[Haskell-cafe] Proposal: Rational read

[David Feuer]

Since decimal notation is defined in terms of fromRational, it seems a bit
strange that the notation doesn't work for read::String->Rational. I'd be
in favor of allowing it. I wouldn't expect too severe a performance penalty.

On Aug 14, 2016 5:28 PM, "Sean Leather" <sean.leather at gmail.com> wrote:

> Hi Erik,
>
> On Sun, Aug 14, 2016 at 11:01 PM, Erik Hesselink wrote:
>
>> On 14 August 2016 at 16:00, Sean Leather wrote:
>> > On Sun, Aug 14, 2016 at 9:48 PM, Tom wrote:
>> >>
>> >> Is there a reason to not allow '.' as a separator for Rational's Read
>> >> instance, so that both "1%2" and "0.5" can be read?
>> >
>> > While it would make read :: String -> Rational more convenient, I'm not
>> sure
>> > it would be a good idea to have read accept input that will never be
>> > produced by show :: Rational -> String.
>>
>> This already happens everywhere, right? For example, if you have a Foo
>> { bar :: Int, baz :: String } with the derived read instance, this
>> works:
>>
>> read "Foo{bar=0x01,      baz     =      \"h\101llo\"}" :: Foo
>>
>
> True. I'm not sure how far down the decimal/scientific notation parsing
> hole you'd want to take the Ratio Read instance. Anyway, my point was that
> there is an existing alternative that doesn't require changing the instance
> and is not that inconvenient.
>
> Regards,
> Sean
>
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