TBD

1. Hackarade #04
"Create your own interpreter"
Yusuke Endoh (@mametter)
2018/10/05 (Fri.)

2. Today's Goal
• Write a Ruby interpreter in Ruby

3. Today's Goal
• Write a Ruby interpreter in Ruby
✕ eval(File.read(ARGV[0]))

4. Today's Goal
• Write a Ruby interpreter in Ruby
✕
✔
eval(File.read(ARGV[0]))
evaluate(
parse(
File.read(ARGV[0])))

5. A simple structure of interpreter
Parser Evaluator
puts 3*(2+2)
Program
12
Output
+
• Built-in features (String, Array, etc.)
• Memory management (GC)
• Libraries
Interpreter

6. Parser
• Converts a program text to
"abstract syntax tree" (AST)
Parser
puts 3*(2+2)
Program
func_call
AST
*
+
"puts"
3
2 2

7. Evaluator
• Runs the instruction by walking AST
func_call
AST
*
+
"puts"
3
2 2
Evaluator

8. Evaluator
• Runs the instruction by walking AST
func_call
AST
*"puts"
3
Evaluator
4

9. Evaluator
• Runs the instruction by walking AST
func_call
AST
"puts" 12
Evaluator

10. Evaluator
• Runs the instruction by walking AST
AST
nil
Evaluator
12
Output

11. Today's Goal
• Write a Ruby interpreter in Ruby

12. Today's Goal
• Write a Ruby interpreter in Ruby
– Impossible in one day!

13. Today's Goal
• Write a Ruby interpreter in Ruby
– Impossible in one day!
• More precisely:
Write a "MinRuby" interpreter in Ruby

14. Today's Goal
• Write a Ruby interpreter in Ruby
– Impossible in one day!
• More precisely:
Write a "MinRuby" interpreter in Ruby
– Runs a program written in MinRuby
(Target language)
– Is written in Ruby (Host language)

15. MinRuby
• Very small subset of Ruby
– Arithmetic, comparison:
– Statements, variables:
– Branches, loop:
– Function call:
– Function definition:
– Array and Hash:
1+2*3 42>40
x=42; x=x+1; p(x)
if x < 2 while x < 10
func(1,2) p(42)
def func(x,y); …; end
a=[1,2,3]; a[1]=42; p(a[0])
3*4==5+7
h={}; h["foo"]="bar"; p(h["foo"])

16. MinRuby
• Very small subset of Ruby
– Arithmetic, comparison:
– Statements, variables:
– Branches, loop:
– Function call:
– Function definition:
– Array and Hash:
• Class? Method call? Block? Not needed.
1+2*3 42>40
x=42; x=x+1; p(x)
if x < 2 while x < 10
func(1,2) p(42)
def func(x,y); …; end
a=[1,2,3]; a[1]=42; p(a[0])
3*4==5+7
h={}; h["foo"]="bar"; p(h["foo"])

17. Your task
• Write "interp.rb"
• Check if it works as the same as Ruby
# test1-1.rb
p(1 + 1)
def parse(src); …; end
def evaluate(tree); …; end
evaluate(parse(File.read(ARGV[0])))
$ ruby test1-1.rb
2
$ ruby interp.rb test1-1.rb
2

18. Focus: Evaluator
Parser Evaluator
puts 3*(2+2)
Program
12
Output
Interpreter
Use
"minruby"
gem
A skeleton
code is
provided

19. "minruby" gem
• Provides a MinRuby parser
require "minruby"
p minruby_parse( "3*(2+2)" )
#=> ["*",
["lit", 3],
["+",
["lit", 2],
["lit", 2]
]
]
*
+3
2 2
Observe!

20. A evaluator skeleton
• MinRuby interpreter with many "holes"
# An implementation of the evaluator
def evaluate(exp, env)
case exp[0]
when "+"
evaluate(exp[1], env) + evaluate(exp[2], env)
when "-"
raise(NotImplementedError) # Problem 1
…
end
end
evaluate(minruby_parse(minruby_load()), {})

21. Your task
• Complete "interp.rb" and pass all
test programs ("test*-*.rb")
$ ruby interp.rb test1-1.rb
2
$ ruby interp.rb test1-2.rb
Traceback (most recent call last):
2: from interp.rb:168:in `<main>'
1: from interp.rb:94:in `evaluate'
interp.rb:23:in `evaluate': NotImplementedError
Fix it!

22. Problems 1..6
1. Arithmetics (test1-*.rb)
2. Statements and variables (test2-*.rb)
3. Branches and loops (test3-*.rb)
4. Function calls (test4-*.rb)
5. User-defined functions (test5-*.rb)
6. Arrays and Hashes (test6-*.rb)
– You can implement and test them in turn

23. Problem 7
• Self-Hosting
– Write a MinRuby interpreter in MinRuby
• MinRuby is limited but still powerful to
implement a MinRuby interpreter!
$ ruby interp.rb interp.rb test1-1.rb
ary.each do |x|
...
end
i = 0
while ary[i]
x = ary[i]
...
end
$ ruby interp.rb interp.rb interp.rb test1-1.rb

24. Advanced Problems
• Write your own MinRuby parser
• Support advanced features
– Blocks, class/modules, eval, callcc, …
– Increment, lazy eval., type analysis, …
• Write a XXX interpreter in XXX
– MinSwift, Kotlin, Python, JavaScript, …
• Do true self-hosting
– Write a MinRuby parser in MinRuby
• Write a MinRuby compiler in Ruby

25. Good luck!
• The files and details are available at:
– https://github.com/mame/cookpad-
hackarade-minruby/
• Feel free to ask me any questions
– Or some experts

26. Answer [PR]
• A book to learn Ruby
by writing Ruby
interpreter in Ruby
– (written in Japanese)