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Custom operators: "This value is not a function and cannot be applied..."
f#

I'm trying to create a user-defined operator according to p.27 of Foundations of F#, and have run up against an error:

> let (!!) a b = a - b;;
val ( !! ) : int -> int -> int
> 1 !! 1;;
(1 !! 1);;
^^^^^^^^^

stdin(11,11): error: FS0003: This value is not a function and cannot be applied. Did you forget a ';' or 'in'?

Are the rules for valid operator characters on p.27 correct?

--
Jeff S.

.

Hi Jeff,

I test your code and I'm getting exactly the same error as you. It seems at the moment operations can't start with !, I tried several other operators, the following worked correctly:

1

let (-!!) a b = a - b;;
1

let (-!) a b = a - b;;

But this operator fails with the same error:

1

let (!-) a b = a - b;;

I suppect this is a bug in the compiler rather than a bug in my book, since the rules seem to be in line with the informal specification [1] and ! is sometime used as a special operator. I'll send a bug report to the F# team (although they'll probably read it here soon anyway).

Cheers,
Rob

[1] [link:research.microsoft.com]

By on 7/23/2007 11:17 AM ()

Hi,

This is not a bug. Operators type and priority are defined by their first character (the first character, ignoring leading . and $). An operator starting by !, ? or ~ is an infix operator. Remember: you write !foo (when foo is a ref).

From lexyacc.aspx, priorities are:

1
2
3
4
5
6
7
8
9

-OP +OP -. %left

*OP /OP %OP %left

**OP %right

?? %left

!OP ?OP ~OP %prefix

Edit: ok, Don was faster. :)
I believe that if each operator could be used either as infix or as prefix, that wouldn't lead to conflicts in the grammar (like with infix + and prefix +). And I think that would not lead to incompatibilities with old code.

Laurent.

By on 7/23/2007 1:15 PM ()

Thanks Rob! Good to know. It's worth mentioning that I can't get operators to start with '?' either.

--
Jeff S.

By on 7/23/2007 11:27 AM ()

Operators beginning with "!", "?" and "~" are always prefix - this is a design decision taken by OCaml and one that we have stuck with for F#.

See [link:research.microsoft.com].

I'll made a note to make this clearer in the spec as we revise it (soemthing we're currently doing).

cheers

Don

By on 7/23/2007 1:02 PM ()

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