RE: Thinking Aloud: Refactoring from nested loops to enumerators and lambda's

I tend to think of loops -- at least loops with blocks -- as a code smell.

So the first step for me is to make the body of each of those loops a function.  After that, if you are finding that there are too many parameters being passed around, then that might be an indication that you the design is not optimal (only _might_).  For example, perhaps you could use a helper class that carries some of those parameters as persistent state.

Then, too, if your loops have external side effects, that's another line of attack.  I kind of want each of my functions to have an external effect, or return a value, but definitely not both.

Of course
```
Result = []
func_x( stuff) do |a|
  result << func_y( a)
end
result
```

Is really `func_x(stuff).map(&:func_y)`, so there's that, too.


-----Original Message-----
From: ruby-talk [mailto:[email protected]] On Behalf Of John Carter
Sent: 01 July 2019 05:13
To: Ruby users
Subject: Thinking Aloud: Refactoring from nested loops to enumerators and lambda's

So I'm just thinking out aloud here.

Being an Old School Programmer I tend to naturally to write ever more
deeply nested loops.

I hate myself when I do this because it’s hard to test especially if
some of the loops have nasty external side effects, it’s hard to
reuse, it’s hard to refactor.

So I’m trying two new patterns....

* Passing enumerators as parameters so I can pull out inner loops as
standalone functions without calling the entire function every time..

* Creating functions that return lambdas, so I can pass an inner loop
in as a parameter.

Walk with me this is going to be long…. the example is a teaching
example / dojo exercise for myself, so excuse me it it slightly
contrived.

(In the following, a line of ===== indicates the next, slightly
different version of the code)

==========================================

Here is a typical chunk of my code…

def nested( a, b, c)

   stuff_a = func_a(a)
   stuff_b = func_b(b)
   stuff_c = func_c(c)
   result = {}
   func_1( stuff_a) do |a1|
      stuff_d = func_d( a1 + stuff_b)
      func_2( stuff_d) do |a2|
         stuff_e = func_e( a2 + stuff_c)

         func_3( stuff_e) do |a3|

            result[a3] = func_f( a3)
         end
      end
   end

   result
end

It’s fairly clear but has a few gotchas.

* It’s unclear which part of the code actually depends on which parameters.
* In this toy example, the function is small… but a Real Life nested
loop function like this can quickly grow hideously large.
* As a “premature optimization” I have factored out subexpressions
that do not alter within the loops, resulting in large scopes for
variables that are only used inside the loops.
* func_1(), func_2(), func_3() yield a stream of things….but a stream
of things should just be an enumerable!

==========================================

Ok, so try 2… reduce the scope of the stuff_* variables, a pessimation..

def nested( a, b, c)

   result = {}
   stuff_a = func_a(a)
   func_1( stuff_a) do |a1|
      stuff_b = func_b(b)
      stuff_d = func_d( a1 + stuff_b)
      func_2( stuff_d) do |a2|
         stuff_c = func_c(c)
         stuff_e = func_e( a2 + stuff_c)

         func_3( stuff_e) do |a3|

            result[a3] = func_f( a3)
         end
      end
   end

   result
end


==========================================

I can extract the inner loop as a function, but my parameter list balloons…

def inner_2( a2, c, result)
   stuff_c = func_c(c)
   stuff_e = func_e( a2 + stuff_c)

   func_3( stuff_e) do |a3|
      result[a3] = func_f( a3)
   end
end

def nested( a, b, c)

   result = {}
   stuff_a = func_a(a)
   func_1( stuff_a) do |a1|
      stuff_b = func_b(b)
      stuff_d = func_d( a1 + stuff_b)
      func_2( stuff_d) do |a2|
         inner_2( a2, c, result)
      end
   end

   result
end

and I still reevaluate func_c for every loop!

==========================================
If I use a closure instead, my parameter list collapses again…

def nested( a, b, c)

   result = {}
   stuff_a = func_a(a)
   func_1( stuff_a) do |a1|
      stuff_b = func_b(b)
      stuff_d = func_d( a1 + stuff_b)
      inner_2 = ->( a2){
         stuff_c = func_c(c)
         stuff_e = func_e( a2 + stuff_c)

         func_3( stuff_e) do |a3|
            result[a3] = func_f( a3)
         end
      }
      func_2( stuff_d) do |a2|
         inner_2.call( a2)
      end
   end

   result
end

==========================================
But I still have a pessimization, so if I could pass an enumerator
around…. So lets try convert func_2 to an enumerator….

def func_2( j)
   return to_enum( __method__, j) unless block_given?
   .....lots of code and a...
       yield  a2
   ...lots more code
end

def nested( a, b, c)

   result = {}
   stuff_a = func_a(a)
   func_1( stuff_a) do |a1|
      stuff_b = func_b(b)
      stuff_d = func_d( a1 + stuff_b)
      inner_2 = ->( a2){
         stuff_c = func_c(c)
         stuff_e = func_e( a2 + stuff_c)

         func_3( stuff_e) do |a3|
            result[a3] = func_f( a3)
         end
      }
      func_2( stuff_d).each do |a2|
         inner_2.call( a2)
      end
   end

   result
end

==========================================

Then pass the enumerator in, and then we can stop the silly
re-evaluation of func_c on every loop…

def nested( a, b, c)

   result = {}
   stuff_a = func_a(a)
   func_1( stuff_a) do |a1|
      stuff_b = func_b(b)
      stuff_d = func_d( a1 + stuff_b)
      inner_2 = ->( e){
         stuff_c = func_c(c)

         e.each do |a2|
            stuff_e = func_e( a2 + stuff_c)

            func_3( stuff_e) do |a3|
               result[a3] = func_f( a3)
            end
         end
      }

      inner_2.call( func_2( stuff_d))
   end

   result
end

==========================================

And I can keep going with func_1….

def nested( a, b, c)

   result = {}
   stuff_a = func_a(a)
   inner_1 = ->( e1, inner_2){
      stuff_b = func_b(b)
      e1.each do |a1|
         stuff_d = func_d( a1 + stuff_b)

         inner_2.call( func_2( stuff_d))
      end
   }

   inner_2 = ->( e2){
      stuff_c = func_c(c)

      e2.each do |a2|
         stuff_e = func_e( a2 + stuff_c)

         func_3( stuff_e) do |a3|
            result[a3] = func_f( a3)
         end
      end
   }


   inner_1.call( func_1( stuff_a), inner_2)


   result
end

==========================================

I can reduce scope of result and move it to the outermost level….

def nested( a, b, c)

   stuff_a = func_a(a)

   inner_1 = ->( e1, inner_2, &block){
      stuff_b = func_b(b)
      e1.each do |a1|
         stuff_d = func_d( a1 + stuff_b)

         inner_2.call( func_2( stuff_d), &block)
      end
   }

   inner_2 = ->( e2,&block){
      stuff_c = func_c(c)

      e2.each do |a2|
         stuff_e = func_e( a2 + stuff_c)

         func_3( stuff_e,&block)
      end
   }


   result = {}
   inner_1.call( func_1( stuff_a), inner_2) do |a3|
      result[a3] = func_f( a3)
   end
   result
end

==========================================

I can convert the inner_1 lambda to a vanilla method and convert that
to an Enumerator…

def inner_1( b, e1, inner_2, &block)
   return to_enum( __method__, b, e1, inner_2) unless block_given?
   stuff_b = func_b(b)
   e1.each do |a1|
      stuff_d = func_d( a1 + stuff_b)

      inner_2.call( func_2( stuff_d), &block)
   end
end

def nested( a, b, c)

   stuff_a = func_a(a)


   inner_2 = ->( e2,&block){
      stuff_c = func_c(c)

      e2.each do |a2|
         stuff_e = func_e( a2 + stuff_c)

         func_3( stuff_e,&block)
      end
   }


   result = {}
   inner_1( b, func_1( stuff_a), inner_2).each do |a3|
      result[a3] = func_f( a3)
   end
   result
end

==========================================

Since inner_1 is just a vanilla enum, I can use each_with_object

I can also extract inner_2 as a method that returns a lambda…

def inner_1( b, e1, inner_2, &block)
   return to_enum( __method__, b, e1, inner_2) unless block_given?
   stuff_b = func_b(b)
   e1.each do |a1|
      stuff_d = func_d( a1 + stuff_b)

      inner_2.call( func_2( stuff_d), &block)
   end
end

def inner_2( c)
   stuff_c = func_c(c)

   ->( e2,&block){

      e2.each do |a2|
         stuff_e = func_e( a2 + stuff_c)

         func_3( stuff_e,&block)
      end
   }
end

def nested( a, b, c)

   stuff_a = func_a(a)

   inner_1( b, func_1( stuff_a), inner_2( c)).each_with_object({}) do
|a3, result|
      result[a3] = func_f( a3)
   end
end

Note func_c is now only evaluated once again, It’s testable it, it’s
reusable in the vanilla “everything is just Enumerable” sense.

Comments, suggestions, nitpicks welcome....

As I said, I'm just thinking aloud trying to fix a common anti-pattern
in my own code.

--
John Carter
Tait Electronics
PO Box 1645 Christchurch
New Zealand

--
This Communication is Confidential. We only send and receive email on the

basis of the terms set out at www.taitradio.com/email_disclaimer
<http://www.taitradio.com/email_disclaimer>

Unsubscribe: <mailto:[email protected]?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-talk>


Click here to view Company Information and Confidentiality Notice.<http://www.jameshall.co.uk/index.php/small-print/email-disclaimer>

Please note that we have updated our privacy policy in line with new data protection regulations. Please refer to our website to view the ways in which we handle your data.

Unsubscribe: <mailto:[email protected]?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-talk>