Batteries user guide : BatOrd.Incubator

# Module BatOrd.Incubator

    module Incubator: sig .. end

    val eq_by : ('a -> 'b) -> 'a BatOrd.eq

    val comp_by : ('a -> 'b) -> 'a BatOrd.comp

    val ord_by : ('a -> 'b) -> 'a BatOrd.ord

Build a `eq`, `cmp` or `ord` function from a projection function. For example, if you wanted to compare integers based on their lowest 4 bits, you could write `let cmp_bot4 = cmp_by (fun x -> x land 0xf)` and use cmp_bot4 as the desired integer comparator.