Simon Peyton-Jones wrote:
> Folks
>
> The concrete is setting fast, but Ross points out that the instance for
> Enum (Ratio a) is inconsistent with that for Enum Float and Enum Double.
> (It's strange that these non-integral types are in Enum, but we're stuck
> with that.)
>
> All three use 'numericEnumFrom' etc for the enumFrom method, but Float
> and Double define succ and pred thus:
>
> instance Enum Float where
> succ x = x+1
> pred x = x-1
>
> But Ratio does not... it uses the default method for succ/pred which
> gives
>
> instance Integral a => Enum (Ratio a) where
> succ x = fromInt (1 + (toInt x))
>
> Thus (succ (1.5 :: Float)) is 2.5
> but (succ (3%2)) is 2%1
>
> Furthermore, you'd expect that
> [x..y]
> would mean the same as
> [x,succ x .. y]
> and that is true for Float/Double but not for Ratio.
>
> So I propose to modify the instance decl for Ratio by adding explicit
> defns for succ/pred just like those in Float/Double.
>
> Any objections?
>
> Simon
No objection, but here are additional related corrections to be made:
(1) In section A (Standard Prelude), in the definition of
`numericEnumFromThenTo`, change "n' > n" to "n' >= n", to agree with the
last bullet in section 6.3.4. That is, the function definition should read:
numericEnumFromThenTo n n' m = takeWhile p (numericEnumFromThen n n')
where
p | n' >= n = (<= m + (n'-n)/2)
| otherwise = (>= m + (n'-n)/2)
(2) In section 3.10, change "which Prelude type are in Enum" to "which
Prelude types are in Enum".
-- Dean
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