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

[Hécate]

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 
> <mailto: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
>     <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
>     <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  <https://glitchbra.in>
>     RUN: BSD
>
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-- 
Hécate ✨
🐦: @TechnoEmpress
IRC: Uniaika
WWW: https://glitchbra.in
RUN: BSD

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