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Re: first-class functions?

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  Re: first-class functions? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-03-12 03:07 +0000
    Re: first-class functions? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-03-12 11:06 +0100

#23562 — Re: first-class functions?

ram@zedat.fu-berlin.de (Stefan Ram) writes:

>   I start with the following code in a web page:
>
> document.writeln( 4 );
>
>   . It prints »4«. Ok. Next, I enclose the function expression
>   in parentheses:
>
> ( document.writeln )( 4 );
>
>   Still prints »4«. Fine. But now, I try to get closer to what
>   I actually wanted to write:
>
> ( true ? document.writeln : document.writeln )( 4 );
>
>   And this gives an error message here! Where are my
>   first-class "functions as values", when I need them?

document.writeln is a function but, being a method, it will refer to
properties in the document object (it does so using "this" of course).

A simpler example:

  var wl = document.writeln;
  wl(4);  // This is an error -- to what does 'this' refer in wl?

You can get your first-class function back by binding the member
function to the object:

  var wl = document.writeln.bind(document);
  wl(4);

Something similar will fix your conditional expression, too.

-- 
Ben.

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#23565

Ben Bacarisse wrote:

> ram@zedat.fu-berlin.de (Stefan Ram) writes:
>>   I start with the following code in a web page:
>>
>> document.writeln( 4 );
>>
>>   . It prints »4«. Ok. Next, I enclose the function expression
>>   in parentheses:
>>
>> ( document.writeln )( 4 );
>>
>>   Still prints »4«. Fine. But now, I try to get closer to what
>>   I actually wanted to write:
>>
>> ( true ? document.writeln : document.writeln )( 4 );
>>
>>   And this gives an error message here! Where are my
>>   first-class "functions as values", when I need them?
> 
> document.writeln is a function but, being a method, it will refer to
> properties in the document object (it does so using "this" of course).

“Of course”?  No.  It is a method of a host object.  All bets are off.
 
> A simpler example:
> 
>   var wl = document.writeln;

This could already be an error, referring to a host objects method not in a 
CallExpression.  BTST.  The host object referred to by “document” does not 
have to implement [[Get]] in the way of native objects. [1]

>   wl(4);  // This is an error -- to what does 'this' refer in wl?

We cannot know that, if there is such a value, and if it would be relevant.  
(Assuming native object-like behavior, “this” would either refer to the 
global object, or in strict mode would be the “undefined” value.)

> You can get your first-class function back by binding the member
> function to the object:
> 
>   var wl = document.writeln.bind(document);
>   wl(4);

Unreliable at best: a host object’s method does not have to refer to a 
Function instance, it does not even have to have Function.prototype in its 
prototype chain which provides .bind() [2].  It just needs to be callable, 
i.e. implement [[Call]] (not: .call()) in a useful way. [3, 4]

Certainly incompatible as Function.prototype.bind() was not introduced 
before ECMAScript Edition 5. [5]
 
> Something similar will fix your conditional expression, too.

You simply cannot know that.

_________
[1] <http://ecma-international.org/ecma-262/5.1/#sec-8.6.2>
[2] <http://ecma-international.org/ecma-262/5.1/#sec-15.3.4.5>
[3] <http://ecma-international.org/ecma-262/5.1/#sec-11.2.3>
[4] <http://ecma-international.org/ecma-262/5.1/#sec-15.3.4.4>
[5] <http://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%203rd%20edition,%20December%201999.pdf>
-- 
PointedEars
FAQ: <http://PointedEars.de/faq> | SVN: <http://PointedEars.de/wsvn/>
Twitter: @PointedEars2 | ES Matrix: <http://PointedEars.de/es-matrix>
Please do not Cc: me. / Bitte keine Kopien per E-Mail.

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