Why neural networks are implemented using OOP? Isn't functional paradigm more appropriate?

I'm learning OCaml, hence I'm learning functional paradigm too.

These days I ways thinking: "A neural network uses lots of concepts used in functional programming... Inputs should be immutable, connected layers (by matmul) are composible functions, functions should be pure ... " Then I realise why Python is so popular? Why using OOP in neural nets?

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Anyone have an answer for this questions?

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