Proposal: Applicative => Monad: Call for consensus

John Smith voldermort at
Sun Jan 2 13:04:30 CET 2011

The patches attached to make Applicative a superclass of Monad. Default 
definitions are provided for backwards compatibility.

Advantages: The class hierarchy correctly models the logical relationship between Applicative and Monad.
             Boilerplate Applicative instances that duplicate Monad functions will no longer be required.

Disadvantage: All Monads will be Applicatives, even if you're only interested in the Monad interface. This is akin to 
all Ords being Eqs, even if you're not using ==.

Backwards Compatibility: Existing Monad definitions will have to be changed to declare Applicative. The original 
function bodies can be used unchanged, but they are now declared in a different class. Calling code should work unchanged.

This ticket has already been discussed on the mailing lists. The purpose of this message is to call for consensus by the 
deadline of 1 February.

Please note that although the ticket references the wiki page at, the patches attached to the ticket are much more 
conservative than the more ambitious reforms on the wiki. (I would like to change the ticket description to make this 
clearer, but I can't edit it.)

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