Originally posted by Michael_S
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The obvious problem in Haskell like languages is, you can't extend a data definition. If you do, you might do it via composition which makes handling the structure ugly even if you use lenses. Or some OOP like extensions (existential types). Just changing the data type back-propagates API/ABI changes and might break stuff. I'm not arguing one needs to develop in this contrived way, but random extensions often happen in long life enterprise software.
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