Debugging and testing in OCaml

Are you tired of spending hours debugging your OCaml code? Do you want to ensure that your code is error-free before releasing it to the world? Look no further because we have got you covered! In this article, we will explore the world of debugging and testing in OCaml.

When it comes to developing robust and reliable software, debugging and testing are crucial parts of the process. Bugs can be frustrating, difficult to track down and can result in unexpected behavior or crashes. Therefore, it's essential to catch them early in the development process. Fortunately, OCaml provides a set of powerful tools for debugging and testing your code. Let's dig in and discover these tools.


Debugging is the process of finding and fixing errors in software. It's a crucial part of the development process that ensures the quality of the code you produce. Fortunately, OCaml provides an excellent debugger, the OCaml Debugger, which enables you to debug programs interactively. The OCaml Debugger provides the following features:

The debugger is pretty simple and easy to use. To start it, you need to add the -g flag when compiling your OCaml program. This flag lets the compiler generate debugging information, which is necessary for the debugger to work correctly. Once you have compiled the program with the -g flag, you can start the debugger by entering the following command:

ocamldebug <program_name>

The debugger prompt will appear, and you can start stepping through your program. You can set breakpoints, step through the code, inspect variables, and examine the stack trace. You can also inspect the contents of the heap and interactively modify variables.

For example, say you have the following code:

let rec fib n =
  match n with
  | 0 -> 0
  | 1 -> 1
  | n -> fib (n - 1) + fib (n - 2)
let result = fib 5 in
print_int result

You set a breakpoint at the beginning of the fib function by typing break fib. Then you step through the code by typing step and next. You can inspect the value of the variable n by typing print n. Once you have found the bug, you can interactively modify variables by typing set. Finally, you can exit the debugger by typing quit.

The OCaml Debugger is an incredibly powerful tool that lets you analyze and debug your programs interactively. It's a must-have tool for any OCaml developer.


Testing is the process of verifying that a software program meets its intended behavior and performs as expected. Through testing, you can ensure that your program works correctly and is free of bugs. OCaml provides an excellent set of testing tools that you can use to test your programs thoroughly. In this section, we will explore these testing tools.


OUnit is a popular unit-testing framework for OCaml. It's easy to use, and you can test your code directly within your codebase. With OUnit, you can write test cases for your OCaml functions, test these functions for expected behavior, and ensure that your program is working correctly.

To use OUnit, you need to install it first. You can do this by running the command opam install ounit.

Once you have installed OUnit, you need to create a test file. For example, say you have the following code:

let even n =
  n mod 2 == 0

You can test the even function by creating a test file as follows:

open OUnit2
let tests = "even tests" >::: [
  "even 0" >:: (fun _ -> assert_equal true (even 0));
  "even 2" >:: (fun _ -> assert_equal true (even 2));
  "even 3" >:: (fun _ -> assert_equal false (even 3));
  "even -2" >:: (fun _ -> assert_equal true (even -2));

The code above creates a test suite named even tests, which contains four test cases. The test cases ensure that the even function works correctly for various inputs.

You can run the test suite by invoking the command make or make test in your terminal. OUnit will run your test cases and report whether they pass or fail.


QuickCheck is a property-based testing framework that enables you to test your code for expected behavior. With QuickCheck, you can create a set of random inputs for your functions and ensure that your program works correctly under these conditions. It's a powerful tool that can help you find edge cases that you might have missed in your testing.

Let's illustrate using an example. Say you have the following code that sorts a list of integers:

let rec quicksort l =
   match l with
   | [] -> []
   | hd :: tl ->
        let smaller, larger = List.partition (fun x -> x < hd) tl in
        (quicksort smaller) @ [hd] @ (quicksort larger)

You can use QuickCheck to test the quicksort function for various inputs. First, you need to install QuickCheck by running the command opam install qcheck.

Next, you need to create a test file and define a property that the function should satisfy. For example, say you want to ensure that the quicksort function preserves the length of the input list. You can define this property as follows:

open QCheck

let prop_length_preserved =
  let gen_list = Gen.(list (int_range 0 100)) in
  let f l = List.length l = List.length (quicksort l) in
  Test.make ~count:100 gen_list f

The code above creates a property test named prop_length_preserved. The test generates a list of integers between 0 and 100 and ensures that the quicksort function returns a sorted list of the same length.

You can run the property test by invoking the command qcheck in your terminal. QuickCheck will generate 100 random lists and report whether the quicksort function preserves their length.


Bisect is a code coverage tool that lets you measure how much of your code has been executed during your test runs. With Bisect, you can ensure that your tests are thorough and cover all parts of your codebase. Bisect instruments your code and records the execution path of your code during the test runs. You can then run a report that shows how much of your code has been executed during the tests.

First, you need to install Bisect by running the command opam install bisect.

Next, you need to instrument your code by running the command:

bisect-ppx-opt <other-compiler-fragments> -I _build/ <your-ocaml-files>

The command above instruments your code and generates new OCaml files that include the code coverage information.

Finally, you can run your tests and generate a report using the following commands:

make clean
make test
bisect-ppx-report html

The commands above clean your build directory, run your tests, and generate a web-page report with the code coverage information.


Debugging and testing are crucial parts of software development. With OCaml, you have a set of powerful tools at your disposal that can help you maintain the quality of your codebase. The OCaml Debugger is an interactive debugger that enables you to step through your code, inspect variables, and modify them on the fly. OUnit, QuickCheck, and Bisect are testing frameworks that provide various forms of testing for your code, such as unit testing, property testing, and code coverage. By combining these tools with good coding practices, you can produce reliable and robust software that meets your customers' needs.

Thank you for reading! We hope that this article has been informative and engaging. Don't forget to subscribe to our site for more exciting content about OCaml development!

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