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pierric/lens.lhs

Last active October 21, 2020 11:44
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lens magic
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lens.lhs
m = HashMap.fromList [
("X", HashMap.fromList [("a", 0), ("b", 1)]),
("Y", HashMap.fromList [("a", 1)]),
("Z", HashMap.fromList [("b", 2)])
]
I want to extract all value with key "a" in the nested HashMap, and keep the root key.
> m ^.. (itraversed <. at "a" . folded) . withIndex
result is [("X", 0), ("Y", 1)]
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