  override x.Equals(yobj) =  
     match yobj with 
     | :? TreeOfPosition as y -> 
        // TODO: Check whether both y and x are leafs/branches
        // and compare their content (not them directly)
     | _ -> false 
  override x.GetHashCode() = // TODO: hash values stored in leaf/branch

  interface System.IComparable with 
     member x.CompareTo yobj =  

       // Declare helper function inside the 'CompareTo' member
       let mycompare x y = 
         match x, y with
         // Compare values stored as part of your type
         | LeafP(_, n1), LeafP(_, n2) -> compare n1 n2
         | BranchP(_, l1), BranchP(_, l2) -> compare l1 l2
         | _ -> -1 // or 1 depending on which is list...

       // Actual implementation of the member
       match yobj with 
       | :? TreeOfPosition as y -> mycompare x y
       | _ -> invalidArg "yobj" "cannot compare value of different types" 

This would work, because every call to compare takes only some part of the data, so you're making some progress.