I hit that again (The F# April 2010 Community Technology Preview, 2.0.0.0). With this:

people.Sort(
{
new IComparer with
member this.Compare(l, r) =
if l.First > r.First then 1
elif l.First = r.First then 0
else −1
} )

And this:

people.Sort(
{
new IComparer with
member this.Compare(l, r) =
if l.First > r.First then 1
elif l.First = r.First then 0
else −1
} )

And error message is:

Error 2 Unexpected character '−' in if/then/else expression C:\Users\...\C6DotNetProgramming.fs 111 45 Funtastic

This is the content of fs file:

module C6DotNetProgramming

// CLI
// ...

// Garbage Collection
// ... (IDisposable)

//*****************************************
// Interfaces
//*****************************************

// Example: Interfaces in F#
type Tastiness =
| Delicious
| SoSo
| TrySomethingElse

type IConsumable =
abstract Eat : unit -> unit
abstract Tastiness : Tastiness

// Protip: Eat one of these a day
type Apple() =
interface IConsumable with
member this.Eat() = printfn "Tastey!"
member this.Tastiness = Delicious

// Not that tastey, but if you are really hungry will do a bind
type CardboardBox() =
interface IConsumable with
member this.Eat() = printfn "Yuck!"
member this.Tastiness = TrySomethingElse

//-----------------------------------------
// Using Interfaces

(*
Unlike other .NET languages, in F#, the methods and properties of implemented interfaces
are not in scope by default, so in order to call interface methods, you must cast
the type to the corresponding interface. Once you have an instance of the interface,
that object can do exactly what the interface specifies, no more and no less.
*)

let apple = new Apple()
// apple.Tastiness;; -> error
let iconsumable = apple :> IConsumable
// iconsumable.Tastiness;;

//-----------------------------------------
// Defining Interfaces

(*
To create an interface, you simply define a new class type with only abstract methods.
The type inference system will infer the type to be an interface. Alternatively, you can
use the interface keyword, followed by abstract members and properties, followed by
the end keyword.
*)

// Define a type inferred to be an interface
type IDoStuff =
abstract DoThings : unit -> unit

// Define an interface explicitly
type IDoStuffToo =
interface
abstract member DoThings : unit -> unit
end

(*
Interfaces can inherit from one another, with the derived interface extending the contract
of the first. However, consumers must implement the entire inheritance chain,
implementing each interface fully.
*)

//*****************************************
// Object Expressions
//*****************************************

//-----------------------------------------
// Object Expressions for Interfaces

// Example 6-3. Sorting a list using IComparer<'a>
open System.Collections.Generic

type Person =
{ First : string; Last : string }
override this.ToString() = sprintf "%s, %s" this.Last this.First

let people =
new List<_>(
[|
{ First = "Jomo"; Last = "Fisher" }
{ First = "Brian"; Last = "McNamara" }
{ First = "Joe"; Last = "Pamer" }
|] )

let printPeople () =
Seq.iter (fun person -> printfn "\t %s" (person.ToString())) people

printfn "Initial ordering:"
printPeople ()

(*
Error 2 Unexpected character '−' in if/then/else expression C:\Users\ShahKaveh\Documents\Visual Studio 2008\Projects\Funtastic\Funtastic\C6DotNetProgramming.fs 111 45 Funtastic
*)

// Sort people by first name
people.Sort(
{
new IComparer with
member this.Compare(l, r) =
if l.First > r.First then 1
elif l.First = r.First then 0
else −1
} )

printfn "After sorting by first name:"
printPeople ()

</i><b>How to resolve (hack?):</b><i> I have wrote this code: let minusOne = fun () -> -1 // Sort people by first name people.Sort( { new IComparer with member this.Compare(l, r) = if l.First > r.First then 1 elif l.First = r.First then 0 else minusOne () } ) And the compiled the solution; It compiled. Then I have changed it to this: // Sort people by first name people.Sort( { new IComparer with member this.Compare(l, r) = if l.First > r.First then 1 elif l.First = r.First then 0 else -1 } ) And It compiled without error again. Note: I have typed this at first (time): people.Sort( { new IComparer with member this.Compare(l, r) = if l.First > r.First then 1 elif l.First = r.First then 0 else -1 } ) And then added tabs (in my VS == 4 spaces) by selecting lines and pressing tabs (if it helps). I apologize in advance if a too basic mistake presented here.