How can we make use of `Infinity` that Haskell returns when you do `2/0`

3
votes

I understand that Haskell tries to do something more beneficial than simply throwing an error when one does division by zero

test :: Int -> Int -> String
test a b = case a/b of
    Infinity -> "fool"
    x  -> Show x

However i was told my ghc that Infinity is not a data constructor. What is it actually and how can i make use of it? I don't want to simply check b for 0

1
haskell
isNaN - user2407038
Just for context or if you want to read more about this, the behavior you're asking about isn't specific to haskell but is an aspect of the IEEE 754 floating point standard - jberryman

1 Answers

7
votes

There are a handful of ways to do this. My preferred one would be to use isInfinite from the Prelude:

test :: Int -> Int -> String
test a b = case fromIntegral a / fromIntegral b of
             x | isInfinite x && x > 0 -> "fool"
               | otherwise -> show x

Alternately, you could define infinity as in this question and compare for equality (since Infinity == Infinity).

Your code also had a couple of issues I assume are unrelated to your question:

  • Show should be show
  • (/) doesn't work for Int arguments, so you need to use fromIntegral to convert a and b to something floating

I also suspect you aware this particular function doesn't require an infinity check...

test :: Int -> Int -> String
test a 0 | a > 0 = "fool"
         | otherwise = show (fromIntegral a / fromIntegral b)