[Haskell-cafe] Lifting IO actions into Applicatives

[Michael Snoyman]

On Tue, Oct 1, 2013 at 10:24 AM, Alexey Uimanov <s9gf4ult at gmail.com> wrote:

> Maybe this is needed new typeclass ApplicativeTrans?
>
>
>
There's actually no problem with defining a MonadTrans instance for
non-monads. Obviously this can't follow the laws directly (since they're
defined in terms of monadic bind and return), but I think we could probably
state Applicative versions of those laws (assuming I haven't made a stupid
mistake):

lift . pure = pure
lift (x <*> y) = lift x <*> lift y

Michael


> 2013/10/1 Michael Snoyman <michael at snoyman.com>
>
>> I'm wondering if anyone's run into this problem before, and if there's a
>> common solution.
>>
>> In Yesod, we have applicative forms (based originally on formlets). These
>> forms are instances of Applicative, but not of Monad. Let's consider a
>> situation where we want to get some user input to fill out a blog post
>> datatype, which includes the current time:
>>
>> data Blog = Blog Title UTCTime Contents
>>
>> myBlogForm :: Form Blog
>> myBlogForm = Blog <$> titleForm <*> something <*> contentsForm
>>
>>  The question is: what goes in something? Its type has to be:
>>
>> something :: Form UTCTime
>>
>> Ideally, I'd call getCurrentTime. The question is: how do I lift that
>> into a Form? Since Form is only an Applicative, not a Monad, I can't create
>> a MonadIO instance. However, Form is in fact built on top of IO[1]. And
>> it's possible to create a MonadTrans instance for Form, since it's entirely
>> possible to lift actions from the underlying functor/monad into Form. So
>> something can be written as:
>>
>> something = lift $ liftIO getCurrentTime
>>
>> This works, but is unintuitive. One solution would be to have an
>> ApplicativeIO typeclass and then use liftIOA. My questions here are:
>>
>> 1. Has anyone else run into this issue?
>> 2. Is there an existing solution out there?
>>
>> Michael
>>
>> [1] Full crazy definition is at:
>> http://haddocks.fpcomplete.com/fp/7.4.2/20130922-179/yesod-form/Yesod-Form-Types.html#t:AForm
>>
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>
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