Passing Functions and Lambdas into Functions with Ruby

Ruby's New Style of Lambda Functions

f = ->( m ) { p m }
f.call( 1 )
#=> 1

Which of course means the same thing as:

f = lambda { |n| p n }
f.call(1)
#=> 1

Ruby Proc Objects

p = Proc.new { |n| p n + 2 }
p.call(2)
#=> 4

Using a Function as a Closure in Ruby

def domo( k )
  ->(m) { p m + k }
end

z = domo( 5 )
z.call( 5 )
#=> 10

Function :domo takes a single parameter. Within :domo, we create a lambda that takes a single parameter, and adds that parameter to the value :domo takes in as its parameter. Then, we assign z to be the result of the lambda in :domo with its 'k' parameter loaded with 5.

When z is called, we pass (another) 5 to it. This parameter loads the lambdas n parameter. The lambda executes, essentially adding n(5) + k(5) and yielding the result of 10.

The thing about closures such as this is, we can load the initial value of the lambda to be whatever we want it to be when assigning the function :domo's return value to z. That becomes a fixed value that z will always have. But..., we can call z with whatever value we wish to add z to. For example:

z = domo(6)
z.call(4)
#=> 10
z.call(6)
#=> 12

etc., etc. That's the power of the closure. You could set up a number of such closures with constant values set up, and then operate on those values with whatever numbers you wish to pass in. Each closure represents its own individual processing with its own loaded value.

Passing Functions to Functions in Ruby

def fritter( f, n )
  func = method( f )
  func.call( :n => n )
end

def mac( opts = {} )
  v = opts[ :n ]
  v + 6
end

p fritter( :mac, 6 )
# => 12

Passing one function to another function as a parameter is how Ruby can be used in a functional programming style. The function :fritter takes the function :mac, as well as a number, as parameters.

Inside :fritter, on the first line, the symbol passed in to parameter f is converted to a method object via the :method function. This method object is stored in the variable, func.
Next, on the second line of :fritter, we call the method object, which is the method, :mac, sending it the parameter 6
To pass a function to another function in Ruby, the name of the function is passed. To name a function, a symbol must be used. It is the symbol that represents the functions name. So, the name of the mac function is :mac.

Now, consider the function, :mac. :mac takes a hash as a parameter. Inside the function, the local variable, v is assigned either the value attached to the key :n, or 1 if :n has no value. :mac then returns the sum of its local variable v, and the number 6.

Finally, the function call of :fritter, with the parameters of the :mac function, and the numeral 6 ( note that this 6 id different from the 6 within the body of :mac. So, :fritter is called, and it returns the return value of :mac called itself with a parameter of 6!

Somewhat convoluted? Admittedly, this example is. But its the idea that is important. You can change the functions to solve whatever problem you're working on. Here I wanted to show just the mechanics of passing a function a function.

Passing Lambdas to a Function in Ruby

@even = ->( n ) { n % 2 == 0 }

def fun( e, n )
  e.call( n )
end

def negate( e, n )
  !e.call( n )
end

The lambda here, @even, simply takes an input parameter of n, and tests to see if it is even.
The function, :fun, simply calls the @even lambda with the numeral parameter, n that is passed in.

The function, :negate, literally does the opposite of :fun. It does this in a very simple way: it literally
negates the @even function called with the numerical parameter, n.

fun( @even, 2 )
#=> true
fun( @even, 1 )
#=> false
negate( @even, 2 )
#=> false
negate( @even, 1 )
#=> true

Both of the functions are called, each with an even value, and an odd value. They return the expected boolean results.