Proposal: Add IsString instance for (Maybe a) to base

Sjoerd Visscher sjoerd at
Thu Jul 11 22:22:27 CEST 2013

Could this be formalised by saying that there should be a function toString such that

  fromString . toString = id

There's no such toString for "Maybe a" with fromString = Just.


On Jul 10, 2013, at 4:02 PM, David Luposchainsky <dluposchainsky at> wrote:

> On 2013-07-10 15:55, Joachim Breitner wrote:
>> Hi,
>> Am Mittwoch, den 10.07.2013, 09:21 +1000 schrieb Ivan Lazar Miljenovic:
>>> -0.5 from me; I think it's too easy to forget that it's a Maybe value
>>> there when refactoring, etc.
>> same here; even with OverloadedStrings Haskell should feel as strongly
>> typed as it is.
> Agreed. Overloaded strings should be reserved for things that *are*
> strings in a certain way. A "Maybe String" is not a special way of
> storing a string (like for example a "Vector Char"), it's a String with
> an additional value. Providing a Maybe instance goes too far in the
> direction of having a general "'return' with strings" function, so I'm
> -1 on this.
> David
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