[Haskell-cafe] Lifting IO actions into Applicatives

[Alexey Uimanov]

Maybe this is needed new typeclass ApplicativeTrans?


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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