Why it matters

Generalised advantage methods like PPO rely on a clipped value loss to keep critics stable. Fehu.Training.value_loss exposes a ~clip_range argument, but the implementation at fehu/lib/training.ml:86 just ignores it and falls back to plain MSE. Outreachy applicants working on policy-gradient baselines hit a silent footgun: the API promises clipping, yet nothing happens.

How to see the gap

Open fehu/lib/training.ml and inspect value_loss; the comment even says “interface may need adjustment”. Run dune runtest fehu and note that fehu/test/test_training.ml only checks the unclipped path—no scenario exercises clipping.

Your task

• Extend value_loss to accept an optional ~old_values array. When ~clip_range is provided you must also have old_values; otherwise raise Invalid_argument with a helpful message.
• Implement the PPO-style clipping: compute value_clipped = old_values + clamp(values - old_values, ±clip_range) and take the max between the unclipped and clipped squared error for each element.
• Update the interface and documentation in fehu/lib/training.mli to describe the new parameter and behaviour.
• Enhance fehu/test/test_training.ml with a regression test that feeds in toy values, old_values, returns, and clip_range, then asserts the clipped result matches the hand-computed expectation. Keep the existing unclipped test passing.

Tips

• Use Array.length checks similar to the existing code to ensure all arrays align; fail fast if they don’t.
• To keep numerical comparisons stable in the test, compare against a precomputed float value with Alcotest.(check (float 1e-6)).
• Document the requirement in the .mli that callers must pass old_values when using clipping so future users understand the contract.

Done when

• value_loss clips correctly and raises if ~clip_range is used without ~old_values.
• The new unit test proves the clipped branch works.
• dune runtest fehu continues to succeed.