Folding and OUnit

Roughly speaking, test suites in OUnit2 are represented by the following ADT.

type 'a test =
  | TestCase of 'a
  | TestList of 'a test list

A test suite is either a test case (created using the (>::) operator) or made up of a list of other test suites (created using the (>:::) operator). The ADT is parametrized by the type variable 'a because there are multiple kinds of test cases.

When we check a test suite, we run each test in that suite from left to right (i.e., in-order traversal) and collect all the results. This is most naturally represented by a fold operation over the test suite.

Implement the function

val fold_left : ('a -> 'b -> 'a) -> 'a -> 'b test -> 'a

so that fold_left op base test is the result of applying op left-associatively to the test cases in test from left to right, starting with base.

let t = TestList [TestCase 1; TestList [TestCase 2; TestCase 3]; TestCase 4]

let _ = assert (fold_left (+) 0 t = 10)
let _ = assert (fold_left (fun acc x -> acc @ [x]) [] t = [1;2;3;4])
let _ = assert (fold_left (fun acc _ -> acc + 1) 0 (TestList []) = 0)