From: "M. Edward (Ed) Borasky" <znmeb@...>
Date: 2006-08-13T15:43:09+09:00
Subject: Re: A use case for an ordered hash

Hal Fulton wrote:
> There have been numerous occasions when I wanted an
> ordered hash, but usually I can't remember to write
> them down.
> 
> Here's just one.
> 
> Once I wanted a "dynamic case statement" of sorts.
> I wanted to use procs as values in a hash. Something
> like:
> 
>    actions = { /abcd/  => lambda { do_this },
>                /xyz/   => lambda { do_that },
>                /abc/   => lambda { other }}
> 
> Then I could just iterate through the keys looking
> for a match, then use that key to find the associated
> proc and call it.
> 
> However, I quickly noticed that this is no good. The
> order of iteration is unpredictable. So I couldn't
> guarantee that (for example) /abcd/ would be tested
> before /abc/, and so on.
> 
> So yeah, I ended up using an array of arrays. But it
> just felt wrong.
> 
> 
> Hal
> 
> 

That sort of code is what Chuck Moore evolved into Forth. :)  Seriously, 
though, isn't there a way to do this with a single array? Store the 
patterns followed by the lambdas in sequence. Find the first index that 
matches the pattern and take the lambda at [index+1]? Or two parallel 
arrays? I use two parallel arrays for this sort of thing a lot -- it's 
easy for me to read, given my FORTRAN background. :)