Text.Printf replacement?

[Bart Massey]

I already tried FlexibleInstances and the IsUnit typeclass trick and
could get neither to work. AFAICT, both require cooperation from the
caller. I may have just done it wrong, though. I have absolutely no
problem with an IsUnit typeclass if that's a clean solution to the
problem somehow. --Bart

On Sat, Sep 7, 2013 at 5:12 PM, Ben Millwood <haskell at benmachine.co.uk> wrote:
> A no-extensions approach would be to duplicate the string trick, with an
> IsUnit class like the IsChar one. However, that's kind of incredibly silly,
> because fromUnit :: () -> a is pretty much the same as hasAValue :: a, so
> you get the least specific class ever.
>
> Plus there already exist classes which have () as a member. How about:
>
>    instance Monoid a => PrintfType (IO a) where ...
>
> with 'return mempty' instead of 'return undefined'?
>
> This is a little bit silly, but lightweight and extensions-free. If you
> think that mentioning Monoid here is a distraction, feel free to introduce a
> new class instead. It would end up being roughly speaking the Default class
> from data-default, which I have gone on record before as calling annoying
> and useless (with, admittedly, a few limited applications).
>
> Here's another idea: just enable FlexibleInstances and use
>
>    instance PrintfType (IO ()) where ...
>
> FlexibleInstances is pretty stable and really the bottom of the hierarchy of
> class system extensions (perhaps just above FlexibleContexts, but certainly
> TypeFamilies and MPTCs tend to sort of assume you've got it enabled
> already). I don't know if there are any inference issues that I'm missing
> here.
>
>
> On Fri, Sep 06, 2013 at 10:56:55AM -0700, Bart Massey wrote:
>>
>> TypeFamilies are kind of heavy machinery here :-), but since it would
>> be confined to the library and seems to solve the problem, I'd be in
>> favor of doing as you suggest. We can conditionally compile if we are
>> worried about non-GHC implementations, I think. Do other folks have an
>> opinion? --Bart
>>
>> On Fri, Sep 6, 2013 at 8:18 AM, Twan van Laarhoven <twanvl at gmail.com>
>> wrote:
>>>
>>> On 06/09/13 04:16, Bart Massey wrote:
>>>>
>>>>
>>>> A final issue has to do with the return type of Text.Printf.printf,
>>>> which is polymorphic between String and IO a. I'm sure this seemed
>>>> like a good idea at the time, but it's not so ideal today: GHC gives a
>>>> warning when printf is used at IO a unless you explicitly ignore the
>>>> result.
>>>
>>>
>>>
>>> You can probably force the `a` to be `()` by using:
>>>
>>>     instance (a ~ ()) => PrintfType (IO a) where ...
>>>
>>> But that is of course not standard Haskell.
>>>
>>>
>>> Twan
>>>
>>>
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