Using Haskell ranges: Why would mapping a floating point function across a range cause it to return an extra element?

Question

I know that floats can lead to odd behavior in ranges due to their imprecise nature. I would expect the possibility of imprecise values. For instance: [0.1,0.3..1]

Answer 1

I answered this for myself as I was writing it.

Haskell uses type inference, so when it sees a floating point function being mapped over a list (or used on an element of that list, as in my example using last), it is going to infer the type of that list to be floating point and therefore evaluate the range as if it were [1,3..10] :: [Float] instead of what I was intending, which is [1,3..10] :: [Int]

At this point, it uses the Float rules for enumerating, as described in the post that I linked to in the question.

The expected behavior can be forced like this:

ghci> map (\x -> (fromIntegral x) / 10) ([1,3..10]::[Int])
[0.1,0.3,0.5,0.7,0.9]

Relying on Haskell's type inference, we can drop the ::[Int] since fromIntegral causes our lambda expression to have the correct type:

ghci> :t (\x -> (fromIntegral x) / 10)
(\x -> (fromIntegral x) / 10)
  :: (Fractional a, Integral a1) => a1 -> a