Proposal #2316: Add Applicative instances for all MTL Monads

Ross Paterson ross at
Thu May 29 10:58:07 EDT 2008

On Thu, May 29, 2008 at 06:44:12AM -0400, Isaac Dupree wrote:
> Spencer Janssen wrote:
>> This proposal is very straightforward: add Applicative instances for all the
>> Monads in the MTL.
> It's not quite as straightforward as it looks:
> If you only need an
> instance (Applicative m) => Applicative (Transformer m),
> not
> instance (Monad m) => Applicative (Transformer m),
> will you use the first one?  (I think yes.)

Yes, and ditto for the Functor instances, where possible.

Also, Alternative instances for everything that has a MonadPlus instance.

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