Proposal: Add (<$>) to Prelude as part of the AMP in GHC 7.10-RC3

[Greg Fitzgerald]

Those operators are for Power Users - why put them in the Prelude?  I
mean, they are so money, I can't find words to describe them.  Below
are a few attempts.  I start with a description in English, then
implement using Functor's bling operators, and then again using the
Prelude as-is.  In each case, the code is shorter with the bling
operators, but if you try to verbalize it, it's nonsense and the
syntax is ungoogleable.


import Data.Functor((<$>), (<$), ($>))

-- Map (+1) on a computation
moneyCentric, moneyCentric2 :: IO Int
moneyCentric  = (+1) <$> return 1
moneyCentric2 = fmap (+1) $ return 1

-- Map (const 2) on a computation
lessMoney, lessMoney2 :: IO Int
lessMoney  = 2 <$ return 1
lessMoney2 = fmap (const 2) $ return 1

-- Run a computation, but then return 2.
moneyGreaterThan, moneyGreaterThan2 :: IO Int
moneyGreaterThan  = return 1 $> 2
moneyGreaterThan2 = return 1 `butReturn` 2
  where
    m `butReturn` x = m >> return x



And check this out:

Prelude> 2 <$ return 1

<interactive>:2:3:
    Not in scope: ‘<$’
    Perhaps you meant one of these:
      ‘<’ (imported from Prelude), ‘<=’ (imported from Prelude),
      ‘$’ (imported from Prelude)


The syntax '<$' is close enough to '<=' and '$' that ghci guesses it
was a typo.  Shouldn't we assume this syntax would make a beginner's
head spin?  If you really want to use those operators, import
Data.Functor.  Why must it be in the Prelude?

-Greg


> Dne út 24. 2. 2015 16:39 uživatel Edward Kmett <ekmett at gmail.com> napsal:
>>
>> 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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