haskell - Construct predicates with lenses -


i want create function a -> bool using lenses of a. instance:

data = { _foo :: int, _bar :: int } makelenses ''a  l :: [a]  l' = filter (\a -> a^.foo > 100) l

the filter predicate looks bit clumpsy. ((>100).(^.foo)) not better. without lenses, use ((>100) . foo).

is there nice way create such predicates lens? ideally allow predicates (\a -> a^.foo > 100 && a^.bar < 50).

i think ((>100).(^.foo)) best can using standard operators. if willing define new comparison operators lenses, like:

import control.lens hiding  ((.>)) import control.monad        (liftm2) import control.monad.reader (monadreader) import data.function        (on)  (.==) :: (monadreader s m, eq a) => getting bool s -> -> m bool (.==) l = views l . (==) infix 4 .==  (.==.) :: (monadreader s m, eq a) => getting s -> getting s -> m bool (.==.) = liftm2 (==) `on` view infix 4 .==.  (.<) :: (monadreader s m, ord a) => getting bool s -> -> m bool (.<) l = views l . flip (<) infix 4 .<  (.<.) :: (monadreader s m, ord a) => getting s -> getting s -> m bool (.<.) = liftm2 (<) `on` view infix 4 .<.  (.<=) :: (monadreader s m, ord a) => getting bool s -> -> m bool (.<=) l = views l . flip (<=) infix 4 .<=  (.<=.) :: (monadreader s m, ord a) => getting s -> getting s -> m bool (.<=.) = liftm2 (<=) `on` view infix 4 .<=.   (.>) :: (monadreader s m, ord a) => getting bool s -> -> m bool (.>) l = views l . flip (>) infix 4 .>  (.>.) :: (monadreader s m, ord a) => getting s -> getting s -> m bool (.>.) = liftm2 (>) `on` view infix 4 .>.  (.>=) :: (monadreader s m, ord a) => getting bool s -> -> m bool (.>=) l = views l . flip (>=) infix 4 .>=  (.>=.) :: (monadreader s m, ord a) => getting s -> getting s -> m bool (.>=.) = liftm2 (>=) `on` view infix 4 .>=.  (.&&.) :: monad m => m bool -> m bool -> m bool (.&&.) = liftm2 (&&) infix 3 .&&.  (.||.) :: monad m => m bool -> m bool -> m bool (.||.) = liftm2 (||) infix 3 .||.

the logic behind operator choices dot signifies side has lens, write either foo .== 5 or foo .==. bar (where foo , bar lenses). unfortunately, lens package defines own (.<) operator, maybe other naming convention better. first idea came mind.

using these new operators, able write stuff like

l' = filter (foo .> 100 .&&. bar .< 50) l

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