[Newbies] Re: error trap

[Klaus D. Witzel]

On Sun, 13 Aug 2006 18:31:23 +0200, Damien Cassou wrote:

>> Yes. And, into the other direction, even in good core methods one often  
>> finds things like
>>   ^ dict at: aKey ifAbsent: [nil]
>>  There seems to be a "natural" confusion between "object" value and  
>> block "value".
>
> What's the problem with that ?

The lack of understanding the expressive power of the software developer  
is the problem. Why always use a hammer? Because everything looks like  
nails?

> Yes, 'nil value' answers nil but 'nil' is not a block and #at:ifAbsent:  
> waits for a block (the parameter is called aBlock).

No, #at:ifAbsent: doesn't wait for an instance of BlockContext. Smalltalk  
is typeless and, if at all, only has *one* (the universal) type.

> It is shorter to write 'nil' directly I agree.

And the compiler is not forced to emit code for a BlockContext and the VM  
is not forced to create a BlockContext only for throwing it away unused  
(unused in the sense that nil == [nil] value). It is like writing (x)  
versus (0+x) or (1*x).

> What is more problematic in my point of view is code like this:
>
> AbstractLauncher>>parameterAt: parName ifAbsent: aBlock
>    "Return the parameter named parName.
>      Evaluate the block if parameter does not exist."
>    ^self parameters
>      at: parName asUppercase
>      ifAbsent: [aBlock value]
>
> It may be me (still a beginner), but it looks equivalent to
>
>    ^self parameters
>      at: parName asUppercase
>      ifAbsent: aBlock

Yes, anObject == [anObject] value.

> which is faster and clearer in my opinion.

Sure.

/Klaus