<div dir="auto">If you're playing around with these ideas, you may appreciate the coercible-utils package[*]. I basically rewrote the package last year to get really good type inference at the cost of some (hard to use) flexibility.<div dir="auto"><br></div><div dir="auto">[*] <a href="https://hackage.haskell.org/package/coercible-utils">https://hackage.haskell.org/package/coercible-utils</a><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Nov 10, 2020, 8:23 AM Hécate <<a href="mailto:hecate@glitchbra.in">hecate@glitchbra.in</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div>
<p>Hello, CLC and haskell-libraries,<br>
<br>
I am opening a proposal process to consider the integration of
several helper functions in `base`, operating on Newtypes, and all
based on `coerce`.<br>
<br>
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.<br>
<br>
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<br>
to have found their use amongst their users, me included.<br>
Their documentation can be found here =>
<a href="https://hackage.haskell.org/package/relude-0.7.0.0/docs/Relude-Extra-Newtype.html" target="_blank" rel="noreferrer">https://hackage.haskell.org/package/relude-0.7.0.0/docs/Relude-Extra-Newtype.html</a>
<br>
but I am reproducing them below for convenience:<br>
<br>
---<br>
<a id="m_4016565528343698598v:un" rel="noreferrer">un :: forall a n. Coercible a n => n
-> a<br>
<br>
Unwraps value from newtype.<br>
<br>
```<br>
>>> newtype Size = Size Int deriving Show<br>
>>> un @Int (Size 5)<br>
5<br>
>>> un (Size 5) == length ['a', 'x', 'b']<br>
False<br>
```<br>
<br>
</a>---<br>
wrap :: forall n a. Coercible a n => a -> n <br>
<br>
Wraps value to newtype. Behaves exactly as 'un' but has more
meaningful name in case you need to convert some value to newtype.<br>
<br>
```<br>
>>> newtype Flag = Flag Bool deriving (Show, Eq)<br>
>>> wrap False == Flag True<br>
False<br>
```<br>
</p>
<p>---<br>
under :: forall n a. Coercible a n => (n -> n) -> a ->
a<br>
<br>
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.<br>
<br>
```<br>
>>> newtype Foo = Foo Bool deriving Show<br>
>>> under not (Foo True)<br>
Foo False<br>
>>> newtype Bar = Bar String deriving Show<br>
>>> under (filter (== 'a')) (Bar "abacaba")<br>
Bar "aaaa"<br>
```<br>
<br>
As well as the coerced composition operator:<br>
<br>
(#.) :: Coercible b c => (b -> c) -> (a -> b) -> (a
-> c)<br>
(#.) _f = coerce<br>
{-# INLINE (#.) #-}<br>
<br>
Which currently lives in
<a href="https://hackage.haskell.org/package/base-4.14.0.0/docs/src/Data.Functor.Utils.html#%23" target="_blank" rel="noreferrer">https://hackage.haskell.org/package/base-4.14.0.0/docs/src/Data.Functor.Utils.html#%23</a>
but is not exported.<br>
<br>
<br>
Regarding the location of these functions, I either see them
living in their own "Data.Newtype", or they could join
Data.Coerce.<br>
I would personally create a new module as to avoid "polluting"
Data.Coerce with non-class functions, but this is my personal
preference.<br>
<br>
<br>
Thank you for reading.<br>
<br>
</p>
<pre cols="72">--
Hécate ✨
🐦: @TechnoEmpress
IRC: Uniaika
WWW: <a href="https://glitchbra.in" target="_blank" rel="noreferrer">https://glitchbra.in</a>
RUN: BSD</pre>
</div>
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</blockquote></div>