[ketil@ii.uib.no: Re: Enum class]

[Ketil Malde]

[Heavy snippage and editing]

>> From: Ketil Malde <ketil@ii.uib.no>
>> Subject: Re: Enum class

>>> i_succ' = succ i'
>>>   -- ghc : 2147483648
>>>   -- hugs: -2147483648

>> I think Hugs is wrong.  Integer shouldn't wrap.

"Sigbjorn Finne" <sof@galconn.com> writes:

> Hugs is indeed wrong, I've checked in the same (obvious) fix
> that was made to GHC's Enum Integer instance a long time ago.

Uh - I meant wrong in an intuitive way, and I was corrected on the
grounds that the Prelude defines:

  class  Enum a  where
    succ, pred       :: a -> a
    toEnum           :: Int -> a
    fromEnum         :: a -> Int
    enumFrom         :: a -> [a]             -- [n..]
    enumFromThen     :: a -> a -> [a]        -- [n,n'..]
    enumFromTo       :: a -> a -> [a]        -- [n..m]
    enumFromThenTo   :: a -> a -> a -> [a]   -- [n,n'..m]

and that the .. "operator", as well as succ and pred is defined in
terms of toEnum and fromEnum:

    -- Minimal complete definition:
    --      toEnum, fromEnum
    succ             =  toEnum . (+1) . fromEnum
    pred             =  toEnum . (subtract 1) . fromEnum
    enumFrom x       =  map toEnum [fromEnum x ..]
    enumFromTo x y   =  map toEnum [fromEnum x .. fromEnum y]
    enumFromThenTo x y z = 
                        map toEnum [fromEnum x, fromEnum y .. fromEnum z]


(All right off the web pages at Haskell.org)

So apparently, it was the intention of the language designers that
"infinite" lists defined by [b..] and the so-called succ only go from
2G long before they wrap, since toEnum and fromEnum maps to Ints and
not Integers.

There's also been a longish thread about the fun consequences for
floating point numbers, I can try to summarize if necessary.  I, being
a mere lay programmer, find it quite surprising that "succ" returns
the next higher integer for positive floats, but the next higher *plus
one* for negatives.  Or that [a..b] produces a set of numbers outside
the specified range (and indeed that these are defined at all for floating
point numbers).

-kzm
-- 
If I haven't seen further, it is by standing in the footprints of giants