Dealing with 'Overlapping Pattern Match' in OCaml

When encountering the Overlapping Pattern Match warning in OCaml, it signals that one of your patterns in a match or function block is redundant because a preceding pattern already matches the same cases.

While OCaml’s type-checking is excellent at detecting such redundancy, this warning is more than just a nitpick—it’s about ensuring code clarity and avoiding logical errors.

To resolve this issue, you must identify the exact pattern causing the redundancy.

First, compile the code with the -w +8 flag (which enables the overlapping pattern warning).

OCaml will usually pinpoint the offending line.

Now, analyze the match cases one by one.

Start by reordering them logically—ensure more specific cases precede the generic ones.

For instance, if you’re matching on a tuple (Some x, _), ensure this appears before a generic wildcard _.

Overlaps often occur with wildcards (_) or default cases that unintentionally shadow valid patterns.

If the redundancy serves as documentation for future-proofing, you can safely suppress the warning using [@@ocaml.warning **-8**].

But be cautious when doing so—it’s better to refactor the code.

Also, consider running tools like ocamlformat or merlin to help identify and clean up overlapping patterns.

Ultimately, addressing these warnings ensures that your pattern matching is both efficient and easier for collaborators to understand.