The goal of Homework #2 is to implement the interpreter of MUWAE, which is the extended version of WAE that deals with multiple values.
1 Supporting Multi-Value
The first thing we should fix is that we’re still define a Num with a single numeric value and use it as the result of run.
One way to resolve this would be to add a new case class called Nums to our AST definition. But this would require reworking new cases for it in a few places. So instead, we will choose an easier solution: just change the existing Num so instead of holding a single integer it will hold a List[Int].
We need to fix the arithmetic operators. This is a bit tricky, since each of them receives two inputs that are both lists, and they should apply the operator on each pair from these two inputs, and collect a list of all of the results. So to make it easy, here is a skeleton of a utility function that will do this work. It is near-complete, and you have a tiny hole to fill:
// applies a binary numeric function on all combinations of integers from // the two input lists, and return the list of all of the results def binOp(
op: (Int, Int) = Int, ls: List[Int], rs: List[Int]
): List[Int] = ls match { case Nil = Nil case l :: rest =
def f(r: Int): Int = ??? // TODO: complete this function rs.map(f) ++ binOp(op, rest, rs) }
Don’t forget to add tests that demonstrate that this works: that using with to bind a name to a multi-valued expression works as expected. Here are some tests that should work once you’re done with this part:
test(run("{+ 3 7}"), List(10)) test(run("{- 10 {3 5}}"), List(7, 5)) test(run("{with {x {+ 5 5}} {+ x x}}"), List(20))
2 Adding More Arithmetic Operators
Next, add two arithmetic operators, min and max that take three expressions and return the minimum and the maximum list of integers, respectively, to MUWAE:
test(run("{min 3 4 5}"), List(3)) test(run("{max {+ 1 2} 4 5}"), List(5)) test(run("{min {1 4} {2 9} 3}"), List(1, 1, 2, 3))
1
test(run("{max {1 6} {2 5} {3 4}}"), List(3, 4, 5, 5, 6, 6, 6, 6))
