From: Robert Klemme <shortcutter@...>
Date: 2007-12-21T16:50:57+09:00
Subject: Re: Using 'when' as a statement modifier

2007/12/19, MonkeeSage <MonkeeSage@gmail.com>:
> On Dec 17, 10:31 pm, Tom Agnew <agne...@nospam.net> wrote:
> > Hello all,
> >
> > I have a suggestion for an enhancement that could add to the readability
> > of my programs.
> >
> > I'm a frequent user of 'if' and 'unless' statement modifiers.  It's a
> > beautiful abbreviation that enhances the readability of my code.
> >
> > Here is a pattern that I use from time to time:
> >
> >         result = func(x) unless func(x)==y
> >
> > This has the potential to invoke func(x) twice in some implementations.
> >
> > It occurred to me that the 'case' semantics could be borrowed to create
> > a useful new modifier.  Here's a suggestion that might be more efficient
> > and possibly more readable:
> >
> >         result = func(x) when result==y

Note, that this does not seem to be the same as above and can be replaced by

result = y if func(x) == y

This has the negative effect that it will change semantics of
assignment which is bad IMHO.  If I understand you properly then the
code above with "when" should do something like

__tmp_1 = func(x)
result = __tmp_1 if __tmp_1 == y

.i.e. assignment is done only after a successful test.

Personally I don't mind to write it exactly like that (with different
name for __tmp_1 probably).  Because it's totally obvious what happens
here.

You could even write a method to do that for you

def choose(val, test, fallback)
  val == test ? val : fallback
end
def except(val, test, fallback)
  val == test ? fallback : val
end
# ...
result = except func(x), y, result

Kind regards

robert

-- 
use.inject do |as, often| as.you_can - without end