[Python-Dev] Simple Switch statement (original) (raw)

Ka-Ping Yee python-dev at zesty.ca
Sun Jun 25 23:35:34 CEST 2006

On Sun, 25 Jun 2006, Guido van Rossum wrote:

In your eagerness to rule out surprises, you're creating the biggest surprise of all: the restriction to literals is certainly a surprise!

I disagree. Perhaps what we mean by "surprise" is different. In Raymond's design, there is a simple rule for what's allowed in a case. The whole statement can be described in very few words:

Each case is a literal integer, string, or tuple of integers
or strings.  Execution jumps to the first case that matches the
value of the switch expression (or to 'default' if no match).

That's it. The simpler the rule, the less surprising it is. It would take a lot more words to explain the behaviour of something like Nick's 'once'-based proposal. Here's an attempt:

Each case consists of an arbitrary expression, but the expression
may not refer to any local variables or arguments of the immediately
surrounding function definition.  The value of each case is computed
when the surrounding function definition is compiled.  If any two
cases have the same value, an exception is thrown at compile time.
At runtime, execution jumps to the case whose previously fixed value
matches the value of the switch expression (or to 'default' if no
match).

Not only is that longer to describe, it's also more difficult for a beginning programmer to understand, since it requires knowing when different parts of a program are compiled (and what happens if the same part is compiled more than once).

-- ?!ng

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