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Stateful span #437
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Stateful span #437
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Very nice idea, thanks! |
Bodigrim
reviewed
May 16, 2022
Bodigrim
reviewed
May 17, 2022
Bodigrim
reviewed
May 17, 2022
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Please rebase.
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Bodigrim
approved these changes
May 17, 2022
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Closes #178 and #362
Depends on PR #436
This PR implements
spanM
(span from the left) andspanEndM
(span from the right) for both strict and lazyText
.spanM
is a combination ofspan
andfoldr
.spanM
, as its name indicates, generalizesspan
, with a monadic predicate, of typeChar -> m Bool
rather thanChar -> Bool
.spanM
generalizes a short-circuitingfoldr
, by also remembering the location where it stopped so you can easily resume with another search from there.spanM
can also implementfoldl'
with the same space and time efficiency, by making the predicate alwaysTrue
and only using the state effect.I argue these two functions pass the Fairbairn threshold as they provide the ability to safely move back and forth inside a
Text
. As mentioned in #178, this is currently not possible to do both efficiently and safely: either you keep track of an offset in code points, which then requires a redundant traversal to dosplitAt
, or you keep track of an offset in bytes, which is unsafe.With
spanM
one can easily implement atakeWhileM
(#362) by ignoring the remaining suffix. I'm not sure if its allocation would be optimized out already, but if not, I can generalize `spanM`` with some continuation-passing trick to do that in a future PR.