Ruby blocks made easy, part II, curry and procs as arguments
Understanding the fundamentals of Ruby blocks
3 min read
In the previous post, we learned that methods can be transformed into procs to be evaluated later.
One thing worth to mention is that, the method itself can be used like a proc: every
Method structure has a method called
Time.now # => 2021-04-10 17:22:05 Time.method(:now).call # => 2021-04-10 17:22:06 Time.method(:now).to_proc.call # => 2021-04-10 17:22:07 # checking the classes Time.method(:now).class # => Method Time.method(:now).to_proc.class # => Proc (lambda)
Given that, the
method structure is good enough to be used in later evaluations. In theory, it is NOT a Proc. But in practice, it behaves like proc lambdas and will evaluate the expression later.
We can also evaluate methods later with arguments:
def multiply(a, b) a + b end method(:multiply).call(2, 4) # => 8
Proc with arguments (curry)
Sometimes it's useful to define a proc with some static arguments, which can be evaluated later along with the dynamic arguments. These arguments can be "curried" to the proc using the method
# creates a proc with no curried arguments # and calls using two dynamic arguments method(:multiply).call(2, 4) # => 8 # creates a proc with the first argument curried # and calls using one remaining dynamic argument method(:multiply).curry.call(4) # => 8 method(:multiply).curry.call(5) # => 15 # checking the class method(:multiply).class # => Proc method(:multiply).curry.class # => It's also a Proc
This feature is powerful because it allows us to write more expressive code:
multiply_by_2 = method(:multiply).curry multiply_by_3 = method(:multiply).curry multiply_by_2.call(4) # => 8 multiply_by_3.call(6) # => 18
Passing a lambda method as argument to another method
Lambda methods can be passed as arguments to methods like any other valid expression.
Let's suppose we want a method which takes a list of numbers and applies some calculation method to each number, returning a new calculated list:
def map_numbers(numbers, calculation) new_list =  for number in numbers # `calculation` is a proc, but we don't care about # its logic: we simply evaluate whatever the # calculation is to the number new_list << calculation.call(number) end new_list end
Now, we want to use the method
map_numbers to take a list of numbers and return a new list with each number multiplied by 2:
# declaring the list numbers = [1, 2, 3] # declaring the "calculation" proc, using the "2" curried # as we've seen in the previous example multiply_by_2 = method(:multiply).curry map_numbers(numbers, multiply_by_2) # => [2, 4, 6]
We can do even better, in a single line, applying other calculations as well:
# multiplying by 2 map_numbers([1, 2, 3], method(:multiply).curry) # multiplying by 4 map_numbers([1, 2, 3], method(:multiply).curry) # multiplying by 42 map_numbers([1, 2, 3], method(:multiply).curry)
We could go beyond, supposing we'd have more calculation methods:
map_numbers([1, 2, 5], method(:sum_by).curry) map_numbers([2, 4, 9], method(:square_of).curry)
We learned that procs can be passed as arguments to another methods and, optionally, can use curried arguments, making our code appear more declarative.
In the next and last post of this series, we will unblock more fundamentals and introduce blocks.