Proposal: Export (#.) and (.#) from Data.Coerce (Libraries Digest, Vol 176, Issue 8)

[Edward Kmett]

Note: this way of indicating which argument is unused means that this
confusingly looks almost precisely backwards relative to the more general
code
<https://hackage.haskell.org/package/profunctors-5.2.2/docs/Data-Profunctor-Unsafe.html>
that created those names and usage pattern in the first place. (I don't
have a particularly strong objection to allowing the same sort of tweak to
that code, assuming type inference works out in practice for the major
consumers of the combinators there. I don't really foresee a problem, but
I've been surprised by interactions before.)

I do have some concern that exporting these incompatible versions from
Data.Coerce would break a subset of the code that is using the more general
combinators in profunctors. e.g. in the lens library these were coined for
the code for prisms imports both.

This is why they were placed in a more obscure internal location to begin
with as just the special case was needed by base and it was easier to write
a copy locally than merge Profunctor to base.

-Edward

On Sat, Apr 21, 2018 at 10:16 AM, Andrew Martin <andrew.thaddeus at gmail.com>
wrote:

> Generalizing the type like that helps a lot. I had skimmed the
> documentation before, but I didn’t immediately comprehend what it meant.
> When I read the type signatures you gave them, it immediately made sense.
>
> Also, +1 on exporting these.
>
> Sent from my iPhone
>
> > On Apr 21, 2018, at 10:06 AM, Li-yao Xia <lysxia at gmail.com> wrote:
> >
> > That seems useful indeed! Using only 'coerce' requires too many type
> annotations.
> >
> > Would it make sense to generalize the type so it's clear that one
> argument is unused?
> >
> > (#.) :: Coercible b c => p b c -> (a -> b) -> (a -> c)
> > (.#) :: Coercible a b => (b -> c) -> p a b -> (a -> c)
> >
> > Li-yao
> >
> >> On 04/20/2018 03:56 PM, Daniel Cartwright wrote:
> >> (#.) :: Coercible b c => (b -> c) -> (a -> b) -> (a -> c)
> >> (#.) _ = coerce
> >> {-# INLINE (#.) #-}
> >> (.#) :: Coercible a b => (b -> c) -> (a -> b) -> (a -> c)
> >> (.#) f _ = coerce f
> >> {-# INLINE (.#) #-}
> >> The first of these is exported from Data.Functor.Util, and used in many
> >> places inside of base for efficiency over '.' (compose), However no
> module
> >> in base actually exports these. I have recently been using Data.Coerce
> more
> >> frequently and think it would be useful to go ahead and export these
> from
> >> somewhere in base.
> >> For convenience, I will paste the note about (#.) from
> Data.Functor.Util:
> >> "Note [Function coercion]
> >> ~~~~~~~~~~~~~~~~~~~~~~~
> >> Several functions here use (#.) instead of (.) to avoid potential
> efficiency
> >> problems relating to #7542. The problem, in a nutshell:
> >> If N is a newtype constructor, then N x will always have the same
> >> representation as x (something similar applies for a newtype
> deconstructor).
> >> However, if f is a function,
> >> N . f = \x -> N (f x)
> >> This looks almost the same as f, but the eta expansion lifts it--the lhs
> >> could
> >> be _|_, but the rhs never is. This can lead to very inefficient code.
> Thus
> >> we
> >> steal a technique from Shachaf and Edward Kmett and adapt it to the
> current
> >> (rather clean) setting. Instead of using  N . f,  we use  N #. f, which
> is
> >> just
> >> coerce f `asTypeOf` (N . f)
> >> That is, we just *pretend* that f has the right type, and thanks to the
> >> safety
> >> of coerce, the type checker guarantees that nothing really goes wrong.
> We
> >> still
> >> have to be a bit careful, though: remember that #. completely ignores
> the
> >> *value* of its left operand.
> >> "
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