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Programming language: Haskell
License: MIT License
Tags: Text     Pandoc    

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This package contains some useful functions for writing Pandoc filters and integrating Pandoc into Haskell applications such as Hakyll.

It provides a composable wrapper for filters acting on nodes of the Pandoc AST and a few functions to convert between filters. The package also provides an attributes builder to work with attributes and some string utility functions to handle the switch from String to Text in pandoc-types 1.20.

Filter conversion/composition

As an example, let us look at the behead and delink filter from Pandoc's tutorial.

behead :: Block -> Block
behead (Header n _ xs) | n >= 2 = Para [Emph xs]
behead x = x

delink :: Inline -> [Inline]
delink (Link _ txt _) = txt
delink x = [x]

Since behead has type Block -> Block, while delink has type Inline -> [Inline], they are not naturally composable. However, this package provides a utility function mkFilter to convert them into a wrapped PandocFilter.

import Text.Pandoc.Utils

beheadFilter :: PandocFilter
beheadFilter = mkFilter behead

delinkFilter :: PandocFilter
delinkFilter = mkFilter delink

PandocFilter is an alias for PartialFilter Pandoc, so you can also have PartialFilter Inline, etc. There is also a monadic version called PartialFilterM for encapsulating monadic filter functions.

The PandocFilter is a monoid so you can do something like,

myFilter :: PandocFilter
myFilter = beheadFilter <> delinkFilter

where myFilter would apply beheadFilter first, then the delinkFilter. You can apply the filter using applyFilter,

import Text.Pandoc
import Data.Default (def)

  :: Text                    -- ^ Input markdown string
  -> Either PandocError Text -- ^ Html string or error
mdToHtml md = runPure $ do
  doc <- readMarkdown def md
  let doc' = applyFilter myFilter doc
  writeHtml5String def doc'

or get an unwrapped Pandoc -> Pandoc filter function using getFilter (this function is also capable of doing implicit conversion from PartialFilter a to b -> b).

myPandocFilter :: Pandoc -> Pandoc
myPandocFilter = getFilter myFilter

If you just want to convert between Pandoc filter functions, e.g. Inline -> [Inline] to Pandoc -> Pandoc without using the wrapped filter, there is also convertFilter and convertFilterM

delinkPandoc :: Pandoc -> Pandoc
delinkPandoc = convertFilter delink

This function is slightly more powerful than walk and walkM in that it is also able to handle filter functions of type a -> [a] and a -> m [a].

For applying multiple filters, there is also a function called sequenceFilters, which takes a list of wrapped filters and apply it to a Pandoc document (or subnode) sequentially, from left to right.

myFilters :: [PandocFilter]
myFilters =
  [ beheadFilter
  , delinkFilter

  :: Text                    -- ^ Input markdown string
  -> Either PandocError Text -- ^ Html string or error
mdToHtml' md = runPure $ do
  doc <- readMarkdown def md
  let doc' = sequenceFilters myFilters doc
  writeHtml5String def doc'

Attribute builder

pandoc-utils also provides an attribute builder for handling attributes. You can create a new attributes by

ghci> import Text.Pandoc.Utils
ghci> import Text.Pandoc.Definition
ghci> nullAttr `setId` "id" `addClass` "class" `addKVPair` ("key","value")

or modifying an existing attributes by

ghci> attr = ("id",[],[])
ghci> attr `setId` "newId"

For more examples, please read the [spec][spec].