[Haskell-beginners] special polymorphic default of typeclass depending on other typeclasses?

[Silent Leaf]

typos in my previous message (including in the code), don't pay attention,
any error is an error :P

2017-06-29 20:08 GMT+02:00 Silent Leaf <silent.leaf0 at gmail.com>:

> you're right my example is more like Show, it's because i change two times
> the order of input and output, and did not check at the end if it was what
> i wanted...
>
> Actually i did not mean constraints on instances, especially thereafter
> ad-hoc polymorphic instances.
> I rather meant what i said, as in default implementation (as in, right
> inside the body of the class).
> like that:
>
> class Foo a where
>   bar :: a -> String
>   bar :: Show a => a -> String
>   bar = show -- default implementation: in other terms, if you define an
> instance without defining this method
>
> the idea would be then that if you have Foo (), you can either implement a
> special version, or leave it to show. mind you i'm not even sure we can
> define an instance without any specific implementation of method?
>
> --that would be allowed:
> instance Show => Foo () where
>   -- nothing, if it's even legal by default
>
> -- and of course that would be allowed to if someone wanted a special
> function bar :: Foo () => () -> String
> instance Foo () where
>   bar = ....
>
> obviously, i do not mean *both* instances of Foo (), just, one or the
> other. it would merely be a way to implement ad-hoc polymorphism onto
> *default implementations of methods*, that is, those inside the body of the
> class.
>
>
>
> 2017-06-29 19:45 GMT+02:00 David McBride <toad3k at gmail.com>:
>
>> This is a common mistake that people who try to use type classes run
>> into.  I remember banging my head against it pretty hard when I first
>> started out.
>>
>> There's this temptation that  you should be able to write the following:
>>
>> class Foo a where
>>   bar :: a  -> String
>>
>> instance Read a => Foo a where
>>   bar a = read a
>>
>> instance Foo () where
>>   bar _ = "bar"
>>
>> But the problem with that is that now () applies to two conflicting
>> classes.  It is both a Read and a Foo. So when you go bar (), which
>> instance should fire?  The Foo instance or the Read () => Foo
>> instance?
>>
>> There are a multitude of ways you could try to resolve this.  Let's
>> say obviously the Read constrainted instance is more specific, we
>> should use that.  But then what if the user of your library happens to
>> have a instance Ord a => Foo a in his library, now which one of those
>> is more specific?  Read or Ord?
>>
>> Because of all these ambiguities during type checking ghc doesn't even
>> look at the constraint.  It would see instance Foo a, and instance Foo
>> (), and then say oh! those are overlapping instances because () could
>> apply to either class before you consider what constraints apply.
>>
>> There's actually several very in depth answers on stackoverflow for
>> this questions like this, such as this one:
>> https://stackoverflow.com/a/3216937/1500583  It might give you some
>> ideas on what to do about this.
>>
>> On Thu, Jun 29, 2017 at 1:15 PM, Silent Leaf <silent.leaf0 at gmail.com>
>> wrote:
>> > hi,
>> >
>> > say i have the following typeclass:
>> >
>> > class Foo a where
>> >   bar :: a  -> String
>> >
>> > looks a lot like the Read typeclass, right? (at least i think it
>> should?)
>> > well say it's a different meaning (in other terms i can't or do not
>> want to
>> > use Read, but i'd like to implement a default version of bar for those
>> > instances that also implement Read. is there a way to do so?
>> >
>> > _______________________________________________
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>> >
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>
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