Proposal: Improving the IsString String instance

[Edward Kmett]

That seems worth pursuing.

If that works, it'd turn the defaulting rules fix from a fishing expedition
to simply adding a couple of extra classes to the list of approved basic
defaultings.

-Edward


On Mon, Aug 26, 2013 at 4:54 PM, Ganesh Sittampalam <ganesh at earth.li> wrote:

> Couldn't it work (with suitable extensions to the list of blessed
> classes) with Henning's IsCharList suggestion? The unresolved constraint
> would be IsCharList a.
>
> On 26/08/2013 21:47, Edward Kmett wrote:
> > The problem is ExtendedDefaultRules only works when the whole type is
> > unknown. If we know part of the type e.g. that it is [a] for some a as
> > we figure out from length, or that it is (f a), then it doesn't kick in.
> >
> > print works because "hello" :: (IsString a, Show a) => a
> >
> > knows nothing about the argument a
> >
> > and Show is included in the EDR list of blessed classes.
> >
> > If at the repl you turn on OverloadedStrings and EDR
> >
> > showList "hello" ""
> >
> > will still blow up, because it can know you have an argument [a] with a
> > Show a constraint on it, and IsString [a], so it fails to meet the
> > fairly simplistic conditions required by EDR.
> >
> > -Edward
> >
> >
> > On Mon, Aug 26, 2013 at 4:34 PM, Dag Odenhall <dag.odenhall at gmail.com
> > <mailto:dag.odenhall at gmail.com>> wrote:
> >
> >     |ExtendedDefaultRules| already provides this to some extent, for
> >     example it makes |print "hello"| work without type information.
> >
> >
> >
> >     On Mon, Aug 26, 2013 at 10:09 PM, Dan Burton
> >     <danburton.email at gmail.com <mailto:danburton.email at gmail.com>>
> wrote:
> >
> >         Just to throw another curveball at this conversation, what about
> >         a general, ad-hoc solution for 'typeclass defaults'? The logic
> >         goes like this: Is there an "ambiguous type variable" error due
> >         to the use of the IsString typeclass? Try defaulting to the
> >         String instance. If that doesn't work, try defaulting to Text.
> >         Repeat for a finite, explicit list of instances. If none of
> >         these defaults are successful, type error.
> >
> >         We have hard-coded this sort of behavior into the Num class. Why
> >         not just provide this general capability for arbitrary classes?
> >         Or at least hard-code it into IsString as well.
> >
> >         In the absence of this sort of solution, +1 to Edward's original
> >         proposal.
> >
> >         The "o" proposal is not viable because the oft-needed parens
> >         make it confusing and irritating to the new Haskeller.
> >
> >         -- Dan Burton
> >
> >
> >         On Mon, Aug 26, 2013 at 12:41 PM, Henning Thielemann
> >         <lemming at henning-thielemann.de
> >         <mailto:lemming at henning-thielemann.de>> wrote:
> >
> >
> >             On Mon, 26 Aug 2013, Gabriel Gonzalez wrote:
> >
> >                 I'm guessing this is sarcastic, but I just want to
> >                 clarify what I understood Henning's proposal to be.
> >                  He's not saying we should provide an `o` function in
> >                 the standard library, but rather encourage users to
> >                 define their own.
> >
> >
> >             Yes. I would be ok if packages provide this function, but I
> >             would urge programmers to import that explicitly.
> >
> >
> >                  This one liner would take the place of the current line
> >                 that they devote right now to `OverloadedStrings` .
> >
> >
> >             right
> >
> >
> >                 However, the analogy is still apt since the exact same
> >                 line of reasoning applies to overloaded numeric
> >                 literals where we currently rely on defaulting to solve
> >                 this problem.
> >
> >
> >             I always use -Wall and thus I am warned about when
> >             defaulting takes place. Thus I am confident that I do not
> >             rely on defaulting in my code.
> >
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