Recipe: OUnit assertions

content/languages/ocaml/recipes.md

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+---
+title: OCaml recipes for Codewars
+tags: [authoring, recipe, ocaml]
+---
+
+
+## Informative assertion messages
+
+Default assertion messages of OCaml OUnit are usually very bad. OUnit assertions accept two optional arguments:
+- `~printer` defines a printer (stringifier) for compared values.
+- `~msg` is used to provide additional information about failure, if necessary.
+
+With both arguments used, test output becomes more explicit:
+
+```ocaml
+"(square 3) should return 9" >:: (fun _ ->
+  let actual = square 3 in
+  assert_equal 9 actual ~printer: string_of_int ~msg: "Incorrect answer for n=3"
+)
+```
+
+```text
+Tests for square
+    (square 3) should return 9
+        Incorrect answer for n=3
+        expected: 9 but got: 10
+    Completed in 0.22ms
+Completed in 0.24ms
+```
+
+### References
+
+- [Improving OUnit Output](https://cs3110.github.io/textbook/chapters/data/ounit.html#improving-ounit-output)
+- [OUnit: xUnit testing framework for OCaml](https://ocaml.org/p/ounit2/2.2.3/doc/index.html#error-reporting)

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