PEP308: Proposing function-call syntax

Bengt Richter bokr at oz.net
Tue Feb 11 05:41:22 CET 2003


On Mon, 10 Feb 2003 10:44:41 +0100, "Anders J. Munch" <andersjm at dancontrol.dk> wrote:

>Yet-another PEP 308 proposal: 
>    ifelse(condition, iftrue, iffalse)
>
>That is, use plain old function call notation, but with 'ifelse' being
>a keyword recognised by the interpreter, so that arguments are only
>evaluated as necessary.
>
>Example:
>  messsage = "Access " + ifelse(key=correct_key, "granted", "denied")
I'd like to counter with my latest ;-)
   messsage = "Access " + (key==correct_key -> "granted" -> "denied")

>
>Pro: 
+ >Syntax is straightforward yet terse.
+ >Order of evaluation left-to-right.
+ >There's no confusing it with a statement.
? >'ifelse' is easy for newbies to search for in documentation.
>
>Con:
+ not: >Looks like a function call but isn't.  People might expect arguments
>to be fully evaluated.
+ not:>Adds new keyword.
>
>A possible variation: Call it 'cond' instead and allow for multiple
>conditions.
>    cond(cond1, expr1, ..., condN, exprN, exprOtherwise)
Got that too:
     cond1 -> expr1 >< condN -> exprN >< True -> exprOtherwise

and final ">< True" is optional, so
     cond1 -> expr1 >< condN -> exprN -> exprOtherwise

and the basic ternary becomes
     cond1 -> expr1 -> exprOtherwise


IOW, the syntax is

    multiary ::= cond "->" expr ("><" cond "->" expr)* [ "->" expr]

where cond is an expr and its bool(expr) value is used to decide whether
to evaluate the associated expr after the "->". If "><" appears next, another
cond is evaluated, but if another "->" appears instead, the cond is treated
as True and the immediately following expr is evaluated.

Regards,
Bengt Richter




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