On Fri, Apr 18, 2014 at 12:54:48PM -0700, Bob Ippolito wrote:
ghci> [0.1, 0.3 .. 1] [0.1,0.3,0.5,0.7,0.8999999999999999,1.0999999999999999]
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Can anyone explain me why it works for the first few values, and not "completely"?
It doesn't "work" for any of the values, it's just an artifact of how they're rendered. 0.1 can't be exactly represented in binary floating point, so the error compounds. Double probably shouldn't be enumerable in the first place, but that's a decision we have to live with. The reason that the end result is so surprising is that 1.0999999999999999 is less than 1 + 0.1 and for whatever reason the way Enum is defined for Double checks to see if the result is > to+(then-from) rather than <= to.
To clarify, this is what is happening:
λ> takeWhile (< 1 + 0.1) $ iterate (+0.1) 0.1
[0.1,0.2,0.30000000000000004,0.4,0.5,0.6,0.7,0.7999999999999999,0.8999999999999999,0.9999999999999999,1.0999999999999999]
Actually, the check is whether the result is > to + ((then - from)/2). Notice that the original example is stepping by 0.2, not 0.1. -Brent