# Writing a Y-Combinator in Ruby 1.9

**URL:** <https://www.ruby-forum.com/t/writing-a-y-combinator-in-ruby-1-9/169904>\
**Category:** Ruby\
**Created:** [June 10, 2009, 4:07am UTC](https://www.ruby-forum.com/t/writing-a-y-combinator-in-ruby-1-9/169904 "2009-06-10T04:07:23Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![night\_stalker](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/f0a364/32.png) [@night\_stalker](https://www.ruby-forum.com/u/night_stalker)\
**Post date:** [June 10, 2009, 4:07am UTC](https://www.ruby-forum.com/t/writing-a-y-combinator-in-ruby-1-9/169904/1 "2009-06-10T04:07:23Z")

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i’ve read  
[http://weblog.raganwald.com/2007/02/guest-blogger-tom-moertel-derives-y.html](http://weblog.raganwald.com/2007/02/guest-blogger-tom-moertel-derives-y.html)  
and finally figured out this:

* * *

# a trick to make currying easier

class Proc  
def -@  
self.curry  
end  
end

## def Y &f g = → h, x, \*xs { f[h[h], x, \*xs] } g[g] end

and had a try:

* * *

factorial = Y {|f, n| n == 0 ? 1 : f[n - 1] \* n}  
puts factorial[12]

# =\> 479001600

fibbonaci = Y {|f, n| n \<= 2 ? 1 : f[n - 1] + f[n - 2]}  
puts fibbonaci[10]

# =\> 55

* * *

the intresting part is:  
if you replace “x, \*xs”(like pattern matching in functional languages  
^\_^) with “\*args”, it overflows the stack.