[Haskell-cafe] Applicative and Monad transformers

Martijn van Steenbergen martijn at van.steenbergen.nl
Wed Aug 26 12:04:44 EDT 2009

Jeremy Shaw wrote:
> What I would prefer is:
> instance (Monad f, Applicative f) => Applicative (ReaderT r f) where
>     pure a = ReaderT $ const (pure a)
>     f <*> a = ReaderT $ \r -> 
>               ((runReaderT f r) <*> (runReaderT a r))

Right. This doesn't only go for ReaderT, it already goes for Either, 
too: you don't want the 'ap' implementation for <*> there either.

These are beautiful examples of how applicative style gives the caller 
less power, but the callee more information, allowing more information 
to be retained. In this case it allows you to concatenate errors using 

Another example is parsing: I believe Doaitse's parsers allow more 
optimization if they are only used in applicative style (but I'm not 
sure of this).

This shows there can be several sensible implementations of a type 
class. You ask which instance is right--that depends entirely on what 
you want it to do! Setting (<*>) = ap is just one of them, one you 
happen to get for free if your functor is already a monad.

Hope this helps,


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