Search code examples
structf#overloadingwarnings

Can I overload the '=' operator for struct-typed arguments?

I want to get a warning everywhere the equals operator '=' is used on struct-typed arguments. It seems that if I define this operator it does not overload, it just redefines '=' to only work for struct-typed arguments, which is not what I want.

[<Obsolete("Possible performance issue with '=' operator on structs. See https://github.com/dotnet/fsharp/issues/526.")>]
let inline (=) (x:  ^T) (y:  ^T) : bool when   ^T: struct and ^T: (static member (=) :  ^T *  ^T -> bool) =
    x = y

let a = obj()
let x = Guid.NewGuid()
let y = Guid.NewGuid()

a = a |> ignore // oh no - "A generic construct requires that the type 'obj' is a CLI or F# struct type."

x = y |> ignore // ok - gets desired warning

Can this be done in F# as is?

Update: found a possible workaround: simply use the [<NoEquality>] attribute on the affected structs; it does mean that all structs need to be annotated but at least it does help.

Solution

  • No, you cannot redefine a globally-scoped (i.e. non-member) function only for some cases. Once a function is in scope, it will always be used, there is no fallback to the previously defined one.