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digitalmars.D.learn - calling functions without parentheses

reply =?ISO-8859-1?Q?Adam_Cig=E1nek?= <adam.ciganek gmail.com> writes:
Hello,

It seems that if function has no parameters, it's possible to omit the
parentheses when calling it:

    string sayHello() {
      return "hello";
    }

    void main() {
      writeln(sayHello); // same as writeln(sayHello());
    }

Is this an actual defined (and documented) behaviour that I can expect
to keep working in the future, or just some random unintended side
effect which might disappear in a next version?


adam.
Nov 08 2010
parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Mon, 08 Nov 2010 14:48:20 -0500, Adam Cigánek <adam.ciganek gmail.com>  
wrote:


 Hello,

 It seems that if function has no parameters, it's possible to omit the
 parentheses when calling it:

     string sayHello() {
       return "hello";
     }

     void main() {
       writeln(sayHello); // same as writeln(sayHello());
     }

 Is this an actual defined (and documented) behaviour that I can expect
 to keep working in the future, or just some random unintended side
 effect which might disappear in a next version?


It is documented behavior in D1 and originally in D2.  D2 will eventually  
require you to mark function calls omitting parentheses with  property.   
This is valid D2 today, but the  property part is not required to call the  
function:

 property string sayHello() {
     return "hello";
}

D1 will remain the same, i.e. it will not require  property.

-Steve
Nov 08 2010
parent reply =?ISO-8859-1?Q?Adam_Cig=E1nek?= <adam.ciganek gmail.com> writes:
Ok, thanks. I knew about  property, but not that it works even without
it. Actually, is  property even needed? I think that the ability to
call functions without parentheses could be useful for non-property
like function as well:

    widget.hide;

instead of:

    widget.hide();

The empty parentheses are just noise anyway. Unless it would conflict
with something else. But the only thing that comes to my mind is if
one wants to get the function itself, not call it. But that's what the
& operator is for anyway:

    button.onClick =3D &widget.hide;

a.

2010/11/8 Steven Schveighoffer <schveiguy yahoo.com>:

 On Mon, 08 Nov 2010 14:48:20 -0500, Adam Cig=E1nek <adam.ciganek gmail.co=


m>

 wrote:


 Hello,

 It seems that if function has no parameters, it's possible to omit the
 parentheses when calling it:

 =A0 =A0string sayHello() {
 =A0 =A0 =A0return "hello";
 =A0 =A0}

 =A0 =A0void main() {
 =A0 =A0 =A0writeln(sayHello); // same as writeln(sayHello());
 =A0 =A0}

 Is this an actual defined (and documented) behaviour that I can expect
 to keep working in the future, or just some random unintended side
 effect which might disappear in a next version?


 It is documented behavior in D1 and originally in D2. =A0D2 will eventual=


ly

 require you to mark function calls omitting parentheses with  property.
 =A0This is valid D2 today, but the  property part is not required to call=


 the

 function:

  property string sayHello() {
 =A0 =A0return "hello";
 }

 D1 will remain the same, i.e. it will not require  property.

 -Steve
Nov 08 2010
parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Mon, 08 Nov 2010 15:29:15 -0500, Adam Cigánek <adam.ciganek gmail.com>  
wrote:


 Ok, thanks. I knew about  property, but not that it works even without
 it. Actually, is  property even needed? I think that the ability to
 call functions without parentheses could be useful for non-property
 like function as well:

     widget.hide;

 instead of:

     widget.hide();


The point is that parentheses imply a function, where lack-of implies  
property.  For example:

if(x.read)

Could mean, read something from x, see if it's valid, or check if x has  
been read.

With forcing omitting parentheses or using them, the author of x is able  
to keep code that uses his object consistent.  The only alternative to  
this is to make the function/property a compound word like 'isRead' or  
'readValue', and we are back to Java.

For functions that return no value, it's pretty obvious that those are  
functions and not properties.  I have proposed that it might be possible  
to allow omitting parentheses on parameter-less functions that return  
void, but I have no idea if that will be implemented.


 The empty parentheses are just noise anyway. Unless it would conflict
 with something else. But the only thing that comes to my mind is if
 one wants to get the function itself, not call it. But that's what the
 & operator is for anyway:

     button.onClick = &widget.hide;


When the compiler is changed to require omission of parentheses for  
properties, you will not be able to get the address of the function via  
the & operator.  This is a limitation of the requirement.  Otherwise, if  
the property returns an lvalue, how do you know what the user wants, an  
address to the lvalue or the function address?

I've proposed to work around this using __traits syntax:

auto dg = __traits(getDelegate, x.read);

Haven't got any feedback on that idea.  It could also make getting  
delegates to specific overloads much more friendly.

-Steve
Nov 08 2010