Re: [Caml-list] ignore semantics

[John Whitington <john@coherentgraphics.co.uk>]

Hi,

Philippe Veber wrote:
> This is a rather minor issue, but I was surprised by the following
> answers of the interpreter:
>
>          OCaml version 4.00.1
>
> # ignore succ;;
> Warning 5: this function application is partial,
> maybe some arguments are missing.
> - : unit = ()
> # (fun _ -> ()) succ;;
> - : unit = ()
>
> I naively used to think the two expressions were equivalent. Reading
> pervasives.mli, I found that ignore is a primitive:
>
> external ignore : 'a -> unit = "%ignore"
>
> which means it is treated as a special case. Just for curiosity, what is
> the rationale for the warning in the first case?

Imagine you wrote:

ignore (output_something_and_return_something)

When what you wanted was

ignore (ouput_something_and_return_something ())

You'd want the partial application error there, even though you are 
(intentionally) ignoring the return value of the function.

Cheers,

-- 
John Whitington
Director, Coherent Graphics Ltd
http://www.coherentpdf.com/