Something like let in Ruby

Question

I used to write let-like expressions -- with lexical scope.

So I write my own (sad, but it will fail with multiple threads):

# Useful thing for replacing         


        

Answer 1

An idea:

class Object
  def let(namespace, &block)
    namespace_struct = Struct.new(*namespace.keys).new(*namespace.values)
    namespace_struct.instance_eval(&block)
  end
end

message = let(language: "Lisp", year: "1958", creator: "John McCarthy") do
  "#{language} was created by #{creator} in #{year}"
end

Single-value scopping is more explicit because you name the variable(s) in the block arguments. This abstraction has been called as, pipe, into, scope, let, peg, ..., you name it, it's all the same:

class Object
  def as
    yield self
  end
end

sum = ["1", "2"].map(&:to_i).as { |x, y| x + y } #=> 3

Answer 2

You can't specify the value that you want to initialize, but you can declare a variable as explicitly local to that block:

x = 'external value'
puts x
[1,2,3].each do |i; x|
  x = i
  puts x
end
puts x

This will result in:

external value
1
2
3
external value