Haskell Lenses: Difference of two HashSets in Folds

Question

Given:

f1, f2 :: Fold Box (HashSet Thing)

how do I write a fold that focuses the Things that are in f1 but not in f2? It would (of course) be the same Box, but f1 and f2 filter its contents differently.

I can see that filtered and hasn't might come into it and HashSet.difference would do the gruntwork, but I can't quite figure out the plumbing.

I'm not sure how that comes into it. I was hoping some lens magic would do it very concisely. — Adrian May, May 13 '22 at 11:26

score 0 · Answer 1 · answered May 13 '22 at 13:27

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It was a silly question. f1 is just a field of Box but f2 is a filter of f1's contents, and the answer would be the converse filter. It's not meaningful to speak of stuffing the answer back into Box, so I don't really want an optic.

answered May 13 '22 at 13:27

Adrian May

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Haskell Lenses: Difference of two HashSets in Folds

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