From: David Alan Black <dblack@...>
Date: 2002-06-22T17:29:56+09:00
Subject: Re: What do you think of this idiom?

Hi --

On Sat, 22 Jun 2002, Hal E. Fulton wrote:

> I was writing a little script which prompted
> the user for a directory. I wanted to repeat
> until a valid directory was entered.
>
> At first I was going to write an actual loop.
> Then I thought of this little technique. What
> do you think? How would you do it?
>
> Cheers,
> Hal Fulton
>
>
> begin
>   print "Dir = "
>   mydir = gets.chomp
>   dir = Root + mydir
>   raise "No such directory" if not File.stat(dir).directory?
> rescue => err
>   puts err
>   retry
> end

I think it isn't doing what you think :-)

  $ ruby getdir.rb  # with "Root = './'" added
  Dir = non-existent
  No such file or directory - "./non-existent"

Note that it's not using your error message, because the "if not"
branch never happens: the stat on a bad directory name raises an
exception of its own.  You could use test(?d,dir) to get a true/false
test.

Anyway, I like the way "retry" fits in here, and this seems like a
good way to avoid the annoying pattern of repeating a test, which I
always seem to fall into:

  until some_test(str)
    print "Input: "
    str = gets.chomp
    puts "Invalid" unless some_test(str)
  end

Ultimately, though, it would be nice to wrap all this stuff up in a
method.  I was playing around with that a little, using yours as a
starting point.  Nothing profound here, just curious how the wrapping
might go.

  def getstr(prompt,error="Invalid input")
    str = ""     # not strictly needed, but clearer
    begin
      print prompt
      str = gets.chomp
      raise error unless yield(str)
    rescue => err
      puts err
      retry
    end
    str
  end

  s = getstr("Dir = ", "No such directory") {|dir| test(?d,dir)}


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav