The 'then' Ruby Keyword - What is it?
Have you heard of the then method? I’d never heard of it, till now:
then allows you to curry:
# this is a contrived example
# experiment with it in an IRB console.
# we want to:
# take a string
# add convert it to an int
# add 1 to that
# and then cube it
# returning the result
def operation_1(a)
a_int = (a.to_i)
sum = a_int + 1
cube = sum ** 3
return cube
end
# is it pleasant to read?
puts "operation_1 is: #{operation_1("3")}"
# => operation_1 is: 64
# notice how the input of one function
# is an input into another?
# we can write the same thing like this:
The code works, but it ain’t very readable. Let’s try another approach:
def operation_2_dense(a)
(a.to_i + 1) ** 3
end
puts "operation_2_dense is: #{operation_2_dense("3")}"
# => operation_1 is: 64
# but is it readable?
IMO it’s terser, but there’s still some mental overhead required to read and understand it. Let’s try currying:
# you can also curry it like so:
def curry(a)
a.then { |i| i.to_i }
.then { |i| i + 1 }
.then { |i| i ** 3 }
end
puts "curry is #{curry("3")}"
# => curry is 64
That works! Let’s improve upon it:
def add_one(i)
i + 1
end
def cube(i)
i ** 3
end
def curry_2(a)
a.then { |i| i.to_i }
.then { |i| add_one(i) }
.then { |i| cube(i) }
end
puts "curry_2 is #{curry_2("3")}"
# => curry_2 is 64
# if you want something more readable.
# and if you like, you could put add_one and cube
# into a mixin. The implementation details are hidden
# and you can cleanly / elegantly express
# what you want.
Which do you prefer?
Extracted in the Wild:
The best way of learning is by implementing it in some code base you are reading - preferably your own (at least that’s how I learn and experiment). Otherwise what tends to happen: you’ll read, and then completely forget about it. This is necessarily a problem looking for a solution, but that’s the only way you can advance by adding another tool in your ruby repertoire. At the very least, consider undertaking a similar exercise yourself:
I peaked into 37 Signal’s Writebook offering, from their Once suite of products.
### first_run.rb
User.create!(user_params.merge(role: :administrator))
.tap do |user|
DemoContent.create_manual(user)
end
They’re using tap. Can improve upon the above, by using then? What would you do?
# my answer
User.create_admin(user_params).then do |user|
DemoContent.create_manual(user)
end
Or take the pagy codebase:
def next_cursor
hash = keyset_attributes_from(@records.last)
json = @vars[:jsonify_keyset_attributes]&.(hash) || hash.to_json
B64.urlsafe_encode(json)
end
# or would you prefer it like this?
def next_cursor
keyset_attributes_from(@records.last)
.then { |attributes| @vars[:jsonify_keyset_attributes]&.(attributes) || attributes.to_json }
.then { |json| B64.urlsafe_encode(json) }
end
It all depends on what style you prefer.
yield_self
then seems identical to the yield_self method.
Source Code for Experimentation
Here is the full ruby file if you’re interested.
Updates
I fixed some errors, added some details.