Stijn De Saeger wrote:
data Bound = I Double | E Double deriving (Eq, Show, Ord) data Interval = Il Bound Bound | Nil Bound Bound deriving (Eq,Ord)
isIn :: Double -> Interval -> Bool isIn r (Nil x y) = not (isIn r (Il x y)) isIn r (Il (I x) (I y)) = r >= x && r <= y isIn r (Il (I x) (E y)) = r >= x && r < y isIn r (Il (E x) (I y)) = r > x && r <= y isIn r (Il (E x) (E y)) = r > x && r < y
If performance is the main concern, I would flatten the data structure: data Interval = IlII Double Double | IlIE Double Double | IlEI Double Double | IlEE Double Double | NilII Double Double | NilIE Double Double | NilEI Double Double | NilEE Double Double isIn :: Double -> Interval -> Bool isIn r (IlII x y) = r >= x && r <= y isIn r (IlIE x y) = r >= x && r < y isIn r (IlEI x y) = r > x && r <= y isIn r (IlEE x y) = r > x && r < y isIn r (NilII x y) = r < x || r > y isIn r (NilIE x y) = r < x || r >= y isIn r (NilEI x y) = r <= x || r > y isIn r (NilEE x y) = r <= x || r >= y Depending on your application you might not need all of those cases. Another neat trick you can pull is to take advantage of the fact that Double is actually a discrete type, like Int, and you can therefore get away with closed intervals only: data Interval = Il Double Double | Nil Double Double isIn :: Double -> Interval -> Bool isIn r (Il x y) = r >= x && r <= y isIn r (Nil x y) = r < x || r > y But this requires nextLargestDouble and nextSmallestDouble functions. I don't know if Haskell provides them. Also, you could run into trouble with wider-than-Double intermediate values. Finally, if you never do anything with intervals except pass them to isIn, you can do this: type Interval = Double -> Bool isIn r i = i r -- Ben