[Haskell-cafe] 123

[Isaac Elliott]

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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