Reducers are functions that tell the partition algorithm how
to reduce the data. as_reducer() is a helper function to create new
reducers to be used in partitioners. partitioners can be created with
as_partitioner().
a function that returns either a numeric vector or a data.frame
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Extra arguments passed to .f.
returns_vector
logical. Does .f return a vector? TRUE by default.
If FALSE, assumes that .f returns a data.frame.
first_match
logical. Should the partition algorithm stop when it finds
a reduction that is equal to the threshold? Default is TRUE for reducers
that return a data.frame and FALSE for reducers that return a vector