[Proposal] Integration of helpers to operate on Newtypes in base

[David Feuer]

Certainly you can work around not having good inference. But at what point
are you better off just using `coerce` directly?

On Wed, Nov 11, 2020, 5:01 AM Hécate <hecate at glitchbra.in> wrote:

> Do you think that they will produce unwanted inference even with
> -XTypeApplications?
> Le 11/11/2020 à 08:42, David Feuer a écrit :
>
> For the most part, I oppose adding these to base. I'm most sympathetic to
> adding (.#) and (#.), originally from the profunctors package, as they seem
> to pop up quite a lot. For the rest, inference can be a real problem.
> coercible-utils goes to quite a lot of trouble to make these functions
> reasonably usable, but I don't think we want that sort of machinery in
> `base`.
>
> On Tue, Nov 10, 2020, 8:23 AM Hécate <hecate at glitchbra.in> wrote:
>
>> Hello, CLC and haskell-libraries,
>>
>> I am opening a proposal process to consider the integration of several
>> helper functions in `base`, operating on Newtypes, and all based on
>> `coerce`.
>>
>> My motivations are that we ought to provide a minimum set of tools in
>> order to work effectively with one of our most beloved and ubiquitous
>> language features.
>>
>> Now, these functions that I am about to present to you all do not come
>> out of nowhere. They have been integrated to Kowainik's alternative prelude
>> "Relude", and seem
>> to have found their use amongst their users, me included.
>> Their documentation can be found here =>
>> https://hackage.haskell.org/package/relude-0.7.0.0/docs/Relude-Extra-Newtype.html
>> but I am reproducing them below for convenience:
>>
>> ---
>> un :: forall a n. Coercible a n => n -> a
>>
>> Unwraps value from newtype.
>>
>> ```
>> >>> newtype Size = Size Int deriving Show
>> >>> un @Int (Size 5)
>> 5
>> >>> un (Size 5) == length ['a', 'x', 'b']
>> False
>> ```
>>
>> ---
>> wrap :: forall n a. Coercible a n => a -> n
>>
>> Wraps value to newtype. Behaves exactly as 'un' but has more meaningful
>> name in case you need to convert some value to newtype.
>>
>> ```
>> >>> newtype Flag = Flag Bool deriving (Show, Eq)
>> >>> wrap False == Flag True
>> False
>> ```
>>
>> ---
>> under :: forall n a. Coercible a n => (n -> n) -> a -> a
>>
>> Applies function to the content of newtype. This function is not supposed
>> to be used on newtypes that are created with the help of smart constructors.
>>
>> ```
>> >>> newtype Foo = Foo Bool deriving Show
>> >>> under not (Foo True)
>> Foo False
>> >>> newtype Bar = Bar String deriving Show
>> >>> under (filter (== 'a')) (Bar "abacaba")
>> Bar "aaaa"
>> ```
>>
>> As well as the coerced composition operator:
>>
>> (#.) :: Coercible b c => (b -> c) -> (a -> b) -> (a -> c)
>> (#.) _f = coerce
>> {-# INLINE (#.) #-}
>>
>> Which currently lives in
>> https://hackage.haskell.org/package/base-4.14.0.0/docs/src/Data.Functor.Utils.html#%23
>> but is not exported.
>>
>>
>> Regarding the location of these functions, I either see them living in
>> their own "Data.Newtype", or they could join Data.Coerce.
>> I would personally create a new module as to avoid "polluting"
>> Data.Coerce with non-class functions, but this is my personal preference.
>>
>>
>> Thank you for reading.
>>
>> --
>> Hécate ✨
>> 🐦: @TechnoEmpress
>> IRC: Uniaika
>> WWW: https://glitchbra.in
>> RUN: BSD
>>
>> _______________________________________________
>> Libraries mailing list
>> Libraries at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>
> --
> Hécate ✨
> 🐦: @TechnoEmpress
> IRC: Uniaika
> WWW: https://glitchbra.in
> RUN: BSD
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20201111/ff1942f9/attachment.html>