[Python-3000] Wild idea: Deferred Evaluation & Implicit Lambda

[Toby Dickenson]

On Tuesday 30 May 2006 11:13, Talin wrote:

> For example - suppose I have two dictionaries that I wish to 
> search, such that if I don't find a value in the first, I want to look 
> in the second; And if I don't find a value in the second, I want to 
> default to zero. (This pattern happens to me rather a lot - because I 
> tend to do a lot of stuff with parsing and nested symbol tables.)
> 
> One simple but inefficient way to code this would be:
> 
>     value = a.getitem( key, b.getitem( key, 0 ) )

This reminds me of one feature of the last scripting language I designed prior 
to Python taking over my world - an exception handler operator which could be 
used in expressions.

The ?? operator first evaluated its left operand. If that succeeds its value 
is returned. If that raised an exception it evaluates and returns its right 
operand. That allowed your example to be written:

	value = a[key] ?? b[key] ?? 0

That was handy for the scripts which this language was designed to support: It 
was a common requirement that values had to be found by scavenging through 
increasingly more desperate sources of information. All of the sources were 
fallible, and some default value needed to be substituted in case all other 
methods failed.

-- 
Toby Dickenson

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