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 2:

>>> 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.