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

[Silent Leaf]

Sylvain Henri's link did the trick, that's what i was looking for (they
call it generic implementation).

in terms of terminology, is there an intrinsic difference between generic
and polymorphic?

2017-06-29 20:10 GMT+02:00 Silent Leaf <silent.leaf0 at gmail.com>:

> 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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