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... 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.
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