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

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-U... 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 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

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On Apr 21, 2018, at 10:06 AM, Li-yao Xia 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

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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. "