If one were to add switch into Python, wouldn't it be desirable to
make a pattern matching switch (think the "match" or "case" construct
from Haskell or ML)? Python currently has poor support for union/sum
types in general, not just enumerations. It feels weird to add better
support for enumerations while ignoring the broader use case.
If a switch statement gets added to Python and I still need to examine
ASTs by a long chain of if-elif and isinstance and so on I will cry.
-- Devin
On Wed, Apr 2, 2014 at 7:53 AM, Lucas Malor <3kywjyd...@snkmail.com> wrote:
> Hi all. I would proposeto you all a switch-case syntax for Python. I already
> read PEP 3103 and I'm not completely satisfied by any of the proposed
> solutions. This is my proposal:
>
> switch_stmt ::= "switch" identifier "case" expression_list ":" suite
> ("case" expression_list ":" suite)*
> ["else" ":" suite]
>
> or, more simply:
>
>
>
> switch x case var1:
> ....
> case var2:
> ...
> case var3:
> ...
> else:
> ...
>
>
>
> Expression list should yield an iterable. The case suite will be executed if
> the variable of the identifier is a member of the iterable.
>
> For example, in a "switch x" statement, the code "case iterable: " is
> identical to "if x in iterable: " (or elif etc). So if you want to perform
> the same case block for more than one value, you have only to specify a
> tuple, a range etc.
> I would suggest to add an exception for non-iterable variables, so that you
> don't have to write "case var, : " but simply "case var: " and it will be
> identical to "if x == var: ". It's a bit tricky but the alternative is ugly.
>
> Fallthrough is disabled by default. The continue keyword cause to skip all
> the remaining current case suite. The next case will be checked. You can't
> use the continue keyword in the else clause.
>
> Some random remarks:
> 1. switch is on the same line of the first case. This will avoid any
> unpythonic syntaxes like:
> switch x
> case var1:
> ...
>
> or
>
> switch x:
> case var1:
> ...
>
> 2. Fallthrough is disabled by default because IMHO it's what non-programmers
> expect and that's what programmer usually needs more. I don't think a switch
> with such a syntax needs a break statement.
>
> 3. As an alternative, the continue statement could be written only at the end
> of a case suite; it will be less powerful and not consistent with the other
> compound statements, but it will improve readability.
>
> 4. I decided to not use already existing keyword like "if" or "in", since it
> will be misleading and problematic for syntax highlighters.
>
>
> Tell me what you think about.
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