f :: (Num a) => a -> a
f x = abs(42-x)^2
 
improve :: (Fractional a) => (a -> b) -> a -> (a, b)
improve f x =
    let newX = x+0.1
    in (newX, f newX)
 
step :: (Fractional a, Ord b) => (a -> b) -> b -> a -> (a, b)
step f goal x =
    let
        newX = x+0.1
        err = f newX
    in
        if err < goal then (newX, err) else step f goal newX
 
optimize :: (Fractional a, Ord b) => (a -> b) -> b -> (a, b)
optimize f goal = step f goal 0
 
main :: IO ()
main = print $ optimize f 0.5

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