Lucid: templating DSL for HTML

By Chris Done

I’m not big on custom templating languages, for reasons I’ll write about another time. I prefer EDSLs. I preferred the xhtml package back when that was what everybody used. It looked like this:

header << thetitle << "Page title"

thediv noHtml ! [theclass "logo"] << "…"
thediv noHtml ! [identifier "login"]

Pretty line-noisy to read, write and hard to edit in a reasonable manner.

Later, blaze-html became the new goto HTML writing library. It improved upon the XHTML package by being faster and having a convenient monad instance. It looks like this:

page1 = html $ do
    head $ do
        title "Introduction page."
        link ! rel "stylesheet" ! type_ "text/css" ! href "screen.css"
    body $ do
        div ! id "header" $ "Syntax"
        p "This is an example of BlazeMarkup syntax."
        ul $ mapM_ (li . toMarkup . show) [1, 2, 3]

Much easier to read, write and edit thanks to the monad instance.

However, after several years of using that, I’ve come to write my own. I’ll cover the infelicities about Blaze and then discuss my alternative approach.

Reading back through what I’ve written below, it could be read as a bit attacky, and some of the issues are less philosophical and more incidental. I think of it more that the work on writing HTML in a DSL is incomplete and to some degree people somewhat gave up on doing it more conveniently at some point. So I’m re-igniting that.

The combination of having a need to write a few HTML reports and recent discussions about Blaze made me realise it was time for me to come at this problem a-fresh with my own tastes in mind. I also haven’t used my own approach much, other than porting some trivial apps to it.


Names that conflict with base

The first problem is that Blaze exports many names which conflict with base. Examples:

div, id, head, map

The obvious problem with this is that you either have to qualify any use of those names, which means you have to qualify Blaze, and end up with something inconsistent like this:

H.div ! "logo" $ "…"

Where H and A come from importing the element and attribute modules like this:

import qualified Text.Blaze.Html5            as H
import qualified Text.Blaze.Html5.Attributes as A

Or you don’t import Prelude and only import Blaze, but then you can’t do a simple map without qualification.

You might’ve noticed in the old xhtml package that thediv and identifier are used instead. The problem with using different names from the actual things they refer to is that they’re hard to learn and remember, both for regular Haskellers and newbies coming to edit your templates.

Names that are keywords

This is a common problem in DSLs, too. In Blaze the problem is: class or type (perhaps others I don’t recall). Blaze solves it with: class_ or type_

Again, the problem with this is that it is inconsistent with the other naming conventions. It’s another exception to the rule that you have to remember and makes the code look bad.

Conflicting attribute and element names

There are also names which are used for both attributes and elements. Examples are style and map. That means you can’t write:

H.head $ style "body { background: red; }"
body $ p ! style "foo" $

You end up writing:

H.head $ "body { background: red; }"
body $ p ! "foo" $

Inconsistency is difficult and ugly

What the above problems amount to is ending up with code like this:

body $ H.div ! "logo" ! class_ "left" ! hidden $ "Content"

At this point users of Blaze give up with second-guessing every markup term they write and decide it’s more consistent to qualify everything:

H.body $ H.div ! "logo" ! A.class_ "left" ! A.hidden $ "Content"

Or, taken from some real code online:

H.input H.! A.type_ "checkbox"
        H.! A.checked True
        H.! A.readonly "true"

This ends up being too much. Inconvenient to type, ugly to read, and one more step removed from the HTML we’re supposed to be generating.

The Monad instance isn’t

The monad instance was originally conceived as a handy way to write HTML nicely without having to use <> or lists of lists and other less wieldy syntax.

In the end the monad ended up being defined like this:

instance Monad MarkupM where
  return _ = Empty
  {-# INLINE return #-}
  (>>) = Append
  {-# INLINE (>>) #-}
  h1 >>= f = h1 >> f
      (error "Text.Blaze.Internal.MarkupM: invalid use of monadic bind")
  {-# INLINE (>>=) #-}

And has been for some years. Let’s take a trivial example of why this is not good. You render some HTML and while doing so build a result to be used later:

do xs <- foldM (\c i -> …)
   mapM_ dd xs


*** Exception: Text.Blaze.Internal.MarkupM: invalid use of monadic bind

The overloaded strings instance is bad

The previous point leads onto this next point, which is that due to this phantomesque monad type, the instance is like this:

instance IsString (MarkupM a) where
    fromString = Content . fromString
    {-# INLINE fromString #-}

How can it make this value? It cannot. If you want to go ahead and extract that `a’, you get:

*** Exception: Text.Blaze.Internal.MarkupM: invalid use of monadic bind

Additionally, this instance is too liberal. You end up getting this warning:

A do-notation statement discarded a result of type GHC.Prim.Any

Suppress this warning by saying _ <- "Example" or by using the flag -fno-warn-unused-do-bind

So you end up having to write in practice (again, taken from a real Blaze codebase by one of the authors):

void "Hello!"

Which pretty much negates the point of using IsString in the first-place. Alternatively, you use -fno-warn-unused-do-bind in your module.

Working with attributes is awkward

The ! syntax seems pretty convenient from superficial inspection:

link ! rel "stylesheet" ! type_ "text/css" ! href "screen.css"

But in practice it means you always have the same combination:

div ! H.class_ "logo" $ "…"

Which I find—personally speaking—a bit distasteful to read, it’s not far from what we saw in the old xhtml package:

thediv ! [theclass "logo"] << "…"

Did we really save that much in the attribute department? Operators are evil.

But mostly presents an editing challenge. Operators like this make it tricky to navigate, format in a regular way and do code transformations on. All Haskell code has operators, so this is a general problem. But if your DSL doesn’t actually need these operators, I consider this a smell.

Attributes don’t compose

You should be able to compose with. For example, let’s say you want to define a re-usable component with bootstrap:

container inner = div ! class_ "container" $ inner

Now you can use it to make a container. But consider now that you also want to add additional attributes to it later. You can do that with another call to with:

container ! class_ "main" $ "zot"

In Blaze this produces:

λ> main
"<div class=\"container\" class=\"main\">My content!</div>"

Browsers ignore the latter main, so the composition didn’t work.

Ceremony is tiring

Here’s the example from Blaze’s package, that’s introduced to users.

import Prelude hiding (head, id, div)
import Text.Blaze.Html4.Strict hiding (map)
import Text.Blaze.Html4.Strict.Attributes hiding (title)
import Text.Blaze.Renderer.Utf8 (renderMarkup)

page1 :: Markup
page1 = html $ do
    head $ do
        title "Introduction page."
        link ! rel "stylesheet" ! type_ "text/css" ! href "screen.css"
    body $ do
        div ! id "header" $ "Syntax"
        p "This is an example of BlazeMarkup syntax."
        ul $ mapM_ (li . toMarkup . show) [1, 2, 3]

main = print (renderMarkup page1)

Apart from the import backflips you have to do to resolve names properly, you have at least three imports to make just to render some HTML. Call me lazy, or stupid, but I never remember this deep hierarchy of modules and always have to look it up every single time. And I’ve been using Blaze for as long as the authors have.


A smaller complaint is that it would sometimes be nice to transform over another monad. Simplest example is storing the read-only model information in a reader monad and then you don’t have to pass around a bunch of things as arguments to all your view functions. I’m a big fan of function arguments for explicit state, but not so much if it’s the same argument every time.

No Show instance

It would be nice if you could just write some markup in the REPL without having to import some other modules and wrap it all in a function just to see it.


My new library, Lucid, attempts to solve most of these problems.

Naming issues

Firstly, all names which are representations of HTML terms are suffixed with an underscore _:

p_, class_, table_, style_

No ifs or buts. All markup terms.

That solves the following problems (from the issues described above):

No import problems or qualification. Just write code without worrying about it.

How it looks

Plain text is written using the OverloadedStrings and ExtendedDefaultRules extensions, and is automatically escaped:

λ> "123 < 456" :: Html ()
123 &lt; 456

Elements nest by function application:

λ> table_ (tr_ (td_ (p_ "Hello, World!")))
<table><tr><td><p>Hello, World!</p></td></tr></table>

Elements are juxtaposed via monoidal append:

λ> p_ "hello" <> p_ "sup"

Or monadic sequencing:

λ> div_ (do p_ "hello"; p_ "sup")

Attributes are set using the with combinator:

λ> with p_ [class_ "brand"] "Lucid Inc"
<p class="brand">Lucid Inc</p>

Conflicting attributes (like style_) work for attributes or elements:

λ> html_ (head_ (style_ "body{background:red}") <>
                 with body_ [style_ "color:white"]
                      "Look ma, no qualification!")
<body style="color:white">Look ma, no qualification!</body></html>

The Blaze example

For comparison, here’s the Blaze example again:

page1 = html $ do
    head $ do
        title "Introduction page."
        link ! rel "stylesheet" ! type_ "text/css" ! href "screen.css"
    body $ do
        div ! id "header" $ "Syntax"
        p "This is an example of BlazeMarkup syntax."
        ul $ mapM_ (li . toMarkup . show) [1, 2, 3]

And the same thing in Lucid:

page2 = html_ $ do
    head_ $ do
      title_ "Introduction page."
      with link_ [rel_ "stylesheet",type_ "text/css",href_ "screen.css"]
    body_ $ do
        with div_ [id_ "header"] "Syntax"
        p_ "This is an example of Lucid syntax."
        ul_ $ mapM_ (li_ . toHtml . show) [1,2,3]

I’m not into operators like ($) and swung indentation like that, but I followed the same format.

I’d write it in a more Lispy style and run my hindent tool on it:

page1 =
  html_ (do head_ (do title_ "Introduction page."
                      with link_
                           [rel_ "stylesheet"
                           ,type_ "text/css"
                           ,href_ "screen.css"])
            body_ (do with div_ [id_ "header"] "Syntax"
                      p_ "This is an example of Lucid syntax."
                      ul_ (mapM_ (li_ . toHtml . show)

But that’s another discussion.

It’s a real monad

Normal monadic operations work properly:

λ> (return "OK!" >>= p_)

It’s basically a writer monad.

In fact, it’s also a monad transformer:

λ> runReader (renderTextT (html_ (body_ (do name <- lift ask
                                            p_ (toHtml name)))))
             ("Chris" :: String)

Overloaded strings instance is fine

The instance is constrained over the return type being (). So string literals can only be type HtmlT m ().

λ> do "x" >> "y" :: Html ()

λ> do x <- "x"; toHtml (show x)


Attributes are simply written as a list. That’s all. Easy to manipulate as a data structure, easy to write and edit, and automatically indent in a predictable way:

λ> with p_ [id_ "person-name",class_ "attribute"] "Mary"
<p id="person-name" class="attribute">Mary</p>

No custom operators are required. Just the with combinator. If you want to indent it, just indent it like normal function application:

with p_
     [id_ "person-name",class_ "attribute"]

And you’re done.

Composing attributes

You should be able to compose with. For example, let’s say you want to define a re-usable component with bootstrap:

λ> let container_ = with div_ [class_ "container "]

Now you can use it to make a container:

λ> container_ "My content!"
<div class="container ">My content!</div>

But consider now that you also want to add additional attributes to it later. You can do that with another call to with:

λ> with container_ [class_ "main"] "My content!"
<div class="container main">My content!</div>

Duplicate attributes are composed with normal monoidal append. Note that I added a space in my definition of container anticipating further extension later. Other attributes might not compose with spaces.


Another part I made sure was right was lack of import nightmare. You just import Lucid and away you go:

λ> import Lucid
λ> p_ "OK!"
λ> p_ (span_ (strong_ "Woot!"))
λ> renderBS (p_ (span_ (strong_ "Woot!")))
λ> renderToFile "/tmp/foo.html" (p_ (span_ (strong_ "Woot!")))

If I want to do more advanced stuff, it’s all available in Lucid. But by default it’s absolutely trivial to get going and output something.


Actually, despite having a trivial implementation, being a real monad and a monad transformer, it’s not far from Blaze. You can compare the benchmark reports here. A quick test of writing 38M of HTML to file yielded the same speed (about 1.5s) for both Lucid and Blaze. With such decent performance for very little work I’m already ready to start using it for real work.


So the point of this post was really to explain why another HTML DSL and I hope I did that well enough.

The code is on Github. I pushed to Hackage but you can consider it beta for now.