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Message-ID: <15678947.80IZ92ngzi@PointedEars.de>
From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Reply-To: Thomas 'PointedEars' Lahn <cljs@PointedEars.de>
Organization: PointedEars Software (PES)
Date: Wed, 12 Mar 2014 11:06:24 +0100
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Subject: Re: first-class functions?
Newsgroups: comp.lang.javascript
References: <first-class-functions-20140312030629@ram.dialup.fu-berlin.de> <0.4b944fce00eef4685de9.20140312030725GMT.874n34ktci.fsf@bsb.me.uk>
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Xref: csiph.com comp.lang.javascript: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>
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