The document is an invitation to a presentation about Maccro, a Ruby macro system that allows rewriting Ruby methods by registering rules. It summarizes Maccro's capabilities such as registering before/after matchers, applying rules to methods, matching method ASTs to rules, and rewriting method code by replacing placeholders. It also lists some limitations including only supporting Ruby 2.6+, not handling method call order dependencies, and not updating method visibility. The presentation aims to recruit more developers to Maccro to handle its remaining challenges.

1. Invitation to
the Dark Side of Ruby:
Maccro Strikes Back
#tqrk13
Satoshi Tagomori (@tagomoris)

2. Satoshi Tagomori (@tagomoris)
Fluentd, MessagePack-Ruby, Norikra, Woothee, ...
Arm Ltd.

4. "If you only knew the power of the dark side"
<< >>

5. Vote!!!!!!

6. Maccro

7. Demonstration

8. Maccro: register
• Register rules with "before" and "after" matchers
• "before"/"after": code in Ruby, with DSL for code placeholders
• Placeholders (N: integer, >= 1):
• eN: expressions which returns a value or values
• vN: a single value (e.g., variable, constant, literal, etc)

9. Maccro: Apply on Methods
• Macro rules applied to methods
• manually: apply(ModName,
ModName.method(:method_name)
• automatically: using TracePoint (:end)

10. • Registered matchers:
• replace placeholders w/ matcher nodes for
RubyVM::AST::Node
Maccro: Prepare
VCALL: e1 VCALL: e2
VCALL: e3
Matcher: e1 Matcher: e2
Matcher: e3
OPCALL: <
OPCALL: <
OPCALL: <
OPCALL: <
Matcher code
e1 < e2 < e3
Placeholders
Matcher tree

11. • Applied methods:
• parsed into AST using
RubyVM::AbstractSyntaxTree#of
• get the source of the method
#<UnboundMethod: Mod#foo>
def foo(v)
if bar(v) < 1
# ...
elsif bar(v) == 1
# ...
elsif 1 < bar(v) < 2
# ...
else
# ...
Maccro: Match to Method

12. def foo(v)
if bar(v) < 1
# ...
elsif bar(v) == 1
# ...
elsif 1 < bar(v) < 2
# ...
else
# ...
end
end
• Matched tree:
• get code snippets for matcher nodes
• e1: 1 (literal)
• e2: bar(v) (func call with v)
• e3: 2 (literal)
e1 < e2 < e3
Matcher: e1 Matcher: e2
Matcher: e3
Maccro: Rewrite code (1)

13. • Matched tree:
• replace patterns in "after" code with captured snippets
• e1: 1 (literal)
• e2: bar(v)
• e3: 2 (literal)
e1 < e2 && e2 < e3
"After" code
1 < bar(v) && bar(v) < 2
"After" code for this method
"Before" code
e1 < e2 < e3
Maccro: Rewrite code (2)

14. • Matched tree:
• rewrite original method code
with updated "after" code
def foo(v)
if bar(v) < 1
# ...
elsif bar(v) == 1
# ...
elsif 1 < bar(v) < 2
# ...
else
# ...
end
end
e1 < e2 && e2 < e3
"After" code
1 < bar(v) && bar(v) < 2
"After" code for this method
"Before" code
e1 < e2 < e3
Original code of this method
Maccro: Rewrite code (3)

15. Maccro: Rewrite code (3)
• Matched tree:
• rewrite original method code
with updated "after" code
1 < bar(v) && bar(v) < 2
"After" code for this method
def foo(v)
if bar(v) < 1
# ...
elsif bar(v) == 1
# ...
elsif 1 < bar(v) && bar(v) < 2
# ...
else
# ...
end
end
Updated code of this method
e1 < e2 && e2 < e3
"After" code
"Before" code
e1 < e2 < e3

16. • Rewritten method source:
• call Modue#module_eval to replace the method
definition
def foo(v)
if bar(v) < 1
# ...
elsif bar(v) == 1
# ...
elsif 1 < bar(v) < 2
# ...
else
# ...
end
end
def foo(v)
if bar(v) < 1
# ...
elsif bar(v) == 1
# ...
elsif 1 < bar(v) && bar(v) < 2
# ...
else
# ...
end
end
Overwrite the method definition
by Module#module_eval
Maccro: Rewrite Method

17. Nice stuffs
• Less performance penalty
• Macro processor works only when methods are defined
• Ruby version independent Macro rules
• Compatible between Ruby versions
• Because of rules written in Ruby code
• Built-in rules, useful for some cases
• Continuous inequality operators (<=, <, >, >=)
• ActiveRecord utilities

18. • Supported only on Ruby 2.6 or later
• Only code in def in module/class are rewritable
• Calling class method just after definition can't be updated
• Same method can't be rewritten twice
• Non-idempotent methods causes unexpected result
• Updating Lambda and local variable name matching is not implemented yet
• Stack trace of updated methods would be confusing
• Source from STDIN or command line (and irb/pry) cause errors
• Rule-order-dependent issues will be caused
• Method visibility are not taken care right now
• Non-self singleton method definition causes errors
• Placeholder validations are not implemented yet
Limitations
Just Few Limitations
at April (RubyKaigi 2019)

19. Limitations
• Supported only on Ruby 2.6 or later
• Calling class method just after definition can't be updated
• Non-idempotent methods causes unexpected result
• Stack trace of updated methods would be confusing
• Source from STDIN or command line (and irb/pry) cause errors
• Rule-order-dependent issues will be caused
• Method visibility are not taken care right now
• Non-self singleton method definition causes errors
• Placeholder validations are not implemented yet
Just Few Limitations (now)

20. Maccro v0.2.0
• Support to rewrite (+call) any lambda/proc code
• Support new placeholders
for strings, symbols, numbers and regexps

25. #tqrk13
Satoshi Tagomori (@tagomoris)

27. Rails
• ActiveRecord !
• I WANT YOU

28. "Join me and I will complete your training."
<< >>
https://github.com/tagomoris/maccro
https://rubygems.org/gems/maccro

29. •
• #tqrk13 2

30. •
• limited
• 3kg

31. 4kg
• 1kg →
• 1kg →
• 2kg →

32. •
• #asakusarb
•

33. tqrk14... ?