Round to nearest integer strange results

Question

Is there an official specification for the round function in Haskell? In GHCi version 7.0.3 I see the following behaviour:

ghci> round (0.5 :: Double) 0 ghci> round (1.5 :: Double) 2 

Since both 0.5 and 1.5 are representable exactly as floating point numbers, I expected to see the same behaviour as in Python:

>>> round(0.5) 1.0 >>> round(1.5) 2.0 

Is there a rationale for the difference, or is it a quirk of GHCi?

Answer 1

It's in the specification. You can see it in section 6.4.6 of the Haskell report:

round x returns the nearest integer to x, the even integer if x is equidistant between two integers.

As pointed out by @dflemstr, this is in accordance with the IEEE Standard for Floating-Point Arithmetic.