[Python-Dev] Switch statement

[Talin]

Greg Ewing wrote:
> skip at pobox.com wrote:
> 
> 
>>    switch raw_input("enter a, b or c: "):
>>        case 'a':
>>            print 'yay! an a!'
>>        case 'b':
>>            print 'yay! a b!'
>>        case 'c':
>>            print 'yay! a c!'
>>        else:
>>            print 'hey dummy! I said a, b or c!'
> 
> 
> Before accepting this, we could do with some debate about the
> syntax. It's not a priori clear that C-style switch/case is
> the best thing to adopt.

Since you don't have the 'fall-through' behavior of C, I would also 
assume that you could associate more than one value with a case, i.e.:

    case 'a', 'b', 'c':
       ...

It seems to me that the value of a 'switch' statement is that it is a 
computed jump - that is, instead of having to iteratively test a bunch 
of alternatives, you can directly jump to the code for a specific value.

I can see this being very useful for parser generators and state machine 
code. At the moment, similar things can be done with hash tables of 
functions, but those have a number of limitations, such as the fact that 
they can't access local variables.

I don't have any specific syntax proposals, but I notice that the suite 
that follows the switch statement is not a normal suite, but a 
restricted one, and I am wondering if we could come up with a syntax 
that avoids having a special suite.

Here's an (ugly) example, not meant as a serious proposal:

    select (x) when 'a':
       ...
    when 'b', 'c':
       ...
    else:
       ...

The only real difference between this and an if-else chain is that the 
compiler knows that all of the test expressions are constants and can be 
hashed at compile time.

-- Talin