Re: [Caml-list] Warning 20: Unused function argument

[Gabriel Scherer <gabriel.scherer@gmail.com>]

My understanding is that the warning is raised when applying an
argument to an expression whose type is a polymorphic variable (which
will become a function type through type instantiation, but it does
come from a syntactic of the form "let exp foo bar").

This can quite commonly happen if you use the following pattern to
mark not-yet-implemented functions

  let func _ = failwith "TODO"
  ...
  let foo bar = func bar (1 + bar)

Calling a toplevel with option "-w +20":

  # let test _ = failwith "TODO";;
  val test : 'a -> 'b = <fun>
  # test (1 + 2) (3 + 4);;
  Warning 20: this argument will not be used by the function.
  Exception: Failure "TODO".

The warning goes away if you write "test" with the expected number of
arguments (let test x y = failwith "TODO").

As of today, this warning will not get raised if you simply add unused
arguments to your function declaration (and do pass them at the call
site).

On Fri, Mar 22, 2013 at 10:16 AM, Francois Berenger <berenger@riken.jp> wrote:
> On 03/22/2013 06:05 PM, Gabriel Scherer wrote:
>>
>> My own guess (pure speculation) as to why this difference is that
>> enabling it by default would have raised too many warnings on existing
>> code, in particular breaking code of people sadly using a "all enabled
>> warnings are errors" discipline (don't!), while unused let-bound
>> variables where much less frequent in existing code.
>
>
> OCaml is not lazy, so that's not very smart if some of my functions
> have unused arguments... I have to fix my code. :(
>
>
>> On Fri, Mar 22, 2013 at 10:02 AM, Francois Berenger <berenger@riken.jp>
>> wrote:
>>>
>>> Hello,
>>>
>>> I understand it is turned off by default.
>>>
>>> I am a bit surprised as it is as important
>>> to me as an unused variable, which triggers a warning
>>> by default.
>>>
>>> Regards,
>>> F.
>>>
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>>
>>
>
>
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