Proc#=== + Symbol#to_proc = A Near Miss

Published 30 October 2008 by Farrel

Here’s a an application of combining my Proc#=== suggestions and Rails’ Symbol#to_proc, both now part of the core library in Ruby 1.9, that I thought of this morning.

1 case number
2 when :even?.to_proc
3   puts "Even!"
4 when :odd?.to_proc
5   puts "Odd!"
6 when :zero?.to_proc
7   puts "Zero!"
8 end

which is equivalent to writing

1 if number.even?
2   puts "Even!"
3 elsif number.odd?
4   puts "Odd!"
5 elsif number.zero?
6   puts "Zero!"
7 end

Although I will admit the if/elsif example is more readable, mainly because of the presence of all those (unecessary) to_procs. Ideally I would liked to have been able to write the when clauses like

1 when :even?
2   puts "Even!"

and so on without those to_procs all over the place.

I initially thought about defining Symbol#=== as

1 class Symbol
2   def ===( *values )
3     self.to_proc.call( *values )
4   end
5 end

but it has implications if we’re using === to compare two Symbols, as there is no way to differentiate if we want to merely compare them using == (as the behaviour is now and which is perfectly logical and correct) or if we want to use the new Symbol#=== functionality defined above. So I’ll admit this is a near miss in terms of Ruby dynamic language coolness.

Farrel Lifson is a lead developer at Aimred.