PEP308 Late Entry

Clark C. Evans cce at clarkevans.com
Sun Mar 9 21:11:40 CET 2003


On Sat, Mar 08, 2003 at 10:35:00AM +0000, Stephen Horne wrote:
| 
|   select        : (c1       : r1; c2       : r2; r3)
|   select s      : (v1       : r1; v2       : r2; r3)
|   select sa, sb : (v1a, v1b : r1; v2a, v2b : r2; r3)
| 
| 's' stands for 'switch expression'.
| 'c' for condition.
| 'r' for result.
| 'v' for switch-value.

Ok. But I think 'when' adds readability, and you don't need
a separate case for your third example.

  a = select         when c1        : r1; when c2        : r2; else r3
  b = select s       when v1        : r1; when v2        : r2; else r3
  c = select (sa,sb) when (v1a, v1b): r1; when (v2a, v2b): r2; else r3

Or, written in multi-line form..

  a = select
        when c1: r1
        when c2: r2
        else: r3

  b = select s
        when v1: r1
        when v2: r2
        else: r3

  c = select (sa,sb)
        when (v1a, v1b): r2 

| Switch values could even support special syntaxes such as relative
| operator prefixes ('<', '>' etc) or perhaps a '..' or 'to' notation
| for ranges - though the non-switch first form might be clearer.

I'd call it 'select' and not 'switch'; the 'switch' expression
in "C" is a statement-level operation, while this proposal seems
still to be an expression.

And yes, I like the operator prefixes:

  select s
    when > 100: 'huge'
    when > 50 : 'big'
    when > 30 : 'medium'
    when > 20 : 'respectable'
    when > 10 : 'small'
    else: 'tiny'

   select x:
     when 'RHS':
         select y
            when 'UP': 'Upper Right'
            when 'DOWN': 'Lower Right'
            else: 'Center Right'
     when 'LHS':
         select y
            when 'UP': 'Upper Left'
            when 'DOWN': 'Lower Left'
            else: 'Center Left'
     else:
         select y
            when 'UP': 'Upper Middle'
            when 'DOWN': 'Lower Middle'
            else: 'Center Middle'

| 1.  It handles 'elif' simply and succinctly.
| 
| 2.  It supports the switch/case functionality as a simple extension,
|     but unlike the earlier proposal does not have to be split over
|     multiple lines unless clarity genuinely demands it.

I don't mind being able to add a ';' to make it a one-line
construct if it is necessary.


| 3.  The use of punctuation keeps it succinct while the initial keyword
|     makes it clear what is happening (easy to look up in documentation
|     etc).

I think the parenthesis is ugly.

| 
| 4.  The initial ':' and the use of ':' and ';' in the parentheses
|     makes it clear that this is not a function, for those who see
|     function-call notation as implying no shortcircuiting.

Yes, the ; is nice.

Clark





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