Hi,

(!) is indeed an indentifier name like aa or foo. You can basically use it everywhere you could use aa.

Try this, it should work also:

let (!) = 2 in (!) + 3

match [1..5] with (!)::li ->(!) | _ -> 0

let f (!) = (!) + 1

Laurent

Yes and No :-)

No:
MSDN says that "!" is indeed OPERATOR:
[link:msdn.microsoft.com]
So in my factorial example really we are defining prefix operator.

Yes:
What will be done with (!) really depends on the context.
To see this we only need to print AST (abstract syntax tree) for the some examples:

printf "%A" <@ let f (!) = (!) + 1
f 5 @>

printf "%A" <@ let (!) x = x + 1
!5 @>

printf "%A" <@ let (!) = 15
(!) @>

What we get is:

Let (f,
Lambda (op_Dereference,
Call (None, Int32 op_Addition[Int32,Int32,Int32](Int32, Int32),
[op_Dereference, Value (1)])), Application (f, Value (5)))

Let (op_Dereference,
Lambda (x,
Call (None, Int32 op_Addition[Int32,Int32,Int32](Int32, Int32),
[x, Value (1)])), Application (op_Dereference, Value (5)))

Let (op_Dereference, Value (15), op_Dereference)

----
NOTE that (!) is translated into "op_Dereference" keyword into AST. This also shows already that "!" is INDEED operator :-)
But yes - what will be done with ! operator depends on the context of execution.

1. example - operator ! acts as function parameter.
2. example - operator ! acts as lambda function.
3. example - operator ! acts as some value.

So it seems when we are defining operators, in reality we are defining some lambda function.

I have a quasi-related question. Does anyone know how to define postfix operators in F#? or are we limited to prefix and infix operators only?

Thanks.