Aelve Guide | Haskell

Cons

Quite heavy if the only thing you want is rendering Markdown to HTML.
Doesn't handle pathological inputs well (e.g. parsing [[[[[[[[[[[[[[[[[ foo ]]]]]]]]]]]]]]]]] takes 40s on my machine).

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Ecosystem

Scholdoc – a fork of Pandoc for academic writing
Filters: pandoc-include (for including contents of referenced files), pandoc-csv2table (for rendering CSV tables), pandoc-crossref (for numbering figures/tables and cross-referencing), pandoc-citeproc. A bigger list is on the Pandoc wiki.

Ecosystem

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Notes

Imports

import Text.Pandoc

Usage

Markdown to HTML

Here's a program that reads Markdown input and outputs rendered HTML:

import Text.Pandoc

main = do
  s <- getContents
  parsed <- case readMarkdown def s of
    Left  err -> error (show err)
    Right doc -> return doc
  putStrLn (writeHtmlString def parsed)

We might also generate a full HTML document instead of simply some markup. To do that, we'll use writerStandalone = True and load a template:

import Text.Pandoc

main = do
  s <- getContents
  template <- either (error . show) id <$>
                getDefaultTemplate Nothing "html"
  parsed <- case readMarkdown def s of
    Left  err -> error (show err)
    Right doc -> return doc
  let writerOpts = def {
        writerStandalone = True,
        writerTemplate   = template }
  putStr (writeHtmlString writerOpts parsed)

Working with parsed Markdown

For transforming Markdown, use functions from Text.Pandoc.Walk (from pandoc-types). There are some examples available in the docs.

You can construct your own documents by using functions from Text.Pandoc.Builder.

There are some helper functions (such as stringify or capitalize) in Text.Pandoc.Shared.

cmark (Hackage)

other

Summary

A lightweight Markdown library from the author of Pandoc, implementing the CommonMark standard (which is just a more precisely specified version of Markdown). Can parse Markdown and convert it to various formats (including HTML).

Binds to a C library (libcmark), but doesn't require it to be installed – the sources are shipped with the Haskell package.

Summary

Pros

Very fast (the author's benchmarks: 82× faster than cheapskate, 59× faster than markdown, 105× faster than pandoc, 3× faster than discount).
Can deal with any input, including garbage, with linear performance. (Some Markdown parsers have quadratic complexity on some inputs, which gives an attacker an opportunity to slow down your site.)
Can render to several formats: apart from HTML, it also supports LaTeX, groff man, and a custom XML format.
The only library here that lets you get position info for parsed blocks.
Has a sibling Javascript library that implements the same specification (so that the results of client-side and server-side rendering would fully match).

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Cons

Can't automatically recognise links (i.e. if you write go to https://google.com, the link won't be highlighted).
Doesn't sanitize HTML output by default (you have to use xss-sanitize if you want that).

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Ecosystem

cmark-highlight (highlights code blocks)

Ecosystem

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Notes

Imports

import CMark

Usage

In the simplest case you just use commonmarkToHtml (or commonmarkToLaTeX, etc) which takes text, parses it, and renders it. You can also parse Markdown with commonmarkToNode, transform it, and then render with nodeToHtml.

cheapskate (Hackage)

other

Summary

Another lightweight Markdown library from the author of Pandoc. Unlike cmark, it's implemented in pure Haskell. Can parse Markdown and convert it to HTML.

Summary

Pros

Can deal with any input with linear performance.
HTML output is sanitized by default (to protect against XSS attacks).

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Cons

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Ecosystem

cheapskate-terminal (renders to console), cheapskate-highlight (highlights code blocks), cheapskate-lucid

Ecosystem

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Notes

Imports

import Cheapskate
import Cheapskate.Html

And these are imports from blaze-html that you'll need if you want to render HTML:

import Text.Blaze.Html (Html)

-- for rendering to ByteString
import Text.Blaze.Html.Renderer.Utf8 (renderHtml)

-- or for rendering to Text
import Text.Blaze.Html.Renderer.Text (renderHtml)

Usage

First of all, you should know what options cheapskate will use. By default, sanitize and allowRawHtml are enabled:

data Options = Options {
  sanitize           :: Bool,   -- ^ Sanitize raw HTML, link/image attributes
  allowRawHtml       :: Bool,   -- ^ Allow raw HTML (if false it gets escaped)
  preserveHardBreaks :: Bool,   -- ^ Preserve hard line breaks in the source
  debug              :: Bool }  -- ^ Print container structure for debugging

There are 2 functions we'll primarily use: markdown :: Options -> Text -> Doc parses Markdown, and renderDoc :: Doc -> Html renders it to HTML. Doc is a type that consists of Blocks, and blocks usually have Inlines inside, or other blocks. You can traverse and transform those structures manually, or you can use walk and friends.

Rendering Markdown to HTML

Here's a program that reads Markdown input, outputs rendered HTML, and doesn't allow raw HTML. renderHtml is a function that takes Html and produces Text from it, and it comes from blaze-html.

import Cheapskate
import Cheapskate.Html

import Text.Blaze.Html (Html)
import Text.Blaze.Html.Renderer.Text (renderHtml)

import qualified Data.Text.Lazy    as TL
import qualified Data.Text.Lazy.IO as TL

-- Type signatures are added for clarity.
main = do
  md <- TL.getContents
  let parsed :: Doc
      parsed = markdown def{allowRawHtml = False} (TL.toStrict md)
  let rendered :: Html
      rendered = renderDoc parsed
  TL.putStr (renderHtml rendered)

If you were outputting it to a file, you'd want to use renderHtml from Text.Blaze.Html.Renderer.Utf8 instead. And if you want pretty indented HTML, use Text.Blaze.Html.Renderer.Pretty.

Working with parsed Markdown

The Inline and Block types are defined like this:

data Block = Para Inlines
           | Header Int Inlines
           | Blockquote Blocks
           | List Bool ListType [Blocks]
           | CodeBlock CodeAttr Text
           | HtmlBlock Text
           | HRule

data Inline = Str Text
            | Space
            | SoftBreak
            | LineBreak
            | Emph Inlines
            | Strong Inlines
            | Code Text
            | Link Inlines Text {- URL -} Text {- title -}
            | Image Inlines Text {- URL -} Text {- title -}
            | Entity Text
            | RawHtml Text

Inlines is defined as Seq Inline, Blocks – Seq Block. You can use for_ or fmap if you want to traverse them.

There are two functions for traversing Markdown – walk and walkM. The general signature is walk :: (Data a, Data b) => (a -> a) -> b -> b, so it can traverse all Inlines in a Doc, or all Blocks in a Block, or any other combination. walkM is more powerful, as it allows monadic functions – you can use it if your transforming function uses IO, for instance, or you can do gathering with it. For instance, here's how to turn some Markdown into plain text, using the Writer monad:

-- Using a DList Text instead of Text might be faster

stringify :: Inlines -> Text
stringify = execWriter . walkM go
  where
    go :: Inline -> Writer Text Inline
    go i = do
      case i of
        Str x     -> tell x
        Code x    -> tell x
        Space     -> tell " "
        SoftBreak -> tell " "
        LineBreak -> tell " "
        -- We should've handled the case for Entity as well
        -- (by converting it to a character), but let's ignore it
        -- for the sake of simplicity.
        _other    -> return ()
      return i

Highlighting code blocks

To highlight code in blocks, use cheapskate-highlight:

import Cheapskate.Highlight

In the basic case you can just apply highlightDoc to the parsed Doc before rendering it. You'd also have to include CSS into your page – you can get the CSS to include by applying styleToCss :: Style -> String to one of the styles defined in Text.Highlighting.Kate.Styles (reexported by Cheapskate.Highlight), for instance pygments.

markdown (Hackage)

other

Summary

A library from Michael Snoyman (the author of Yesod). Can parse Markdown and convert it to HTML. Has additional features that make it good for publishing (you can customise the parser, for instance) but simultaneously can get stuck on some inputs and that's pretty bad (unless you explicitly implement a timeout or something like that).

Summary

Pros

Sanitizes input by default.
The parser can be customised to add new kinds of fencing in addition to ``` and ~~~ – for instance, @@@. Moreover, the contents can be parsed as Markdown as well (so you could set it up so that e.g. @@@ would mean “spoiler” or “important note”).
Has an option for adding target=_blank to all links so that they'd open in new tabs.

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Cons

Doesn't handle pathological inputs well (e.g. parsing [[[[[[[[[[[[[[[[[[[[[[[ foo ]]]]]]]]]]]]]]]]]]]]]]] takes 46s on my machine), which makes it unsuitable for e.g. sites with user-submitted content.

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Ecosystem

yesod-text-markdown

Ecosystem

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Notes

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sundown (Hackage)

other

Summary

Bindings to Github's (former) Markdown library, sundown. (The sources are bundled with the package, so the library doesn't need to be installed separately.) Can convert Markdown to HTML, but doesn't give access to parsed Markdown.

Summary

Pros

Very fast (since it's a C library). Can deal with any input with linear performance, and has been battle-tested extensively (since it's been used on Github).
Supports automatic link recognition.
Has support for tables, superscripts, strikethrough.
Can generate a table of contents.

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Cons

The underlying library (sundown) has been deprecated.
Doesn't provide a Haskell type for parsed Markdown (so you can't inspect or modify it).

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Ecosystem

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Notes

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discount (Hackage)

other

Summary

Bindings to another Markdown library, discount. Can convert Markdown to HTML, but doesn't give access to parsed Markdown.

The documentation for Markdown extensions it supports can be found here.

Summary

Pros

Supports tables, superscripts, strikethrough, footnotes.
Has some non-standard extensions too: definition lists, paragraph centering, specifying image sizes, and styling text (e.g. [foo](class:bar) would wrap “foo” into a <span> tag with class="bar" and then you could apply CSS styling to it).
Seems to be able to deal with any input with linear performance (since it renders this stress-test just fine).
Supports math in pages (with MathJax).
Can generate table of contents.

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