[Caml-list] Why warning?

[Caml-list] Why warning?

[SooHyoung Oh]

Why is this warning occurred?
Where can I get infomation about this? (ex: 'nnn' page on manual, articles
or books)

# 1; 2;;
Warning: this expression should have type unit.
- : int = 2

--- SooHyoung Oh

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Re: [Caml-list] Why warning?

[Christian RINDERKNECHT]

Hi Soo-Hyoung,

On Wed, Sep 12, 2001 at 03:22:32PM +0900, SooHyoung Oh wrote:
> 
> Why is this warning occurred?
> Where can I get infomation about this? (ex: 'nnn' page on manual, articles
> or books)
> 
> # 1; 2;;
> Warning: this expression should have type unit.
> - : int = 2

The sequence operator ; (semi-colon) is used for composing expressions
that may perform some side-effects. This is the typical construct of
imperative languages like Pascal. In Caml, such expressions are given
the type unit, and the only value of type unit is noted (). It
is generally considered as good practice to:

  (1) make sure that the expressions are of type unit, except maybe
      the last one, ie. given <e1>; <e2>; .... ; <en>; <e'>, then
      <e1>, <e2>, ..., <en> should have type unit;
  (2) use keywords "begin" and "end" to enclose your sequence:
      begin
        <e1>;
        <e2>;
        ...
        <en>;
        <e'>
      end

So, in your example, 1 is of type int, not unit (hence the warning).

For historical reason, this behaviour is not mandatory, and you can
turn off this warning in batch mode usign the -W s command-line
option. Please refer to
http://caml.inria.fr/ocaml/htmlman/manual021.html for other warnings.

As far as I am concerned, all warnings are turned into errors:)

Hope this helps,

-- 

Christian

-----------------------------------------------------------------------
Christian Rinderknecht                     Phone +82 42 866 6147
Network Architecture Laboratory            Fax   +82 42 866 6154
Information and Communications University  WWW   http://nalab.icu.ac.kr
58-4 Hwaam-dong, Yuseong-gu, Daejeon, 
305-752, Korea
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Re: [Caml-list] Why warning?

[Sven]

On Wed, Sep 12, 2001 at 03:22:32PM +0900, SooHyoung Oh wrote:
> 
> Why is this warning occurred?
> Where can I get infomation about this? (ex: 'nnn' page on manual, articles
> or books)
> 
> # 1; 2;;
> Warning: this expression should have type unit.
> - : int = 2

Because you are throwing away the 1 value, and mostly you don't want to do
this, and it is a symptom of a typo or other kind of bug in your program.
There trully is no real difference bewteen 1; 2 and simply 2, they are
the exact same values. The only reason you would want to do something like
that is if you use functions with side effect, in this case, you can either
switch off this warning (not recomended) or use the ignore : 'a -> unit
function :

# ignore (1); 2;;
- : int = 2

Which does exactly what you want, without the warning.

Friendly,

Sven Luther
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Re: [Caml-list] Why warning?

[Jean-Christophe Filliatre]

SooHyoung Oh writes:
 > 
 > Why is this warning occurred?
 > Where can I get infomation about this? (ex: 'nnn' page on manual, articles
 > or books)
 > 
 > # 1; 2;;
 > Warning: this expression should have type unit.
 > - : int = 2

In  the sequence  "expr1; expr2",  the result  of expr1  is discarded.
Thus, whenever the  result of expr1 is not "()" (i.e.  expr1 is not of
type "unit") the Caml compiler gives you a warning (you may have typed
";" instead of "," for instance, or any similar typo)

This warning may be disabled, as explained in the manual:

     http://caml.inria.fr/ocaml/htmlman/manual021.html

But you should rather write your code in a different way, having expr1
of type unit, or explicitly  ignoring its result. There are (at least)
two possibilities:

   let _ = expr1 in expr2

or better

   ignore (expr1); expr2

"ignore" is part of the Caml code library; see

   http://caml.inria.fr/ocaml/htmlman/manual031.html#@manual94

Best regards,
-- 
Jean-Christophe Filliatre (http://www.lri.fr/~filliatr)

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Re: [Caml-list] Why warning?

[David Mentre]

"SooHyoung Oh" <shoh@duonix.com> writes:

> Where can I get infomation about this? (ex: 'nnn' page on manual, articles
> or books)

I've not found any pointer that explains this specific point.

> # 1; 2;;
> Warning: this expression should have type unit.
> - : int = 2

If you decompose this expression, you have:
# 1;;
- : int = 1 <-- notice the type of '1' is 'int'
# 2;;
- : int = 2

In an expression 'expr1 ; expr2', expr1 should have type unit as its
result is not accessible to expr2.

Hope it helps,
d.
-- 
 David.Mentre@inria.fr
 Opinions expressed here are only mine.
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Re: [Caml-list] Why warning?

[Pierre Weis]

> Why is this warning occurred?
> Where can I get infomation about this? (ex: 'nnn' page on manual, articles
> or books)
> 
> # 1; 2;;
> Warning: this expression should have type unit.
> - : int = 2
> 
> --- SooHyoung Oh

Have a look to the FAQ:

http://pauillac.inria.fr/caml/FAQ/pgl-eng.html#avertissements

paragraph entitled ``Sequence warnings''.

Different ways of getting rid of the warning are discussed, including
the most demanding one (and, to my knowledge, the best one): ``understand why
(the warning) is emitted by the compiler''.

Best regards,

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/


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