Simple Currying in Ruby

Question

I'm trying to do some currying in ruby:

def add(a,b)
  return a+b
end

plus = lambda {add}
curry_plus = plus.curry
plus_two = curry_plus[2] #Line 24
puts plus_two[3]

I get the error

func_test.rb:24:in `[]': wrong number of arguments (1 for 0) (ArgumentError)

from func_test.rb:24:in `'

But if I do

plus = lambda {|a,b| a+ b}

It seems to work. But by printing plus after the assigning with lambda both ways return the same type of object. What have I misunderstood?

Answer 1

lambda {|a,b| a+ b}

Creates a lambda which takes two arguments and returns the result of calling + on the first, with the second as its arguments.

lambda {add}

Creates a lambda which takes no arguments and calls add without arguments, which is an error of course.

To do what you want, you should do

plus = lambda {|x,y| add(x,y)}

or

plus = method(:add).to_proc

Answer 2

You're on the right track:

add = ->(a, b) { a + b }
plus_two = add.curry[2]
plus_two[4]
#> 6
plus_two[5]
#> 7

As others have pointed out, the plus lambda you defined doesn't take any arguments and calls the add method with no arguments.

Answer 3

When you write lambda {add}, you're declaring a Proc that takes no arguments and, as its sole action, calls add with no arguments. It doesn't turn add into a Proc. On the other hand, lambda {|a,b| a + b} returns a Proc that takes two arguments and adds them together — since it takes arguments, it's valid to pass arguments to that one.

I think what you want is method(:add).to_proc.curry.