Re: Proposal: Add (&) to Data.Function

On Tue, Nov 20, 2012 at 9:27 PM, Edward Kmett wrote:

Note: (>>>) is a completely different operator.

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is flipped (.). The proposed & is flipped $.

(>>>) :: Category cat => cat a b -> cat b c -> cat a c

(&) :: a -> (a -> b) -> b

-Edward

...and my quibble with (&) as flip ($) is that I think it would, in fact, be more mnemonic as a synonym for (>>>). Consider: filter even & map (*2) & sum That reads naturally and obviously as "filter by even and double and sum". It's an operation. What if we apply it to an argument? (filter even & map (*2) & sum) theList Still works. Filter by even and double and sum the list. What if (&) is flip ($) instead? theList & filter even & map (*2) & sum The second two are fine enough (same as before), but the first one is weird. What does it mean to "the list and filter by even"? It feels like a type error.

On Tue, Nov 20, 2012 at 3:14 PM, Henning Thielemann wrote:

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On Tue, 20 Nov 2012, Yitzchak Gale wrote:

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It is a common idiom to write a sequence of composed combinators in reverse order to the way they would be written with ($) or (.). That naturally expresses the idea of the combinators as operations being applied in the given order.

This comes up so often, and is commonly used so many times in a single expression, that Control.Arrow.>>> is far too wordy, and even a two- character operator is awkward.

Functional metapost called it (#). But for me (>>>) is ok. It is even more descriptive than (&).

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