Re: [Haskell-cafe] 123

Hey Dmitry, The 'Show1' class accomplishes this for types :: * -> *. https://hackage.haskell.org/package/transformers-0.5.1.0/docs/Data-Functor-C... Then you can write: instance Show1 f => Volume f where... On Thu, 17 Aug. 2017, 4:02 am Jack Henahan, 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 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 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.