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

Hécate hecate at glitchbra.in
Tue Nov 10 13:23:14 UTC 2020

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 

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 => 

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)
 >>> un (Size 5) == length ['a', 'x', 'b']

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

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 

 >>> 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 
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

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