## Programming Praxis – Interval Arithmetic

In today’s Programming Praxis exercise, our goal is to implement some functions to do interval arithmetic. Let’s get started, shall we?

The plus and minus functions are trivial.

plus :: (Num a, Num b) => (a, b) -> (a, b) -> (a, b)
plus (a,b) (c,d) = (a+c, b+d)

minus :: (Num a, Num b) => (a, b) -> (b, a) -> (a, b)
minus (a,b) (c,d) = (a-d, b-c)

As Chun Kin Lee pointed out, my initial attempt at removing duplication doesn’t work for ranges with negative numbers, so I had to go back to way mentioned in the original algorithm.

times :: (Num a, Ord a) => (a, a) -> (a, a) -> (a, a)
times (a,b) (c,d) = let x = [a*c,a*d,b*c,b*d] in (minimum x, maximum x)

divide :: (Fractional a, Ord a) => (a, a) -> (a, a) -> (a, a)
divide (a,b) (c,d) = if c < 0 && d > 0 then error "divide by 0"
else let x = [a/c,a/d,b/c,b/d] in (minimum x, maximum x)

Converting between bounded and centered intervals is also trivial.

toCenter :: Fractional a => (a, a) -> (a, a)
toCenter (a,b) = ((a+b) / 2, (b-a) / 2)

fromCenter :: Num a => (a, a) -> (a, a)
fromCenter (a,b) = (a-b, a+b)

Some tests to see if everything is working properly:

main :: IO ()
main = do let x = (1,2)
y = (3,4)
print \$ plus x y == (4,6)
print \$ minus x y == (-3,-1)
print \$ times x y == (3,8)
print \$ divide x y == (1/4,2/3)
print \$ divide x x == (1/2, 2)
print \$ toCenter x == (3/2,1/2)
print \$ fromCenter (3/2,1/2) == x

Everything seems to be working fine.