Yesod 1.2's cleaner internals

March 11, 2013

GravatarMichael Snoyman

The past 48 hours have been very exciting for me: I've done possibly the largest refactoring of the Yesod codebase to date. Most of it was simply restructuring the internal code and non-user-facing APIs. But there are some user visible changes going on right now, and I think now's a good time to document them. I also have some ideas for even more radical changes, which I'll touch on at the end of this post.

Representations

The most important change I've just implemented was an overhaul of the representation system. This may not even be a concept most Yesod users are familiar with, but it happens to be one of the oldest features of Yesod. The idea is to allow a single function to return a different representation of the data (HTML, JSON, etc) depending on the client's Accept header.

Let's start by defining two datatypes which we'll use below. A ContentType is simply a ByteString holding the raw mimetype. A Content is a bit more complicated: it's a sum type which can be a blaze-builder Builder, a conduit Source, or a few other things which can be easily converted into a response body. There's a ToContent typeclass which provides a function toContent which converts many common datatypes into a Content.

I don't want to dwell on the old approach to much, but it boiled down to every handler function returning something that looked like this:

[ContentType] -> (ContentType, Content)

The input [ContentType] is a list of mime types that the client as requested, ordered by preference. You can then write a function that will determine the appropriate content based on what the user accepts. For example:

myResponse userContentTypes =
    loop userContentsTypes
  where
    loop ("text/html":_) = ("text/html", htmlContent)
    loop ("application/json":_) = ("application/json", jsonContent)
    loop (_:rest) = loop rest
    loop [] = ("text/html", htmlContent)

This works out fairly nicely, except for one detail: it requires you to perform all I/O actions (such as database queries) before you know which representation of the data you need to provide to the user. This unfortunately doesn't work out too well in practice, and therefore representations have not been well utilized in Yesod.

In Yesod 1.2, we're going to have a completely different approach. A handler function will instead need to return an instance of the ToTypedContent typeclass, which looks like this:

data TypedContent = TypedContent ContentType Content
class ToTypedContent a where
    toTypedContent :: a -> TypedContent

We define some sensible instances in yesod-core itself:

instance ToTypedContent Html where
    toTypedContent html = TypedContent "text/html" (toContent html)
instance ToTypedContent Data.Aeson.Value where
    toTypedContent value = TypedContent "application/json" (toContent value)
instance ToTypedContent Text where
    toTypedContent text = TypedContent "text/plain" (toContent text)

With this change, it's now perfectly acceptable to create a JSON response with something as simple as return $ toJSON myValue. But how do we deal with multiple representations? For that, we have a pair of helper functions, which are easiest to understand with a simple example.

getPersonR personid = do
    person <- runDB $ get404 personid
    selectRep $ do
        provideRep $ defaultLayout $ do
            setTitle "Some Person"
            extraInfo <- getExtraInfo person
            $(widgetFile "person")
        provideRep $ toJSON person
        provideRep $ (personName person :: Text)

selectRep will get the parsed contents of the HTTP Accept header and determine which representation should be used. provideRep provides an additional representation for selection. Notice how you don't even need to state the mime type: it's all inferred automatically through the type system. (If you need something more dynamic, provideRepType is available as well.)

In our example, we start off with an HTML representation. This representation requires some extra data to be looked up. We can safely perform the getExtraInfo call inside the provideRep call, and the overhead of that extra call will not affect the JSON and plain text representations.

You can also see the code behind selectRep and provideRep itself.

I've already used this new API to clean up a few nagging issues (error messages and authentication responses are now representation-aware). I'm hopeful that this change makes it much more convenient for developers to create sites catering to both a plain-HTML and rich client view.

Caching

Continuing on the trend of inferring information from the types, we also have a new request-local caching mechanism. A prime use case for this is authentication checking. In a typical Yesod application, you'll need to check if a user is logged in in a number of different places: the authorization code, the defaultLayout function, database functions, and the handler itself. Having to do a database round trip for the same data multiple times is inefficient; we should be able to cache that data somewhere.

Yesod has had request local storage for a while now, but it required generation of a unique key. Possible approaches to this are using Template Haskell to generate one, or Data.Unique and some unsafePerformIO. But both of those approaches are just inconvenient enough that this feature went unused.

Luite, Felipe, Greg and I all discussed this a while ago, and Felipe mentioned using TypeRep (i.e., the Typeable typeclass) as a unique key. This requires you to create a newtype wrapper for each piece of data you want cached, but otherwise is unobtrusive. With that approach in hand, the entire caching API becomes a single function:

cached :: Typeable a => Handler a -> Handler a

To implemented our cached maybeAuth function, we could do something like:

newtype CachedUser = CachedUser { unCachedUser :: Maybe User }
    deriving Typeable

cachedMaybeAuth = fmap unCachedUser $ cached $ fmap CachedUser maybeAuth

A variant of this has already been applied to the real maybeAuth in Yesod.Auth, so no changes are required to your code, except ensuring that your User type is an instance of Typeable.

The implementation itself is actually pretty simple.

Handler typeclasses

The Yesod.Handler module (which will probably become Yesod.Core.Handler soon) has a number of functions that can be used when writing Handler code. This includes looking up GET parameters, sending redirects, and modifying the session. And through Yesod 1.1, all of these functions have lived in the GHandler monad.

There are a few reasons why this is suboptimal:

  1. It doesn't allow us to lift into monad transformers automatically. A common workflow is using defaultLayout or runDB to deal with widgets or Persistent, and then needing to lift some operations from the GHandler monad. It would be nice to have automatic lifting.

  2. GHandler, like IO, is a bit of a "sin bin." There's absolutely no control over what a user may do there. By moving operations into typeclasses instead, we can isolate non-mutating effects, mutating effects, short-circuit effects, and IO actions, giving us more knowledge about what our code is doing from the types themselves. I don't know how many people will be interested in using the typeclasses in this way, but I have heard people express interest in this previously.

The move to typeclasses also opens up the possibility for some more radical changes in the future without massive disruption for users. I'll touch on some of these thoughts at the end of this post.

YesodRequest/YesodResponse

This was actually the most invasive change to the codebase, but as it only affects the internals I've left it toward the end of this post. In the WAI world, we have a very simple model for an application: it takes a Request, and returns a Response. The Yesod world seems to drastically complicate that simple approach.

But in reality, Yesod also has the same simplistic approach available, it's just always been buried under piles of code and strangely named functions/data types. My refactoring makes this much clearer.

  • The Request datatype from WAI does a very minimal amount of parsing. However, in Yesod we require a bit more processing of the incoming request to be performed. We store this extra information in the YesodRequest datatype. This adds information like cookies and the user session.
  • Similarly, The Response datatype from WAI is very low-level. A Yesod app may want to return a much more high-level response in terms of status code, content type, content, and an updated session. To allow for this, the YesodResponse data type allows you to return either this higher-level response, or a low-level WAI response instead.
  • As a parallel to the Application type in WAI, Yesod provides the YesodApp synonym, which simply takes a YesodRequest and returns a YesodResponse.
  • We need to store some kind of environment information which is not request-specific, so that our handler functions have access to it. This includes logging functions, error handlers, and the foundation datatype. All of this goes into RunHandlerEnv.
  • There's also mutable data for each request, like the session and the cache. This goes in GHState.
  • We finally tie up the per-request immutable data, environment data, and mutable data into a single datatype: HandlerData. Note that we wrap up the mutable data in an IORef instead of using a State monad since we need to maintain a consistent state even in the presence of exceptions.
  • With all of that out of the way, our GHandler monad is much less mysterious: it's just a Reader providing access to the HandlerData.
  • runHandler will take GHandler and convert it into a YesodApp by feeding it a HandlerData and converting the output into a YesodResponse. defaultYesodRunner takes this a step further and creates a plain WAI Application.

If there are things that are unclear in the explanation above, please let me know. I intend to include this text, or some version of it, in the code as a high-level architectural view of yesod-core.

Dispatch is a different beast

I like to classify Yesod as a Model-View-Controller (MVC) framework. In this approach, the model is handled by Persistent (or whatever replacement you use for data storage) and the view is handled by Shakesepare (again, this can be replaced). The last piece of the puzzle, Controller, is what Yesod itself- and the yesod-core package in particular- deal with.

Within controller, we can also make a few smaller pieces. The HTTP layer itself is handled by WAI, for example. The final two pieces handled by yesod-core are dispatch and handlers. Other frameworks will combine these two concepts together; I think there's huge value to be gained by keeping them as separate components.

So far, I've been focused entirely on the handler aspect of the puzzle. Dispatch has already somewhat moved out of yesod-core itself into the yesod-routes package which, despite its name, is actually not Yesod-specific, as can be seen by some of its dependencies. My goal is to separate out even more of the dispatch functionality from yesod-core itself, to open the door for others to use even more of the functionality outside of Yesod, and to hopefully make a better product for Yesod as well.

I haven't given this part of the process too much thought yet, but I'm definitely playing around with the thought of merging in the ideas from yesod-pure. I still believe that code generation is the best bet for a robust dispatch system, but having that code generation built on top of a more powerful, user accessible library will make the generated code more transparent.

If anyone has thoughts on this part of the refactoring, please let me know!

Better subsite approach for the future?

I had thought I would discuss some radical ideas for ways to clean up the subsite system in Yesod, but this post is already long enough. I'll come out with a separate post in a few days. I'll give this leader though: this change is significantly more breaking in nature than anything I've implemented so far, so I'm hesitant to move ahead with it. I might hold it off for the 1.2 release, and then have a Yesod 2.0 release in the not-too-distant future with a higher level of breakage.

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