From: Florian Gross <flgr@...>
Date: 2004-09-24T05:54:45+09:00
Subject: Re: Newbie: Case statement

Markus wrote:

>>>def greedy_case(obj, cases)
>>>   cases.find_all do |condition, action|
>>>     condition === obj
>>>   end.inject(Hash.new) do |hash, (condition, action)|
>>>     hash[condition] = action.call; hash
>>>   end
>>>end
>>OK - Does this do the following (roughly)
>>- passes the input variable (x) and a hash of ranges and pointers to 
>>functions (C speak)
>      More or less, though there is no requirement that the conditions be
> ranges; the could be values, classes, regexp's, etc. as needed.

Also the 'pointer' part is a bit misleading. It's just a normal object.

In Ruby one can create lambdas (kinda like blocks turned into Objects) 
and .call them later one. They remember their creation context just like 
blocks do. Here's a sample of a higher-order function (a function 
returning another function) -- note that the argument of the outer 
lambda, 'a', will still be available when the returned lambda is called.

times_fun = lambda do |a|
   lambda { |b| a * b }
end

two_times_fun = times_fun.call(2)
two_times_fun.call(4) # => 8

And greedy_case() doesn't even really need a lambda. Anything that has a 
#call method will work. Here's a bad sample:

result = greedy_case(obj,
   String  => obj.method(:reverse),
   Numeric => obj.method(:to_s)).values.first

>>- No idea what end.inject means, but the effect is to iterate through 
>>the hash of "ranges" (or presumably eny condition), and for each 
>>match (determined by === must look this up) execute the inline 
>>function (block). 
>>- What is the last hash for?
>  [Detailed explanation]

I couldn't have explained this better. Thank you. :)

Regards,
Florian Gross