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Make zero-mean fn. constant a Static class - #500
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This was referenced Jul 26, 2026
thomaspinder
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`Zero()` was trainable and drifted towards the data mean during `fit` (0.0 -> 5.09 on a dataset with mean 5), contradicting its own docstring and silently changing the posterior mean of every model using the default mean function. Regression of #330, fixed once in #500. The cause is a changed trainability contract rather than a lost line. Under nnx, `fit` optimised only `Parameter` instances, so `Zero`'s bare array was inert by construction -- which is why #530 could drop the `Static` wrapper and stay correct. Under Equinox, `fit` partitions on `eqx.is_array`, making every array leaf trainable, and `Zero` was still relying on the old meaning of a bare array. Wrap the constant in `paramax.non_trainable` so the invariant holds by construction rather than by the ambient filter semantics. The guard for this was weakened rather than removed: #614 replaced `test_zero_mean_remains_zero` with a test of the initial value only, and `test_zero_mean_function_uses_raw_value` asserted the defect outright. Restore the end-to-end fit assertion and invert the unit test. Fixes #712 Claude-Session: https://claude.ai/code/session_01Bj9k5fnAZ8JzD4Rg3HMDMj Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
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hatch run dev:formatbefore committing.Description
Set the
constantfield of the zero-mean function to be aStaticparameter to prevent its value being changed from zero during training.Issue Number: #499