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

Edward Kmett ekmett at
Tue Feb 24 15:39:51 UTC 2015

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

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