Question

Is there any way to override the comparison function in a F# set?

I don’t see any set construction functions that take a IComparer<T> or comparison function:

• Set.ofSeq et al don’t take a comparison function
• FSharpSet(IComparer<T> comparer, SetTree<T> tree) constructor is internal, because
• SetTree is internal and
• SetTreeModule.ofSeq<a>(IComparer<a> comparer, IEnumerable<a> c) is obviously internal too.

My actual problem is that I have a set of ('a * 'a) and I want a comparison such that for example (1,3) = (3,1).

I know I could wrap this in a type implementing IComparable<T>, but is there any way to avoid this?

Answer 1

I think the set type available in F# core libraries doesn’t allow specifying your own comparison. However, there is a tagged version of the type in PowerPack that allows this. The following shows how to create a set with comparer that compares integers using modulo 10:

#r @"FSharp.PowerPack.dll"
open System.Collections.Generic
open Microsoft.FSharp.Collections.Tagged

type MyComparer() = 
  interface IComparer<int> with
    member x.Compare(a, b) = (a % 10).CompareTo(b % 10)

// Type alias for a set that uses 'MyComparer'
type MySet = Tagged.Set<int, MyComparer>

let comparer = new MyComparer()
let s1 = MySet.Create(comparer, [1; 2; 3])
let s2 = MySet.Create(comparer, [11; 14])

MySet.Union(s1, s2)