Lab: Subversive Scala

Overview

In this lab, you'll face three puzzles. Each puzzle asks you to implement a new control-flow structure for Scala in Scala.

Implementing a control-flow structure is a lot like implementing a data structure: Just as you implement a new data structure by using pre-existing data structures, you implement a new control-flow structure by using pre- existing control-flow structures.

A more detailed description is below. Here are a few notes about the control- flow structures:

• When solving a puzzle, reverse-engineer it from the interface (i.e., what the user types). Looking at this interface, what kinds of Scala structures (e.g., functions and methods) could be used in that way?
• Some of the puzzles may be tricky at first. Keep at it, and talk to other people about your ideas. Above all, resist the temptation to look for an answer online or in a book. The main benefit of this lab is to start developing the thinking process that you'll use to implement internal DSLs.
• The control-flow structures that you're asked to implement are very un-Scala-y! Their behavior essentially relies on mutable variables, which we try to avoid in traditional Scala. But, of course, this is not traditional Scala. This is subversive Scala :).

Here are a few notes about the files for this lab:

• You should follow the standard assignment workflow for the lab, so that you can issue a pull request to show off your work. (You'll get participation points for your pull request.)
• The files are set up for you to run sbt eclipse, so you can work in ScalaIDE (though you don't have to do it that way).
• Initially, the files won't compile because you haven't implemented the control-flow structures. You may want to comment out some of the code until you have a chance to implement your control-flow structure.

Part 1: loop_until

Implement a control-flow structure called loop_until that can be used like so:

var i = 0 
loop_until (i > 9) {
  if ( (i % 2) == 0 ) 
    println(i)
  i += 1
}

The loop_until control structure first checks its condition. If the condition is false, it executes the body. It then checks the condition again. If the condition is still false, it again executes the body and continues in this fashion until the condition becomes true.

In the case of the example above, the body of the loop_until structure should execute 10 times and print the numbers 0, 2, 4, 6, 8 (with each number appearing on a separate line).

Put your implementation in the file /src/main/scala/internal/LoopUntil.scala.

Part 2: for_loop

Implement a control-flow structure called for_loop that can be used like so:

var i = 0;
for_loop(i = 0)(i < 10)(i += 2) {
  println(i);
}

The for_loop control structure first performs the initialization (i=0). Then, it checks its condition (i < 10). If the condition is true, it executes the body. Then it perfoms the increment (i += 2). It then checks the condition again. If the condition is still true, it again executes the body and continues in this fashion until the condition becomes false.

In the case of the example above, the body of the for_loop structure should execute 10 times and print the numbers 0, 2, 4, 6, 8 (with each number appearing on a separate line).

Note that, unfortunately, we don't have a good way to include the variable definition as part of the for loop. The user has to declare the variable outside the for loop, then (re-)initialize it as part of the for loop.

Put your implementation in the file /src/main/scala/internal/ForLoop.scala.

Part 3: while_c and continue

Implement control-flow structures called while_c and continue that can be used like so:

var i = -1
while_c (i < 9) {
   i += 1
   if ( (i % 2) != 0 )
      continue
   println(i)
} 

The while_c control structure first checks its condition. If the condition is true, it executes the body. It then checks the condition again. If the condition is still true, it again executes the body and continues in this fashion until the condition becomes false.

The continue control structure, when it appears inside a while_c, causes the program to jump directly to the top of that loop, where it will check the loop condition again and continue on like normal.

If continue appears outside a while_c loop, its behavior is undefined. So, in your implementation, you don't need to check whether continue appears inside while_c. You only need to make the program behave properly in the case when a while_c contains a continue.

In the example above, the body of the while_c structure should execute 10 times and print the numbers 0, 2, 4, 6, 8 (with each number appearing on a separate line).

Put your implementation in the file /src/main/scala/internal/WhileContinue.scala.

Part 4: repeat … until

Implement a control-flow structure called repeat … until that can be used like so:

var i = 0
repeat {
   if ( (i % 2) == 0 )
      println(i)
   i += 1
} until (i > 9)

The repeat … until control structure first executes its body. Then it checks its condition. If the condition is false, it executes the body again. It then checks the condition again and continues in this fashion until the condition becomes true.

In the case of the example above, the body of the repeat … until structure should execute 10 times and print the numbers 0, 2, 4, 6, 8 (with each number appearing on a separate line).

Put your implementation in the file /src/main/scala/internal/RepeatUntil.scala.

Bonus: Summations

If you finish the first three parts of the lab, consider taking on this challenge: Design and implement a notation for mathematical summations that is as expressive as those used in math and is as close as possible to the notation that a mathematician would use.

Bonus: ScalaTest

Take a look at the ScalaTest guide, which describes a dizzying array of options for specifying tests. See if you can reverse-engineer their implementation: how are the implementers of ScalaTest providing all these options? Check whether your intuition is correct by browsing the ScalaTest documentation or source code.

Note: ScalaTest is a big library that provides many forms of syntactic sugar for what is essentially the same behavior. As such, there's more software engineering in ScalaTest that there is in your implementation of control-flow structures!

Bonus: Baysick

Try your hand at implementing the syntax for Baysick, an internal DSL that mimics the Basic programming language. (Don't look at the implementation online; reverse engineer it from the examples!)