Re: [Python-Dev] Re: switch statement

21 Apr 2005

      On 4/21/05, Michael Hudson  wrote:
...
Shannon -jj Behrens  writes:
...
On 4/20/05, M.-A. Lemburg  wrote:
...
My use case for switch is that of a parser switching on tokens.
mxTextTools applications would greatly benefit from being able
to branch on tokens quickly. Currently, there's only callbacks,
dict-to-method branching or long if-elif-elif-...-elif-else.
I think "match" from Ocaml would be a much nicer addition to Python
than "switch" from C.
Can you post a quick summary of how you think this would work?
Sure.  

Now that I'm actually trying to come up with an example, I'm noticing
that Ocaml is very different than Python because Python distinguishes
statements and expressions, unlike say, Scheme.  Furthermore, it's
important to minimize the number of new keywords and avoid excessive
punctuation (which Ocaml is full of).  Hence, I propose something
like:

def handle_token(token):
    match token:
        NUMBER:
            return number / a
        WHITESPACE if token.value == "\n":
            return NEWLINE
        (a, b):
            return a / b
        else:
            return token

Hence, the syntax is something like (in pseudo EBNF):

    'match' expr ':'
        {match_expression ':'
            block}*
        'else' ':' 
            block

    match_expr ::= lvalue | constant_expression

Sematically, the above example translates into:

def handle_token(token):
    if token == NUMBER:
        return number / a
    elif token == WHITESPACE and token.value == "\n":
        return NEWLINE
    elif "setting (a, b) = token succeeds":
        return a / b
    else:
        return token

However, unlike the code above, you can more easily and more
aggressively optimize.

Best Regards,
-jj

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