# R – how to implement curry(partial function) in ruby

curryingruby

I need some examples of implementing curry function in ruby(1.8.6 or 1.8.7 not 1.9).

#### Best Solution

So here's how to do currying with blocks, rather than methods:

``````def curry(&block)
arity = (block.arity >= 0) ? block.arity : -(block.arity + 1)
# return an immediate value if the block has one
return block[] if arity == 0

# otherwise, curry it argument by argument
args = []
innermost = lambda do |last,*extra|
args[arity-1] = last
block[*(args+extra)]
end
(0...(arity-1)).to_a.reverse.inject(innermost) do |inner,i|
lambda do |arg_i,*extra|
args[i] = arg_i
# pass extra arguments on to inner calls
if extra.empty?
inner
else
inner[*extra]
end
end
end
end
``````

And it works fairly well in practice. Arguments can be curried or not, and extra arguments are collected as usual:

``````irb> (curry { |x,y| x + y })[1,2]
#=> 3
irb> (curry { |x,y| x + y })
#=> 3
irb> (curry { |x,*ys| ys << x })
#=> 
irb> (curry { |x,*ys| ys << x })[1,2,3]
#=> [2, 3, 1]
irb> (curry { |x,y,*zs| zs << (x+y) })[1,2]
#=> 
irb> (curry { |x,y,*zs| zs << (x+y) })[1,2,4]
#=> [4, 3]
irb> (curry { |x,y,*zs| zs << (x+y) })
#=> 
irb> (curry { |x,y,*zs| zs << (x+y) })[2,4]
#=> [4, 3]
irb> (curry { |a,b,c,d,e| a+b+c+d+e })[1,2,3,4,5]
#=> 15
irb> (curry { |a,b,c,d,e| a+b+c+d+e })
#=> 15
irb> (curry { |a,b,c,d,e| a+b+c+d+e })[1,2]
#=> 15
irb> (curry { |a,b,c,d,e| a+b+c+d+e })[2,3,4]
#=> 15
``````

I made the design decision to have no-arg blocks return an immediate value on currying:

``````irb> curry { 3 }
#=> 3
irb> curry { |*xs| xs }
#=> []
``````

This is necessary to avoid having to end currying with a `[]` every time (and is fairly Haskell-like).