Turn exercise 003 into practice

practice/003/answer/impl.ml

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+# let rec at k = function
+    | [] -> None
+    | h :: t -> if k = 0 then Some h else at (k - 1) t;;
+val at : int -> 'a list -> 'a option = <fun>

practice/003/answer/test/dune

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+(include_subdirs no)
+(test (name run) (libraries ounit2 ex))

practice/003/answer/test/run.ml

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+module Test = Ex.Make(Ex.Answer)
+
+let () = OUnit2.run_test_tt_main Test.v

practice/003/dune

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+(include_subdirs qualified)
+(library (name ex) (libraries ounit2))

practice/003/dune-project

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+(lang dune 3.7)

practice/003/ex.ml

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+
+open OUnit2
+
+module type Testable = sig
+  val last : 'a list -> 'a option
+end
+
+module Make(Tested: Testable) : sig val v : test end = struct
+  let v = "last" >::: [
+    "nil" >:: (fun _ -> assert_equal None (Tested.last []));
+    "cons" >:: (fun _ -> assert_equal (Some "d") (Tested.last ["a"; "b"; "c"; "d"]));
+  ]
+end
+
+module Work : Testable = Work.Impl
+module Answer : Testable = Answer.Impl

practice/003/work/impl.ml

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+let last_three _ = failwith "Not yet implemented"

practice/003/work/test/dune

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+(include_subdirs no)
+(test (name run) (libraries ounit2 ex))

practice/003/work/test/run.ml

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+module Test = Ex.Make(Ex.Work)
+
+let () = OUnit2.run_test_tt_main Test.v