[Haskell-cafe] [ANN] ttc-1.0.0.0 - Textual Type Classes

[Tom Smeding]

On 3 Jun 2021, at 11:18, Henning Thielemann
<lemming at henning-thielemann.de> wrote:
 >> I would not do that. Even orphan instances must be unique. If I
would decide to define my own instances but import a library that
transitively imports Data.TTC.Instances somewhere, I get a clash. Orphan
instances are really only helpful for the case where you need an
instance but you neither maintain the class nor the type definition but
otherwise are sure that your instance is the one and only.

While this is true, as far as I understand, avoiding orphan instances
(be it using MigMit's method below or by directly providing instances in
the library) requires that the library that provides the type class (TTC
in this case) depend on all packages necessary to even be able to write
those instances.

While this is clearly not an issue for Base types like Int, Float etc.,
some libraries seem to have the tendency to provide instances for random
data types that require pulling in additional dependencies that no-one
in the tree actually needs.

As someone who always gets slightly depressed when `cabal install XYZ`
gives a multi-screen list of packages to compile, I would like to
suggest that providing pre-made instances is cool, as long as that
doesn't introduce extra dependencies for the type-class-providing library.

How to solve the orphan problem, I also don't know. :)

Cheers,
Tom

On 03/06/2021 11:44, MigMit wrote:
> Agreed. I like to use DefaultSignatures like this:
>
> class Parse a where
>    parse :: ...
>    default parse :: ParseInternal a => ... -- same ... as above
>    parse = parseInternal
>
> class ParseInternal a where
>    parseInternal :: ... -- again, same ...
>
> instance ParseInternal Double where
>    parseInternal = ... -- specific implementation
> instance ParseInternal Float where
>    parseInternal = ... -- specific implementation
>
> Then all the user needs to do is to say
>
> instance Parse Double
>
> and it would automagically use the provided implementation.
>
>> On 3 Jun 2021, at 11:18, Henning Thielemann <lemming at henning-thielemann.de> wrote:
>>
>>
>> On Thu, 3 Jun 2021, Travis Cardwell wrote:
>>
>>> On Thu, Jun 3, 2021 at 4:49 PM Henning Thielemann wrote:
>>>> On Thu, 3 Jun 2021, Travis Cardwell wrote:
>>>>> The Render and Parse type classes in Data.TTC
>>>>> have no instances, allowing developers to write their own instances for
>>>>> Int for example, but some default instances can optionally be imported
>>>>> from Data.TTC.Instances when they are appropriate.
>>>>
>>>> Are these instances orphan?
>>>
>>> Indeed they are.  I use the following directive to hide the warnings for
>>> that module:
>>>
>>>    {-# OPTIONS_GHC -fno-warn-orphans #-}
>>
>> I would not do that. Even orphan instances must be unique. If I would decide to define my own instances but import a library that transitively imports Data.TTC.Instances somewhere, I get a clash. Orphan instances are really only helpful for the case where you need an instance but you neither maintain the class nor the type definition but otherwise are sure that your instance is the one and only.
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