On 10/01/2013 07:58 AM, Michael Snoyman wrote:
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
Is it really necessary to have a type class to do this? You can always just introduce 'io :: IO a -> Form a' to lift IO actions into a form. Then you just have: myBlogForm = Blog <$> titleForm <*> io getCurrentTime <*> contentsForm In digestive-functors, we have the base monad in the type of the form itself, so we provide 'monadic' which goes from m (Form m a) -> Form m a (hand-waving as there are actually other type constraints). You might get more power by following in those steps, and having the aforementioned 'io' function actually be: io :: IO (Form a) -> Form a - ocharles