Definitely +1 on <$>. +0.5 on <$, I suspect this one being in the Prelude
or not isn't as big a deal as for <$>.
On Fri, Feb 27, 2015 at 9:58 AM, S. Doaitse Swierstra wrote: They definitely should be in both, since the default implementations of
<$> and <$ f <$> x = pure f <*> x
f <$ x = const f <*> x may be less efficient than a special implementation having more knowledge
about applicative. Doaitse PS: for that reason they are for almost 20 years now member of the class
IsParser in
http://hackage.haskell.org/package/uulib-0.9.16/docs/UU-Parsing-Interface.ht... ,
which inspired the introduction of Applicative and Alternative in the first
place. On 27 Feb 2015, at 5:45 , Edward Kmett On Thu, Feb 26, 2015 at 6:37 PM, Greg Fitzgerald Those operators are for Power Users - why put them in the Prelude? .. because without them that whole Applicative thing that we're bringing
in as a superclass of Monad with the AMP is cut off at the knees. Applicative sugar is typically used like both f (x,y) = (,) <$> x <*> y or traverse f (x:xs) = (:) <$> f x <*> traverse f xs
traverse f [] = pure [] Without (<$>), (<*>) from Applicative is quite difficult to use. both f (x,y) = pure (,) <*> x <*> y is much less efficient and both f (x,y) = fmap (,) x <*> y is painfully far from idiomatic. -Edward -Greg Dne út 24. 2. 2015 16:39 uživatel Edward Kmett We have a couple of weeks until the third release candidate for GHC 7.10 goes out the door. Along the way with the last couple of release candidates folks have
found
some problems with the way we implemented the AMP. [1][2] Most notably, we failed to include (<$>) in the Prelude, so the
standard
idiom of foo <$> bar <*> baz <*> quux doesn't work out of the box! I'd like to include (<$>) in the Prelude in RC3. I'd also like to invite discussion about whether folks believe we
should
include (<$) out of the box. (<$) has been a member of Functor for a long time, which is only
visible
if you import it from Data.Functor or bring in Control.Applicative.
There is
an idiom that you use (<*) and (<$) to point to the parts of the
structure
that you want to keep the answers from when building longer such
Applicative
chains. Discussion Period: 2 weeks Thank you,
-Edward Kmett [1] http://www.reddit.com/r/haskell/comments/2wzixa/shouldnt_be_in_prelude/ [2] https://plus.google.com/115504368969270249241/posts/URzeDWd7qMp
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