Surprising compilation error when trying to implement an interface using object expressions.

Repro Steps

type IInterface =
    abstract Function : (int32 * int32) -> unit

{ new IInterface with
    member this.Function (i, j) = ()
}

Expected

Compilation without errors like this:

type IInterface =
    abstract Function : int32 -> unit

{ new IInterface'with
    member this.Function (i) = ()
}

Actual

The compilation error is:

FS0768: The member 'Function' does not accept the correct number of arguments, 1 arguments are expected

Severity

• Confusing for newcomers.
• Cumbersome to remember.
• But easy workaround.

Version

• F# 4.0 (FSharp.Core, 4.4.0.0)
• F# Compiler version 14.0.23413.0
• .NET Runtime, Framwork 4.6
• Windows 10
• Visual Studio Community 2015 version 14.0.2513.00 Update 2

Workaround

Surround the error producing parameter with additional brackets:

type IInterface =
    abstract Function : (int32 * int32) -> unit

{ new IInterface with
    member this.Function ((i, j)) = ()
}