Re: Switch/Case statements revisited

["Jeremy Darling" <jeremy.darling@...>]

I've worked in pascal for many years, and I have to say that the two
MAJOR complaints I've had about the language stem directly from the
implementation details of the case statement.  I like the fall through
abilities of C and wish that they existed in pascal without having to
hack them into place.  The other announce for me is the lack of a
proper case statement for strings.

Granted, with the way Lua is implemented, the 2nd is a mute point its
still valid.

The return idiom is a good one as long as it could easily allow for
the fall through statements to work.  Compiled down it seems that you
would basically have the switch at the top that would drop into the
proper execution code below.  If a code block didn't have a return
then it would simply keep executing till the end, if it did then it
would work like any other code block and return to the caller.

Just my two cents worth,
 - Jeremy

On Nov 7, 2007 2:37 AM, steve donovan <steve.j.donovan@gmail.com> wrote:
> Hi guys,
>
> This is obviously a perenial topic, since there's a wiki page dedicated to it:
>
> http://lua-users.org/wiki/SwitchStatement
>
> My feeling is that switch is the wrong model; we should look at
> Pascal's case statement as more appropriate inspiration. Here are some
> possible forms:
>
>     case (k)
>       is 10,11: return 1
>       is 12: return 2
>       is 13 .. 16: return 3
>       else return 4
>     endcase
>     ......
>     case(s)
>         matches '^hell': return 5
>         matches '(%d+)%s+(%d+)',result:
>             return tonumber(result[1])+tonumber(result[2])
>         else return 0
>     endcase
>
> You can provide a number of values after is, and even provide a range
> of values. matches is string-specific, and can take an extra parameter
> which is filled with the resulting captures. This is implementable
> using token filter macros so people can get a feeling for its use in
> practice.
>
> (See http://lua-users.org/wiki/LuaMacro, which has been
> semi-officially released  - the downloadable source zip contains an
> example implementation. Unfortunately, there is a gotcha; Lua
> complains of a malformed number if there is no whitespace around
> '...'. Also 'result' has to be global. These are implementation
> irritations, not relevant to a design discussion, I think),
>
> This case statement is a little bit of syntactical sugar over a chain
> of elseif statements, so its efficiency is the same.  What the
> compiler actually sees in the first case is:
>
>      do local __case = k; if false then
>      elseif __case == 10 or __case == 11 then return 1
>      ...
>     end; end
>
> But already I've thought of some other possibilities. This feels a
> little more elegant.
>
> case (k)
>   is 10 or 11: return 1
>   is 12: return 2
>   is between(13,16): return 3
>   else return 4
> endcase
>
> What do you think?
>
> steve d.
>



-- 
Jeremy

"Help I suffer from the oxymoron Corporate Security."