Re: [Caml-list] 'should have type unit' warning in 'let _ =' ?

[Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>]

From: Richard Jones <rich@annexia.org>
> On Tue, Jul 15, 2003 at 10:04:36AM +0900, Jacques Garrigue wrote:
> > By the way, there is a reason "let _ =" produces no warning: this is
> > the only way to do a partial application without warning.
> > "expr; ..." and even "ignore (expr); ..." will produce a warning.
> 
> Interesting ... what's the use of a partial application which is
> then ignored?

The same as all application whose result is ignored: side effects.
I know this is quite rare. (I can remember using this only once.)
But it would be disturbing to make it completely impossible.

Or is it not so important?
Actually I wonder whether it would not be safer to simply warn for all
unused partial applications in a compiled program. This is a bit
subtle, as one has to keep track of unused variables too (functions
defined by partial application which are never used).

A middle ground would be to only warn when the wild pattern matches a
function type, assuming that when you name something you know what
you are doing. This would be mostly compatible: you just have to name
the pattern when you want to discard a function. As a nice aside, it
would avoid most uses of "ignore".

By the way, my personal approach would be to let programers declare what
they want: remove the "should be unit" warning (it can be easily
enforced by "let () = ... in"), and remove all warnings on "ignore",
since you could then write "let _ = ... in" if you want the partial
application warning. But this is maybe expecting too much from
programmers.

Comments?

Jacques Garrigue

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