Proposal: Improving the IsString String instance

[Thomas Schilling]

We're having a hackathon here in Zurich next weekend.  I'll be happy to give this a shot (or get someone attending to do it).  Doesn't look too complicated.  The bigger unknown are more potential build issues with GHC.


On 27 Aug 2013, at 09:27, Simon Peyton-Jones <simonpj at microsoft.com> wrote:

> If I understand aright, the problem is that we end up with constraints like
> 	IsString alpha
> or
> 	IsString [beta]
> where alpha or beta are otherwise-unconstrained unification variables.  Under those circumstances we'd like to default to IsString String.  
> 
> This is *exactly* what the current type-class-defaulting mechanism does.  Eg with (show 3) you get (Show alpha, Num alpha) and want to choose alpha to be something sensible.
> 
> We've already extended it for GHCi
> http://www.haskell.org/ghc/docs/latest/html/users_guide/interactive-evaluation.html#extended-default-rules
> 
> I don't think this would be controversial.  The path seems to be:
> 	propose a concrete extension to the rules,
> 	specify when it's enabled
> 	get everyone to agree
> 	provide a patch (relevant code is in TcSimplify.applyDefaultingRules)
> 
> Simon
> 
> | -----Original Message-----
> | From: Libraries [mailto:libraries-bounces at haskell.org] On Behalf Of
> | Ganesh Sittampalam
> | Sent: 26 August 2013 21:55
> | To: Edward Kmett
> | Cc: Haskell Libraries
> | Subject: Re: Proposal: Improving the IsString String instance
> | 
> | 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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