This document discusses selecting a markdown parser for a Scala-based GitHub clone called GitBucket. It evaluates several Java-based markdown parsers but finds them lacking support for features like GitHub Flavored Markdown tables and fences. It describes initially trying to port the JavaScript markdown parser marked.js to Scala but facing issues with its use of regular expressions and mutability. The document then explains the decision to port marked.js to Java instead, resulting in the new markdown parser markedj, which supports GFM and has a simple API. GitBucket plans to switch to using markedj starting in its next version.

1. From Scala to Java
Looking for the best markdown parser and markedj
@takezoen
BizReach, Inc
10 Oct. 2015 #shibuya_java 13th

2. Prelude
● We are developing GitBucket which is a Scala based
GitHub clone
● It should support markdown for many purpose
● We are looking for better markdown processor on JVM

3. A lot of markdown processors in Java
● pegdown
○ https://github.com/sirthias/pegdown
● markdown4j
○ https://code.google.com/p/markdown4j/
● MarkdownJ
○ https://github.com/myabc/markdownj
● MarkdownPapers
○ http://markdown.tautua.org/
● Txtmark
○ https://github.com/rjeschke/txtmark
From: http://www.w3.org/community/markdown/wiki/MarkdownImplementations

4. Problems about GFM support
1. Run following code in REPL:
...
2. Result is:
...
Plugin version | GitBucket version
:--------------|:-----------------
3.6.x | 3.6.x
3.5.x | 3.5.x
Table:
Block in list:
```
$ mvn clean
$ mvn package
```
Fence:

5. Status
Library Table Fence Block in
List
Remarks
pegdown ○ ○ × Previously GitBucket were
using this.
markdown4j × ○ ○ Plugin system
UML support using YUML
MarkdownJ × × ○
MarkdownPapers × △ ○
Txtmark × △ ○ Highly buggy?

6. In other languages
● redcarpet (Ruby)
○ https://github.com/vmg/redcarpet
● marked.js (JavaScript)
○ https://github.com/chjj/marked

7. Road to markedj
● At first, I started porting marked.js into Scala

8. Regular expressions!!

9. Jamie Zawinski said:
Some people, when confronted with a problem, think
"I know, I'll use regular expressions."
Now they have two problems.

10. Side-effect! Mutable!! Not functional!!!
Reassign!!
Break!!
Mutable stack!!

11. Road to markedj
● Many regular expressions and mutables, Why Scala?
● Porting to Java may be useful for many people on JVM
○ No Scala runtime library
○ Available for Java and any other JVM langs
● I decided to port from Scala to Java!

12. markedj is JVM port of marked.js
● GitHub Flavored Markdown support
● Simple and extensible API
● GitBucket switches to markedj in the next version (3.7)!
import io.github.gitbucket.markedj.Marked;
String markdown = ...
String html = Marked.marked(markdown);

13. markedj is a graceful markdown
processor on JVM. Try it!!
https://github.com/gitbucket/markedj