On Tue, Feb 04, 2025 at 08:08:13AM +0000, Richard W.M. Jones wrote:
> On Mon, Feb 03, 2025 at 02:35:20PM -0500, Cole Robinson wrote:
> > + ignore (g#sh cmd);
>
> To be clear what's happening with this line of code:
>
> 'ignore' is an internal OCaml function that throws away the normal
> result of the command.
More on this ...
'ignore' has type:
val ignore : 'a -> unit
where 'a is read as "alpha", and means "any type", and unit is a bit
like void in C. So it's a function that takes a single parameter of
any type and returns nothing, and as a side effect ignores the
parameter.
The OCaml compiler warns if the result of a statement is not used,
since that can indicate a bug. For example:
$ rlwrap ocaml
# let f () =
2+5;
() ;;
Warning 10 [non-unit-statement]: this expression should have type unit.
val f : unit -> unit = <fun>
(The warning applies to the statement '2+5' which is calculated but
then not used).
In virt-v2v we convert these warnings into hard errors using the OCaml
compiler equivalent of -Werror.
When you really want to calculate something and ignore the result, you
have to use ignore (...) around it, eg:
# let f () = ignore (2+5); () ;;
val f : unit -> unit = <fun>
'ignore' is needed above because g#sh returns stdout of the command
(in the non-error case), which we're not interested in.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
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