How to create a custom while block

Question

I know that in ruby we can use a while loop, but I want to know if I can create a custom one so I can make something like this:

custom_while i < 5 do
    puts i
    i += 1
end

I currently have this code:

def custom_while(condition)
    loop do
        break if not condition
        yield
    end
end

i = 0
custom_while i < 5 do
    puts i
    i += 1
end

However, when condition is evaluated, it always get true (because it considers the first evaluation of i < 5 = true only.

Any help will be appreciated!

Note: This is for educational purposes only.

Answer 1

You almost had it. So, your problem is that the condition is only evaluated once? Well, what construct do we know that we can evaluate as often as we want? That's right: functions! So, let's make condition a function (or a Proc in Ruby lingo):

def custom_while(condition)
  loop do
    break unless condition.()
    yield
  end
end

i = 0
custom_while -> { i < 5 } do
  puts i
  i += 1
end
# 0
# 1
# 2
# 3
# 4

This is unfortunately not as nice looking as in other languages. Ruby's syntax and semantics are aggressively optimized for methods that take exactly 1 "function" as an argument. Ruby has a special syntactically and semantically light-weight construct for that, namely blocks. As soon as you have more than one, though, you're out of luck.

Compare this with languages that have proper block literals, like Smalltalk, for example. In Smalltalk, you could write a method while:do:, and call it like this:

i := 0.
while: [i < 5] do: [Transcript write: i. i := i + 1].

In fact, in Smalltalk, the syntax for blocks is so lightweight that there are no control structures at all in the language. if/then/else is simply an instance method of Boolean, for example:

i % 2 == 0 ifTrue: [Transcript write: "even"] ifFalse: [Transcript write: "odd"].

And while is actually an instance method of Block, so in reality, your example would look like this:

i := 0.
[i < 5] whileTrue: [Transcript write: i. i := i + 1]

---

Note: I make no guarantees for the Smalltalk code, I didn't test it.

Answer 2

The problem is, the condition is being evaluated before it's passed in, so it will never change.

Make the condition a function that you evaluate inside the loop, or use a macro to make it cleaner.