Make the imperative shell just thin glue; the functional core pure and heavily tested; they communicate by message passing.
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Imperative shell is like a switchboard operator: not a part of the conversation, just someone with two plugs to connect.
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Yes. Also, it creates state by attaching identity to values, which I don't know how to fit into that metaphor.
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Sorry, that last bit was somewhat unclear. Say you have drawings (structs). Imperative shell defines "the" (vs. "a") drawing.