[Haskell-cafe] Why does [1.0, 3 ..4] contain 5?

[Ben Mellor]

My understanding is that it was intended to support enumerations where you write the exact end point, like [1.0, 1.1 .. 2.3]

Because of floating point quirks, this could end up generating a value close to 2.3 that's slightly larger than the value close to 2.3 constructed by directly converting the decimal expression into a floating point number. You need to allow some "slack" in the upper bound to guarantee a value close to the written end point appears in the list. Increasing the bound by half a delta means there will be something close to 2.3 in the list, and shouldn't go far enough to include the next greater element generated.

I suppose by that reasoning you're "not supposed" to use enumerations like [1.0, 3 .. 4], because 4 isn't the idealised mathematical endpoint of the sequence.


On October 20, 2016 6:24:29 AM GMT+11:00, Brandon Allbery <allbery.b at gmail.com> wrote:
>On Wed, Oct 19, 2016 at 8:13 AM, Bence Kodaj <bence.kodaj at gmail.com>
>wrote:
>
>> Does anybody happen to know why [1.0, 3 ..4 ] is [1.0, 3.0, 5.0] ?
>>
>
>Nobody seems to know, aside from "that's what the Libraries part of the
>Report says". You'd probably have to find the committee that added it
>to
>the Report (good luck...) to learn their logic. (The quirks of FP
>arithmetic don't seem to be involved, since the overshoot is overkill
>for
>that.)
>
>-- 
>brandon s allbery kf8nh                               sine nomine
>associates
>allbery.b at gmail.com                                 
>ballbery at sinenomine.net
>unix, openafs, kerberos, infrastructure, xmonad       
>http://sinenomine.net
>
>
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