the idea of this project

• the expression problem
• learn how to solve expression problem by writing dsl with
  • initial embedding
  • tagged initial embedding
  • tagless initial embedding
• the final aim is basicly to practic final tagless approach

project structure

I split this project into some parts:

• expressionproblem
  • creating algebra
  • add addition operation
  • add expression itself
  • evaluate expression with pattern matching and recursion
  • print expression with pattern matching and recursion
• expressionproblemtagged
  • start thinking about approach
  • trying to add new operation multiply
  • found that adding new operation lead to rewriting function with main logic
  • I mean that after adding new operation I should change my function with pattern matching all the time
  • so I refactor a bit logic
  • I did it in a type classes style
    • dsl (type class)
    • interpreters (type class instances)
    • program (type class interfaces)
• expressionproblemtagged2
  • go further in refactoring
  • each operation move to separate trait
  • so now we have separate type class and interpreter for each operation
  • in this case adding new operation will not lead to changing existing code
  • so no changing code only adding new one
• expressionproblemtagged3
  • in this step I added division
  • and I found that this operation may return error
  • so I should change return type from A for something like Option[A]
  • but by adding return type to Option[A] I found that
    • my solution now is not composable and decomposable
    • so I can not do something like

    •    add(lit(23),lit(23))

    • I cant change only return type of division
    • to make it composable I should change all types (param and return) in all operations

    • trait Addition[A] {
      def add(e1:Option[A], e2:Option[A]):Option[A]
      }

    • so good so far but what if I will decide to change type from option to either ???
• taglessfinal
  • in this step I've added higher kinded type F[_]
  • now my solution become more general
  • now I can change not only element type but container type

I started solving expression problem in first expressionproblem directory and I tried to make solution better adding new feature and refactoring code