[Haskell-cafe] instance Applicative f => Applicative (StateT s f)

[Thomas Davie]

On 5 Dec 2008, at 16:54, Ross Paterson wrote:

> On Fri, Dec 05, 2008 at 04:35:51PM +0100, Martijn van Steenbergen  
> wrote:
>> How do I implement the following?
>>
>>  instance Applicative f => Applicative (StateT s f)
>>
>> The implementation of pure is trivial, but I can't figure out an
>> implementation for <*>. Is it possible at all, or do I need to  
>> require f
>> to be a monad?
>
> Yes, because you need part of the value generated by the first  
> computation,
> namely the state (inside the f), to construct the second one.  You  
> can do
> that in a Monad, but not in an Applicative.

I don't think that's true, although I'm yet to decide if Applicative  
for State is possible.

someState <*> someOtherState should take the value out of the first  
state, take the value out of the second state, apply one to the other,  
and return a new stateful value as the result.  At no point in that  
description do I make mention of the previous state of one of these  
values.

Bob