[Haskell-cafe] Lifting IO actions into Applicatives

Michael Snoyman michael
Tue Oct 1 06:58:54 UTC 2013

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?


[1] Full crazy definition is at:
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