remove 'pure' method from Arrow class

[ross]

The Arrow class as originally defined by John Hughes had methods arr, >>> and first. (>>> has since been moved by #1773 to the Category class.) When writing the Fun of Programming paper, I added pure as a synonym for arr, because Richard Bird preferred it. However this name hasn't caught on, and now it clashes with a method in the Applicative class, so I propose to remove it.

The usual practice would be to deprecate the name in one release and remove it in the following one, but I propose to remove it in one step because

• no-one seems to be using this name, and
• backward compatibility has been broken anyway by the Category split (#1773).

The only people who will be bitten by the change are those who import Control.Arrow hiding pure, and they wouldn't be warned by deprecation.

[ross]

Discussion was limited, with a few supporters and no dissent.

[conal]

Does anyone have a way to make code work before and after the removal of pure from Arrow? I import Arrow with "hiding (pure)" quite a lot, which is now required up to recent Arrow versions and forbidden after.

[ross]

Replying to conal:

Does anyone have a way to make code work before and after the removal of pure from Arrow? I import Arrow with "hiding (pure)" quite a lot, which is now required up to recent Arrow versions and forbidden after.

I suppose you don't want to hear about cpp. You could explicitly import all the stuff from Control.Arrow that you need, or qualify Control.Applicative.pure.