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I think we're overdue on having a new example Yesod application. This time, I want to demonstrate reusable components, specifically Widgets.

I saw a blog post about Yesod's composability recently. My takeaway from the post is twofold:

  1. It's not clear to some users how to create reusable components in Yesod.

  2. We're lagging behind systems like ASP.NET by having a smaller ecosystem of reusable components readily available.

I think it's too easy to misinterpret these two points as saying the Yesod is missing out on features to produce reusable components. As I mentioned in a previous post, there are three main approaches to creating reusable components in Yesod: creating general purpose functions (not Yesod-specific), widgets, and subsites.

Of the three, I think the widgets are currently the area we should focus on the most. They provide the ability to create collections of HTML, CSS, Javascript, and external dependencies that can be placed in other pages. One way in which we take advantage of this in the core Yesod packages is to allow form fields to register Javascript components, such as a jQuery UI datepicker. In this post, I'd like to give another concrete example, inspired by Steve's blog post: charts.

Note: the code used in this post is available as a Github gist. There might be many differences between the version on the blog and the version in the gist.


{-# LANGUAGE OverloadedStrings, TemplateHaskell, RecordWildCards #-}

We're going to provide a module that provides a pure-Haskell interface to the jqplot library. To use it, you don't need to write any HTML, CSS, or Javascript in your code, as we'll see in our example app.

The idea is that all of the low-level Javascript code goes in this module, and apps just deal with the widget.

Note that, for simplicity, we're only implementing a tiny subset of jqPlot's full functionality. A more full-fledged wrapper could be written, but would obviously be more involved.

module Jqplot
    ( YesodJqplot (..)
    , YesodJquery (..)
    , PlotSettings (..)
    , plot
    ) where

import Yesod
import Yesod.Form.Jquery (YesodJquery (..))
import Data.Monoid ((<>))
import Data.Text.Lazy.Builder (toLazyText)
import Data.Aeson.Encode (fromValue)
import Data.Aeson (ToJSON (toJSON))
import Data.Text (Text)
import qualified Data.Text.Lazy
import Text.Julius (juliusFile)

This is just a minor utility function which renders values to JSON text. jqPlot, like many Javascript libraries, allows its functions to take arguments as JSON data. This function could in theory be provided by aeson.

encodeText :: ToJSON a => a -> Data.Text.Lazy.Text
encodeText = toLazyText . fromValue . toJSON

We need to be able to find the jqPlot Javascript files. By putting the location in a typeclass, users are able to provide whichever location they want (e.g., on a local webserver) or use the default value, which in this case uses the jsdelivr CDN.

Note that the YesodJquery typeclass, provided by the yesod-form package, works the same way.

class YesodJqplot master where
    jqplotRoot :: master -> Text
    jqplotRoot _ = ""

Now we begin our Haskell API. For a plot, we'll have three settings: the label for the X and Y axes, and the datapoints to be plotted.

data PlotSettings = PlotSettings
    { psXLabel :: Text
    , psYLabel :: Text
    , psData :: [(Double, Double)]

This function is the meat of our module. It takes the PlotSettings and turns it into a Widget. Note the type signature here: we're using GWidget with arbitrary subsite and master site, so that this widget will work with many different applications. However, since we need to know the location of the jQuery and jqPlot libraries, we have the relevant typeclasses in the context.

plot :: (YesodJquery master, YesodJqplot master) => PlotSettings -> GWidget sub master ()
plot PlotSettings {..} = do

Grab the master site...

    master <- lift getYesod
    let root = jqplotRoot master

so that we can add dependencies to the widget. Note that the calling app will automatically inherit these dependencies, and can remain completely ignorant of what's going on inside this function. Yesod will also ensure that each file is only included in the page once.

    addScriptEither $ urlJqueryJs master
    addScriptRemote $ root <> "jquery.jqplot.min.js"
    addScriptRemote $ root <> "plugins/jqplot.canvasTextRenderer.min.js"
    addScriptRemote $ root <> "plugins/jqplot.canvasAxisLabelRenderer.min.js"
    addStylesheetRemote $ root <> "jquery.jqplot.min.css"

We need to give a unique ID to a div tag where the chart will be placed. We ask Yesod to provide a unique identifier and then both the Julius and Hamlet templates are able to use it. This avoids two problems: typos between the two files, and name collisions when using the same widget twice in a page.

    divId <- lift newIdent

And as a standard best practice, we've placed the Julius and Hamlet in separate files.

    toWidget $(juliusFile "jqplot.julius")
    $(whamletFile "jqplot.hamlet")


Here's the important point to make: when you're creating a widget, you have to write Javascript. In many ways, you can consider this like writing an FFI binding: it's low level and not type safe, but once you've written it properly, you can interact fully from within Haskell-land.

In our case, here's the Julius file that produces the relevant Javascript.

  $.jqplot('#{divId}', [#{encodeText psData}], {
          label:#{encodeText psXLabel}
          label:#{encodeText psYLabel},
          labelRenderer: $.jqplot.CanvasAxisLabelRenderer


The HTML is incredibly simple: it just creates a dummy div tag with the correct id.

<div id=#{divId}>

That can also be written as:

<div ##{divId}>

But that version looks just a bit more confusing to me.


Finally, let's use this library!

{-# LANGUAGE OverloadedStrings, MultiParamTypeClasses, TemplateHaskell, QuasiQuotes, TypeFamilies #-}
import Yesod
import Jqplot

Let's kick off a basic Yesod app with three routes: a homepage and two charts.

data App = App

mkYesod "App" [parseRoutes|
/ HomeR GET
/chart1 Chart1R GET
/chart2 Chart2R GET

Next we create typeclass instances. We can use the default methods for YesodJquery and YesodJqplot and take advantage of the built-in CDNs, or could override and use a local copy instead. For simplicity, we choose the former.

instance Yesod App
instance YesodJquery App
instance YesodJqplot App

Basic handler, nothing special.

getHomeR :: Handler RepHtml
getHomeR = defaultLayout $ do
    setTitle "Homepage"
<p>Demonstration of a reusable widget.
    <a href=@{Chart1R}>First chart
    <a href=@{Chart2R}>Second chart

In this chart, we create a graph of the x² function, without writing a single line of HTML, CSS, or Javascript. In theory, as more widgets become available in the Yesod ecosystem, this type of development could become more standard. But for now, the following example is more normative.

getChart1R :: Handler RepHtml
getChart1R = defaultLayout $ do
    setTitle "Graph of x²"

    plot PlotSettings
            { psXLabel = "x"
            , psYLabel = "x²"
            , psData = map (\x -> (x, x * x)) $ [0, 0.1..10]

Very often, you'll want to take some precomposed widget and embed it inside some other HTML. That's entirely possible with widget interpolation.

getChart2R :: Handler RepHtml
getChart2R = defaultLayout $ do
    setTitle "Made up data"
    toWidget [lucius|
.chart {
    width: 500px;
    height: 300px;
<p>You can just as easily embed a reusable widget inside other source.
<div .chart>^{madeUpData}
<p>And it just works.
    -- Yup, totally bogus data...
    madeUpData = plot PlotSettings
        { psXLabel = "Month"
        , psYLabel = "Hackage uploads"
        , psData =
            [ (1, 100)
            , (2, 105)
            , (3, 115)
            , (4, 137)
            , (5, 168)
            , (6, 188)
            , (7, 204)
            , (8, 252)
            , (9, 256)
            , (10, 236)
            , (11, 202)
            , (12, 208)

And now we just run our app!

main :: IO ()
main = warpDebug 3000 App


I think widgets are one of the best features in Yesod (I think I rank it at #2, right behind type-safe URLs). I haven't seen any other framework provide this kind of packaging of HTML, CSS, and Javascript together.

I think one of the reasons for this is that Haskell's type system makes the implementation so straight-forward. The contents of a Widget form a Monoid, and the Widget itself is essentially a WriterT transformer sitting on top of the standard Handler monad. That means we can run arbitrary code from inside our Widget, such as pulling out the last five blog entries for a "recent activity" widget.

I'm very curious to see how the other Haskell web frameworks approach this problem. If the approaches are similar enough, I think it would be worth investigating the possibility of creating a more universal widget system, so that widgets could be shared among various Haskell frameworks. If anyone's interested in working on this with me, please be in touch.

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