Convert Bird-style .lhs to Markdown.

lxx2md.hs

#!/usr/bin/env stack
-- stack runghc --package text --package turtle

{-# LANGUAGE OverloadedStrings #-}

{-
  lxx2md.hs (-l|--lang LANG)
  A filter that converts Bird-style lhs in Markdown ``` blocks.
  Combines consecutive Bird-tracked lines into single blocks.

  KNOWN ISSUES
  - Does not reject program lines next to text lines.
  - Assumes that newlines are \n.

  COPYING
  Do whatever you want - but it'd be nice to know how things
  get improved.
-}

import Turtle
import qualified Data.Text.Lazy as L
import qualified Data.Text.Lazy.IO as L

data Block = Text L.Text | Code L.Text
  deriving Show

main :: IO ()
main = do
  lang <- (fmap L.fromStrict) (options "lxx2md" opts)
  lines <- (fmap L.lines) L.getContents
  putLines (map (render lang) (coalesce $ parse lines))
  return ()
  where opts = optText "lang" 'l' "The language string Markdown code blocks are given."

parse :: [L.Text] -> [Block]
parse [] = []
parse (l:ls) = case L.stripPrefix ">" l of
  Nothing -> Text l : parse ls
  Just l' -> Code l' : parse ls

coalesce :: [Block] -> [Block]
coalesce [] = []
coalesce [a] = [a]
coalesce (Text a:Text b:cs) =
  coalesce $ Text (a +++ "\n" +++ b) : coalesce cs
coalesce (Code a:Code b:cs) =
  coalesce $ Code (a +++ "\n" +++ b) : coalesce cs
coalesce (a:b:cs) = a : b : coalesce cs

render :: L.Text -> Block -> L.Text
render ____ (Text text) = text
render lang (Code text) = "```" +++ lang +++ "\n" +++ text +++ "\n```"

putLines :: [L.Text] -> IO [()]
putLines = mapM L.putStrLn

(+++) = L.append