the structure is the fold
the fold is the structure
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nobody taking the bait...
it has to do with initial/final encodings of data structures.
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i don't know much CT and i wish there were ways to not use that language...
the pattern is very useful tho.
basic idea is that a data structure, and the function processing that data structure are in some sense the same thing (dual meaning opposite, but same structure).
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practically this means that in many cases where you think "i need a data structure", you probably just want the corresponding iteration pattern.
this is the most important organizing principle i learned from functional programming, directly applicable to low level code work.
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i find it very hard to articulate
i don't think it is just "this one trick i like".