[Haskell-cafe] 123

[Dmitry Olshansky]

It seems that another option could be:

instance Foldable f => Show (A f) where
  ...





2017-08-16 23:53 GMT+03:00 Isaac Elliott <isaace71295 at gmail.com>:

> Hey Dmitry,
>
> The 'Show1' class accomplishes this for types :: * -> *.
>
> https://hackage.haskell.org/package/transformers-0.5.1.0/
> docs/Data-Functor-Classes.html
>
> Then you can write: instance Show1 f => Volume f where...
>
> On Thu, 17 Aug. 2017, 4:02 am Jack Henahan, <jhenahan at me.com> wrote:
>
>>
>> Some mostly unrelated thoughts:
>>
>> An instance head has the form `T a_1 ... a_n`, and the constraint can only
>> apply to the `a_i`s. Consider the Show instance for
>> pairs.
>>
>> instance (Show a, Show b) => Show (a, b) -- Show ((,) a b)
>>
>> The constraints only act on the parameters of the type.
>>
>> It looks like you're taking the constraint to mean "whenever I have a
>> Showable
>> `f String`, this is how to define a Show instance", but a constraint
>> actually means "use this rule to make a Show instance for any `A f`, and
>> it is an error if a Show instance for `f String` is not in scope".
>>
>> In the second error, you are making the strong claim that your Show
>> instance for `A f` holds for any `f` and `a`. Even if you could trick
>> the compiler into allowing that, I don't think it would actually express
>> the constraint that you want it to.
>>
>> Is there something a Show instance gets you that a pretty-print function
>> wouldn't?
>>
>> Dmitriy Matrosov <sgf.dma at gmail.com> writes:
>>
>> > Hi.
>> >
>> > Is there a way to avoid `UndecidableInstances` in following code:
>> >
>> >     data A f        = A {_a1 :: f String}
>> >
>> >     instance Show (f String) => Show (A f) where
>> >
>> > it does not compile with
>> >
>> >     1.hs:4:10: error:
>> >         • The constraint ‘Show (f String)’
>> >             is no smaller than the instance head
>> >           (Use UndecidableInstances to permit this)
>> >         • In the instance declaration for ‘Show (A f)’
>> >
>> > Though, initially, this was
>> >
>> >     {-# LANGUAGE RankNTypes #-}
>> >
>> >     data A f        = A {_a1 :: f String}
>> >
>> >     instance forall f a. Show (f a) => Show (A f) where
>> >
>> > which also does not compile with
>> >
>> >     1.hs:5:10: error:
>> >         • Variable ‘a’ occurs more often
>> >             in the constraint ‘Show (f a)’ than in the instance head
>> >           (Use UndecidableInstances to permit this)
>> >         • In the instance declaration for ‘Show (A f)’
>> >
>> > The error is different and i don't sure, that this two cases are
>> related.
>> >
>> > I want these instances to make a type with many records parametrized by
>> > `Alternative` type, e.g.
>> >
>> >     data Volume t       = Volume
>> >                             { _volName  :: t Name
>> >                             , _volSize  :: t Size
>> >                             , _volPath  :: t Path
>> >                             , _pool     :: t Pool
>> >                             }
>> >
>> > When i try to make instances, which require `*` type, i will end with
>> > above cases.
>>
>>
>> --
>> Jack
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