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Refactoring from nested loops to enumerators and lambda's

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Refactoring from nested loops to enumerators and lambda's

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 a new pattern....

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.

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 the 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.

Discussion 2 comments · 3 points · JohnCarter · 2019-06-28
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