type-of-html: High performance type driven html generation.

[ bsd3, html, language, library, text, web ] [ Propose Tags ]

This library makes most invalid html documents compile time errors and uses advanced type level features to realise compile time computations.

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Change log ChangeLog.md
Dependencies base (==4.10.*), bytestring, double-conversion, ghc-prim, text [details]
License BSD-3-Clause
Copyright 2017, Florian Knupfer
Author Florian Knupfer
Maintainer fknupfer@gmail.com
Category Language
Home page https://github.com/knupfer/type-of-html
Source repo head: git clone https://github.com/knupfer/type-of-html
Uploaded by knupfer at 2017-10-29T15:18:15Z
Distributions LTSHaskell:, NixOS:, Stackage:
Downloads 15648 total (13 in the last 30 days)
Rating 2.25 (votes: 2) [estimated by Bayesian average]
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Maintainer's Corner

For package maintainers and hackage trustees

Readme for type-of-html-

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Type of html

type-of-html is a library for generating html in a highly performant, modular and type safe manner.

Please look at the documentation of the module for an overview of the api: Html

Note that you need at least ghc 8.2.


Part of the html spec is encoded at the typelevel, turning a lot of mistakes into type errors.

Let's check out the /type safety/ in ghci:

>>> td_ (tr_ "a")

<interactive>:1:1: error:
    • 'Tr is not a valid child of 'Td
    • In the expression: td_ (tr_ "a")
      In an equation for ‘it’: it = td_ (tr_ "a")

<interactive>:1:6: error:
    • 'Tr can't contain a string
    • In the first argument of ‘td_’, namely ‘(tr_ "a")’
      In the expression: td_ (tr_ "a")
      In an equation for ‘it’: it = td_ (tr_ "a")

>>> tr_ (td_ "a")


>>> td_A (A.coords_ "a") "b"

<interactive>:1:1: error:
    • 'CoordsA is not a valid attribute of 'Td
    • In the expression: td_A (A.coords_ "a") "b"
      In an equation for ‘it’: it = td_A (A.coords_ "a") "b"

>>> td_A (A.id_ "a") "b"
<td id="a">b</td>

Every parent child relation of html elements is checked against the specification of html and non conforming elements result in compile errors.

The same is true for html attributes.

The checking is a bit lenient at the moment:

  • some elements can't contain itself as any descendant: at the moment we look only at direct children. This allows some (quite exotic) invalid html documents.
  • some elements change their permitted content based on attributes: we always allow content as if all relevant attributes are set.

Never the less: these cases are seldom. In the vast majority of cases you're only allowed to construct valid html. These restrictions aren't fundamental, they could be turned into compile time errors. Perhaps a future version will be even more strict.


Html documents are just ordinary haskell values which can be composed or abstracted over:

>>> let table = table_ . map (tr_ . map td_)
>>> :t table
table :: ('Td ?> a) => [[a]] -> 'Table > ['Tr > ['Td > a]]
>>> table [["A","B"],["C"]]
>>> import Data.Char
>>> html_ . body_ . table $ map (\c -> [[c], show $ ord c]) ['a'..'d']

And here's an example module:

{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds     #-}

module Main where

import Html

import qualified Html.Attribute as A

main :: IO ()
  = print
  . page
  $ map td_ [1..(10::Int)]

  :: 'Tr ?> a
  => a
  -> ('Div :@: ('ClassA := String # 'IdA := String))
        ( 'Div > String
        # 'Div > String
        # 'Table > 'Tr > a
page tds =
  div_A (A.class_ "qux" # A.id_ "baz")
    ( div_ "foo"
    # div_ "bar"
    # table_ (tr_ tds)

Please note that the type of page is inferable, so ask ghc-mod or whatever you use to write it for you. If you choose not to write the types, you don't need the language extensions. I strongly suggest that you don't write type signatures for type-of-html.

All text will be automatically html escaped:

>>> i_ "&"

>>> div_A (A.id_ ">") ()
<div id="&gt;"></div>

If you want to opt out, wrap your types into the 'Raw' constructor. This will increase performance, but can be only used with trusted input. You can use this e.g. to embed some blaze-html code into type-of-html.

>>> i_ (Raw "</i><script></script><i>")


type-of-html is a lot faster than blaze-html or than lucid.

Look at the following benchmarks:

Remember this benchmark from blaze-html?


This is comparing blaze with type-of-html:


To look at the exact code of this benchmark look here in the repo. The big table benchmark here is only a 4x4 table. Using a 1000x10 table like on the blaze homepage yields even better relative performance (~9 times faster), but would make the other benchmarks unreadable.

How is this possible? We supercompile lots of parts of the generation process. This is possible thanks to the new features of GHC 8.2: AppendSymbol. We represent tags and attributes as kinds and map these to (a :: [Symbol]) and then fold all neighbouring Symbols with AppendSymbol. Afterwards we reify the Symbols with symbolVal which will be embedded in the executable as Addr#. All this happens at compile time. At runtime we do only generate the content and append Builders.

For example, if you write:

renderText $ tr_ (td_ "test")

The compiler does optimize it to the following (well, unpackCString# doesn't exist for Builder, so it's slightly more complicated):

decodeUtf8 $ toLazyByteString
  (  Data.ByteString.Builder.unpackCString# "<tr><td>"#
  <> builderCString# "test"#
  <> Data.ByteString.Builder.unpackCString# "</tr>"#

Note that the compiler automatically sees that your string literal doesn't need utf8 and converts directly the "test"# :: Addr# to an escaped Builder without any intermediate structure, not even an allocated bytestring.

renderByteString $ tr_ (td_ "teſt")

Results in

  (  Data.ByteString.Builder.unpackCString# "<tr><td>"#
  <> encodeUtf8BuilderEscaped prim (Data.Text.unpackCString# "te\\197\\191t"#)
  <> Data.ByteString.Builder.unpackCString# "</tr>"#

If you write

renderBuilder $ div_ (div_ ())

The compiler does optimize it to the following:

Data.ByteString.Builder.unpackCString# "<div><div></div></div>"#

This sort of compiletime optimization isn't for free, it'll increase compilation times.

Note that compiling with -O2 results in a ~25% faster binary than with -O and compiling with -O0 compiles about 15 times faster, so be sure that you develop with -O0 and benchmark or deploy with -O2. Be aware, that cabal compiles only with -O if you don't specify explicitly otherwise.

Even faster

Need for speed? Consider following advise, which is sorted in ascending order of perf gains:

If you've got attributes or contents of length 1, use a Char.

This allows for a more efficient conversion to builder, because we know the length at compile time.

div_ 'a'

If you know for sure that you don't need escaping, use Raw.

This allows for a more efficient conversion to builder, because we don't need to escape.

div_ (Raw "a")

If you've got numeric attributes or contents, don't convert it to a string.

This allows for a more efficient conversion to builder, because we don't need to escape and don't need to handle utf8.

div_ (42 :: Int)

If you know that an attribute or content is empty, use ().

This allows for more compile time appending and avoids two runtime appends.

div_ ()

If you know for sure a string at compile time which doesn't need escaping, use a Proxy Symbol.

This allows for more compile time appending and avoids two runtime appends, escaping and conversion to a builder.

div_ (Proxy @"hello")

These techniques can have dramatic performance implications, especially the last two. If you replace for example in the big page with attributes benchmark every string with a Proxy Symbol, it'll run in 10 ns which is 500 times faster than blaze-html. Looking at core shows that this is equivalent of directly embedding the entire resulting html as bytestring in the binary and is therefore the fastest possible output.

Comparision to lucid and blaze-html

Advantages of type-of-html:

  • more or less 10 times faster on a medium sized page
  • a lot higher type safety: nearly no invalid document is inhabited
  • fewer dependencies

Disadvantages of 'type-of-html':

  • a bit noisy syntax (don't write types!)
  • sometimes unusual type error messages
  • compile times (30sec for a medium sized page, with -O0 only ~2sec)
  • needs at least ghc 8.2

I'd generally recommend that you put your documents into an extra module to avoid frequent recompilations. Additionally you can use type-of-html within an blaze-html document and vice versa. This allows you to gradually migrate, or only write the hotpath in a more efficient representation.

Example usage

{-# OPTIONS_GHC -fno-warn-missing-signatures #-}

module Main where

import Html

import Data.Text.Lazy.IO as TL

main :: IO ()
main = TL.putStrLn $ renderText example

example =
    ( body_
      ( h1_
        ( img_
        # strong_ "0"
      # div_
        ( div_ "1"
      # div_
        ( form_
          ( fieldset_
            ( div_
              ( div_
                ( label_ "a"
                # select_
                  ( option_ "b"
                  # option_ "c"
                # div_ "d"
            # button_ (i_ "e")