let’s wr e an interpreter!ryan/presentations/2013_gogaruco_lets… · gogaruco 2013, san...

GoGaRuCo 2013, San Francisco, CA

Ryan Davis, Seattle.rbLet’s Write an Interpreter!

Let’s Wr!e an Interpreter!

Sunday, September 22, 13



Thank you so much!




Setting Expectations

• 168 Slides in 30 minutes. ~5.6 spm.

• Boatloads of content. Almost all code.

• I hope to “Hurt Brains”.

• TODO




How to solve a rubiks cube in 27

minutes




Interpreters in 3 minutes




OMG WHY?!?




BecauseI’m a pervert!




Wa!… No!




Because we can!




"e “Uby”Interpreter




Branching Logic:if/else/elsif

if condition then true_codeelse false_codeend




Loops: whilewhile condition codeend




Local Variablesmeaning = 42pi = 3




Functionsdef fib n if n <= 2 then 1 else fib(n-2) + fib(n-1) endend




Functions

recursive function calls

local variables

“primitive” function callsdef fib n if n <= 2 then 1 else fib(n-2) + fib(n-1) endend




Uby Roadmap

Variables Conditionals

Runtime

Environment

Functions

Parser

Loops




Parsing: ruby_parser




ConditionalsLoops

Runtime

Environment

Functions

Parser

Variables




RubyParser.new.parse " def fib n if n <= 2 then 1 else fib(n-2) + fib(n-1) end end"




:defn

:fib

:args:n

:if

:call

:lvar :n

:<

:lit 2

:lit 1

:call

:call

nil

:fib

:call

:lvar :n

:-

:lit 2:+

:call nil :fib

:call

:lvar :n

:-

:lit 1




:defn

:fib

:args

:n

:if

:call

:lvar :n

:<

:lit 2

:lit 1

:call

:call

nil

:fib

:call

:lvar :n

:-

:lit 2:+

:call nil :fib

:call

:lvar :n

:-

:lit 1




else

:call nil :fib

:call

:lvar :n

:-

:lit 1

:call nil :fib

:call

:lvar :n

:-

:lit 2

:call :+

condition

:call

:lvar :n

:<

:lit 2

:defn

:fib :args

:n

:if

then

:lit 1




s(:defn, :fib, s(:args, :n), s(:if, s(:call, s(:lvar, :n), :<=, s(:lit, 2)), s(:lit, 1), s(:call, s(:call, nil, :fib, s(:call, s(:lvar, :n), :-, s(:lit, 2))), :+, s(:call, nil, :fib, s(:call, s(:lvar, :n), :-, s(:lit, 1))))))




S-Expression Vocabulary

s(:call, s(:lit, 3), :+, s(:lit, 4))

Sexp Sub-sexps

Rest(cdr)

Head/Type(car)




All for free using ruby_parser




OK…No more talk about

parsing.




Further study on parsers:

Michael Jackson’sParsing Expressions in Ruby

MWRC 2011http://www.confreaks.com/videos/582


http://www.confreaks.com/videos/582-mwrc2011-parsing-expressions-in-ruby






http://www.confreaks.com/videos/582

http://www.confreaks.com/videos/582



Interpre#ng via sexp_processor




ConditionalsLoops

Environment

Variables

Functions

Parser

Runtime




How to get 7 from 3+4?

73 + 4 ??????




Start with the source

3 + 4




Parse it

3 + 4s(:call, s(:lit, 3), :+, s(:lit, 4))




s(:call, s(:lit, 3), :+, s(:lit, 4))

Then process by type

3 + 4

process_call

process_lit




s(:call, s(:lit, 3), :+, s(:lit, 4))

First with the inner values

3 + 4

process_lit




s(:call, 3, :+, s(:lit, 4))


3 + 4

process_lit




s(:call, 3, :+, 4)


3 + 4




s(:call, 3, :+, 4)

then with the outer

3 + 4

process_call




proc { |a, b| a + b }[3, 4]

then with the outer

3 + 4




proc { |a, b| a + b }[3, 4]

resulting in a final value

3 + 4 7




SexpProcessorclass UbyInterpreter def process_lit s s.last endend s(:lit, 3)

3




Test Dr$en Interpreters




Loops Conditionals

Runtime

Environment

Functions

Parser

Test

Dri

ven

Variables




Unit Testsdef test_sanity val = lang.eval("3 + 4") assert_equal 7, valend




Unit TestsInput Source

Expected Value

def test_sanity val = lang.eval("3 + 4") assert_equal 7, valend




Premature Refactoring

Input SourceExpected Value

def assert_eval exp, src assert_equal exp, lang.eval(src)end

def test_sanity assert_eval 7, "3 + 4"end




San% Test:3 + 4




ConditionalsLoops

Environment

Functions

Parser

Test

Dri

ven

Runtime

Variables




Test Infrastructureclass TestUbyInterpreter < MiniTest::Unit::TestCase attr_accessor :int

def setup self.int = UbyInterpreter.new end

def assert_eval exp, src, msg = nil assert_equal exp, int.eval(src), msg endend




Our First Test:class TestUbyInterpreter < MiniTest::Unit::TestCase attr_accessor :int


def assert_eval exp, src assert_equal exp, ri.eval(src) end

def test_sanity assert_eval 3, "3" assert_eval 7, "3 + 4" endend

Old Code

New Code




Our First Test:class TestUbyInterpreter < MiniTest::Unit::TestCase

# ...

def test_sanity assert_eval 3, "3" assert_eval 7, "3 + 4" endend

Old Code

New Code




Our first error:# Running tests:

E

Finished tests in 0.000364s, 2747.2527 tests/s, 0.0000 assertions/s.

1) Error:TestUbyInterpreter#test_sanity:NameError: uninitialized constant TestUbyInterpreter::UbyInterpreter ./test/test_ruby_interpreter.rb:11:in `setup'




Define the constantclass UbyInterpreter < SexpInterpreter VERSION = "1.0.0"end




Wash, rinse…# Running tests:

E


1) Error:TestUbyInterpreter#test_sanity:NoMethodError: private method `eval' called for #<UbyInterpreter:0x105be17d8> ./test/test_ruby_interpreter.rb:15:in `assert_eval' ./test/test_ruby_interpreter.rb:19:in `test_sanity'




Implement eval & parseclass UbyInterpreter < SexpInterpreter # ... attr_accessor :parser

def initialize super

self.parser = Ruby19Parser.new end

def eval src process parse src end

def parse src self.parser.process src endend




Finally!# Running tests:

E


1) Error:TestUbyInterpreter#test_sanity:UnknownNodeError: Bug! Unknown node-type :lit to UbyInterpreter ./lib/ruby_interpreter.rb:17:in `eval' ./test/test_ruby_interpreter.rb:15:in `assert_eval' ./test/test_ruby_interpreter.rb:19:in `test_sanity'




Implement process_litclass UbyInterpreter < SexpInterpreter

# ...

def process_lit s s.last endend




Tada! Second Assertion! 1) Error:TestUbyInterpreter#test_sanity:UnknownNodeError: Bug! Unknown node-type :call to UbyInterpreter ./lib/ruby_interpreter.rb:17:in `eval' ./test/test_ruby_interpreter.rb:15:in `assert_eval' ./test/test_ruby_interpreter.rb:20:in `test_sanity'




Define :call genericallyclass UbyInterpreter < SexpInterpreter

# ...

def process_call s raise "Boom: #{s.inspect}" endend




See sub-sexps 1) Error:TestUbyInterpreter#test_sanity:RuntimeError: Boom: s(:call, s(:lit, 3), :+, s(:lit, 4)) ./lib/ruby_interpreter.rb:35:in `process_call' ./lib/ruby_interpreter.rb:19:in `eval' ./test/test_ruby_interpreter.rb:17:in `assert_eval' ./test/test_ruby_interpreter.rb:22:in `test_sanity'




Unpack sub-sexpsclass UbyInterpreter < SexpInterpreter

# ...

def process_call s _, recv, msg, *args = s

raise "Boom: #{recv}, #{msg}, #{args.inspect}" endend




Unpacked sub-sexps 1) Error:TestUbyInterpreter#test_sanity:RuntimeError: Boom: s(:lit, 3), +, [s(:lit, 4)] ./lib/ruby_interpreter.rb:31:in `process_call'




Process sub-sexpsclass UbyInterpreter < SexpInterpreter def process_call s _, recv, msg, *args = s

recv = process recv args.map! { |sub| process sub }

raise "Boom: #{recv}, #{msg}, #{args.inspect}" endend




Processed Values 1) Error:TestUbyInterpreter#test_sanity:RuntimeError: Boom: 3, +, [4] ./lib/ruby_interpreter.rb:43:in `process_call'




Cheat: Send to Rubyclass UbyInterpreter < SexpInterpreter def process_call s _, recv, msg, *args = s


recv.send(msg, *args) # big ol' hack… endend




Sanity!# Running tests:

.


1 tests, 2 assertions, 0 failures, 0 errors, 0 skips



Ryan Davis, Seattle.rbLet’s Write an Interpreter!class UbyInterpreter < SexpInterpreter VERSION = "1.0.0"

attr_accessor :parser

def initialize super self.parser = Ruby19Parser.new end


def parse src self.parser.process src end

def process_lit s s.last end




Yay!

Overblown

Calculator!




Cond!ionals&

Truthiness




Loops

Runtime

Environment

Variables

Functions

Parser

Test

Dri

ven

Conditionals




Add :if testsclass TestUbyInterpreter < MiniTest::Unit::TestCase # ...

def test_if assert_eval 42, "if true then 42 else 24 end" end

def test_if_falsey assert_eval 24, "if nil then 42 else 24 end" assert_eval 24, "if false then 42 else 24 end" endend




Unknown :if 1) Error:TestUbyInterpreter#test_if:UnknownNodeError: Bug! Unknown node-type :if to UbyInterpreter ./lib/ruby_interpreter.rb:17:in èval' ./test/test_ruby_interpreter.rb:15:in àssert_eval' ./test/test_ruby_interpreter.rb:34:in `test_if'

2) Error:TestUbyInterpreter#test_if_falsey:UnknownNodeError: Bug! Unknown node-type :if to UbyInterpreter ./lib/ruby_interpreter.rb:17:in èval' ./test/test_ruby_interpreter.rb:15:in àssert_eval' ./test/test_ruby_interpreter.rb:38:in `test_if_falsey'





Define :if genericallyclass UbyInterpreter < SexpInterpreter # ...

def process_if s raise "Boom: #{s.inspect}" endend




See sub-sexps 1) Error:TestUbyInterpreter#test_if:RuntimeError: Boom: s(:if, s(:true), s(:lit, 42), s(:lit, 24)) ./lib/ruby_interpreter.rb:38:in `process_if' ... ./lib/ruby_interpreter.rb:17:in èval' ./test/test_ruby_interpreter.rb:15:in àssert_eval' ./test/test_ruby_interpreter.rb:34:in `test_if'

2) Error:TestUbyInterpreter#test_if_falsey:RuntimeError: Boom: s(:if, s(:nil), s(:lit, 42), s(:lit, 24)) ./lib/ruby_interpreter.rb:38:in `process_if' ... ./lib/ruby_interpreter.rb:17:in èval' ./test/test_ruby_interpreter.rb:15:in àssert_eval' ./test/test_ruby_interpreter.rb:38:in `test_if_falsey'




Process sub-sexpsclass UbyInterpreter < SexpInterpreter # ...

def process_if s _, c, t, f = s

c = process c

if c then process t else process f end endend

Why evaluate c before t &

f?

if true then puts "happy"else system "rm -rf /"end




Sub-sexps fail 1) Error:TestUbyInterpreter#test_if:UnknownNodeError: Bug! Unknown node-type :true to UbyInterpreter ./lib/ruby_interpreter.rb:40:in `process_if' ./lib/ruby_interpreter.rb:17:in èval' ./test/test_ruby_interpreter.rb:15:in àssert_eval' ./test/test_ruby_interpreter.rb:34:in `test_if'

2) Error:TestUbyInterpreter#test_if_falsey:UnknownNodeError: Bug! Unknown node-type :nil to UbyInterpreter ./lib/ruby_interpreter.rb:40:in `process_if' ./lib/ruby_interpreter.rb:17:in èval' ./test/test_ruby_interpreter.rb:15:in àssert_eval' ./test/test_ruby_interpreter.rb:38:in `test_if_falsey'




Define :nil and :trueclass UbyInterpreter < SexpInterpreter # ...

def process_nil s nil end

def process_true s true endend




Third failure 1) Error:TestUbyInterpreter#test_if_falsey:UnknownNodeError: Bug! Unknown node-type :false to UbyInterpreter ./lib/ruby_interpreter.rb:40:in `process_if' ./lib/ruby_interpreter.rb:17:in `eval' ./test/test_ruby_interpreter.rb:15:in `assert_eval' ./test/test_ruby_interpreter.rb:39:in `test_if_falsey'




Define :falseclass UbyInterpreter < SexpInterpreter # ...

def process_false s false endend




Voilà!# Running tests:

...






Test Driven Process

Processsub-sexps

Tests Pass

Add afailing test

Errornode-type

Add genericnode-type

Failure sub-sexpcomponents




Yes, development this fast…

with autotest.




Brain Hurt Yet?




Good




Wait till I get going!

Where was I?




Local Variables




Loops Conditionals

Runtime

Functions

Parser

Test

Dri

ven

Variables

Environment




Local Variables!class TestUbyInterpreter < MiniTest::Unit::TestCase # ...

def test_lvar assert_eval 42, "x = 42; x" endend




Block?!? 1) Error:TestUbyInterpreter#test_lvar:UnknownNodeError: Bug! Unknown node-type :block to UbyInterpreter ./lib/ruby_interpreter.rb:17:in `eval' ./test/test_ruby_interpreter.rb:15:in `assert_eval' ./test/test_ruby_interpreter.rb:24:in `test_lvar'




Define :blockclass UbyInterpreter < SexpInterpreter # ...

def process_block s raise "Boom: #{s.inspect}" endend




Ohhh… 1) Error:TestUbyInterpreter#test_lvar:RuntimeError: Boom: s(:block, s(:lasgn, :x, s(:lit, 42)), s(:lvar, :x)) ./lib/ruby_interpreter.rb:25:in `process_block' ./lib/ruby_interpreter.rb:17:in `eval' ./test/test_ruby_interpreter.rb:15:in `assert_eval' ./test/test_ruby_interpreter.rb:24:in `test_lvar'




Process all sub-sexpsclass UbyInterpreter < SexpInterpreter # ...

def process_block s result = nil s.rest.each do |sub| result = process sub end result endend




:lasgn is undefined 1) Error:TestUbyInterpreter#test_lvar:UnknownNodeError: Bug! Unknown node-type :lasgn to UbyInterpreter ./lib/ruby_interpreter.rb:27:in `process_block' ./lib/ruby_interpreter.rb:26:in èach' ./lib/ruby_interpreter.rb:26:in `process_block' ./lib/ruby_interpreter.rb:17:in èval' ./test/test_ruby_interpreter.rb:15:in àssert_eval' ./test/test_ruby_interpreter.rb:24:in `test_lvar'




Define :lasgnclass UbyInterpreter < SexpInterpreter # ...

def process_lasgn s raise "Boom: #{s.inspect}" endend




Looks like name/value 1) Error:TestUbyInterpreter#test_lvar:RuntimeError: Boom: s(:lasgn, :x, s(:lit, 42)) ./lib/ruby_interpreter.rb:58:in `process_lasgn' ./lib/ruby_interpreter.rb:27:in `process_block' ./lib/ruby_interpreter.rb:26:in èach' ./lib/ruby_interpreter.rb:26:in `process_block' ./lib/ruby_interpreter.rb:17:in èval' ./test/test_ruby_interpreter.rb:15:in àssert_eval' ./test/test_ruby_interpreter.rb:24:in `test_lvar'




Add a stupid tableclass UbyInterpreter < SexpInterpreter # ...

attr_accessor :env

def initialize # ... self.env = {} # omg this is horrible end

def process_lasgn s _, n, v = s

self.env[n] = process v endend

EnvironmentEnvironment

x 42




Now we need to read 1) Error:TestUbyInterpreter#test_lvar:UnknownNodeError: Bug! Unknown node-type :lvar to UbyInterpreter ./lib/ruby_interpreter.rb:29:in `process_block' ./lib/ruby_interpreter.rb:28:in èach' ./lib/ruby_interpreter.rb:28:in `process_block' ./lib/ruby_interpreter.rb:19:in èval' ./test/test_ruby_interpreter.rb:15:in àssert_eval' ./test/test_ruby_interpreter.rb:24:in `test_lvar'




Access the tableclass UbyInterpreter < SexpInterpreter # ...

def process_lvar s _, name = s

self.env[name] endend




Voilà# Running tests:

....






Just wait…




It gets “better”




Fun&ions




Conditionals

Runtime

Environment

Variables

Parser

Test

Dri

ven

Loops

Functions




Add :defn Testclass TestUbyInterpreter < MiniTest::Unit::TestCase # ...

def test_defn assert_eval nil, <<-EOM def double n 2 * n end EOM

assert_eval 42, "double(21)" endend




Unknown :defn 1) Error:TestUbyInterpreter#test_defn:UnknownNodeError: Bug! Unknown node-type :defn to UbyInterpreter ./lib/ruby_interpreter.rb:19:in `eval' ./test/test_ruby_interpreter.rb:17:in `assert_eval' ./test/test_ruby_interpreter.rb:45:in `test_defn'




Generic :defnclass UbyInterpreter < SexpInterpreter # ...

def process_defn s raise "Boom: #{s.inspect}" endend




Examine sub-sexps 1) Error:TestUbyInterpreter#test_defn:RuntimeError: Boom: s(:defn, :double, s(:args, :n), s(:call, s(:lit, 2), :*, s(:lvar, :n))) ./lib/ruby_interpreter.rb:44:in `process_defn' ./lib/ruby_interpreter.rb:19:in `eval' ./test/test_ruby_interpreter.rb:17:in `assert_eval' ./test/test_ruby_interpreter.rb:45:in `test_defn'




Unpack & Storeclass UbyInterpreter < SexpInterpreter # ...

def process_defn s _, name, args, *body = s

self.env[name] = [args, body]

nil endend


n 8

life 42

pi 3

double

[ s(:args, :n),

[s(:call, s(:lit, 2), :*, s(:lvar, :n))]]




What’s up with :call? 1) Error:TestUbyInterpreter#test_defn:NoMethodError: undefined method `double' for nil:NilClass ./lib/ruby_interpreter.rb:40:in `send' ./lib/ruby_interpreter.rb:40:in `process_call' ./lib/ruby_interpreter.rb:19:in `eval' ./test/test_ruby_interpreter.rb:17:in `assert_eval' ./test/test_ruby_interpreter.rb:54:in `test_defn'




No NilClass#doubleclass UbyInterpreter < SexpInterpreter # ...







Isolate the Problemclass UbyInterpreter < SexpInterpreter # ...



if recv then recv.send(msg, *args) # less of a hack else raise "argh" end endend




Error, Isolated 1) Error:TestUbyInterpreter#test_defn:RuntimeError: argh ./lib/ruby_interpreter.rb:43:in `process_call' ./lib/ruby_interpreter.rb:19:in `eval' ./test/test_ruby_interpreter.rb:17:in `assert_eval' ./test/test_ruby_interpreter.rb:54:in `test_defn'




Set vars & run the codeclass UbyInterpreter < SexpInterpreter # ...



if recv then recv.send(msg, *args) # less of a hack else decls, body = self.env[msg]

decls.rest.zip(args).each do |name, val| self.env[name] = val end

process_block s(:block, *body) end endend




args... decls... zip bwuh?





n 21

life 42

pi 3

double

[ s(:args, :n),

s(:call, s(:lit, 2), :*, s(:lvar, :n))]


n 8

life 42

pi 3

double

[ s(:args, :n),

s(:call, s(:lit, 2), :*, s(:lvar, :n))]

s(:call, nil, :double, s(:lit, 21))

decls.rest.zip(args).each do |k, v| self.env[k] = vend

[:n].zip([21]).each do |k, v| self.env[k] = vend

self.env[:n] = 21

decls

args




Voilà!# Running tests:

.....






“Getting to green just means you don’t have enough

tests.”– Kent Beck




Fibonacci




Conditionals

Runtime

Parser

Test

Dri

ven

Environment

Loops

Functions

Variables




Finally Freakin’ Fibonacci!class TestUbyInterpreter < MiniTest::Unit::TestCase # ...

def test_fib assert_eval nil, <<-END def fib n if n <= 2 then 1 else fib(n-2) + fib(n-1) end end END

assert_eval 8, "fib(6)" endend




Whoa… it ran… but failed 1) Failure:TestUbyInterpreter#test_fib [./test/test_ruby_interpreter.rb:71]:Expected: 8 Actual: 3






n 4

life 42

pi 3

fib

[ s(:args, :n),

s(:if, ...)]


n 6

life 42

pi 3

fib

[ s(:args, :n),

s(:if, ...)]


n 8

life 42

pi 3

fib

[ s(:args, :n),

s(:if, ...)]

That stupid Table?Yeah...

def fib n if n <= 2 then 1 else fib(n-2) + fib(n-1) endendn = 8; fib(6)n = 6; fib(4) + fib(5)n = 4; ... and so on...




[ s(:args, :n),

s(:if, ...)]

fib

3pi

42life

n 8

[ s(:args, :n),

s(:if, ...)]

fib

3pi

42life

n 5

[ s(:args, :n),

s(:if, ...)]

fib

3pi

42life

n 3

[ s(:args, :n),

s(:if, ...)]

fib

3pi

42life

n 1

Wanted: Stacked Hashn 1

n 3

n 5

[ s(:args, :n),

s(:if, ...)]

fib

3pi

42life

n 8

fib(1)

fib(3)

fib(5)

original

but acts like:




6z

5y

4x

2b

3c

42x

1a

z 6

y 5

x 42

3c

2b

1a

Environment Classenv[:x] == ?

env[:x] = 42

all:

scope:

scope:

scope:



Ryan Davis, Seattle.rbLet’s Write an Interpreter!class UbyInterpreter < SexpInterpreter # ...

class Environment def [] k self.all[k] end

def []= k, v @env.last[k] = v end

def all @env.inject(&:merge) end

def scope @env.push({})

yield ensure @env.pop end

def initialize @env = [{}] end endend

Writes to top layer only

Reads from everything

Flattened. Newest wins

Automatic layering




Replace stupid tableclass UbyInterpreter < SexpInterpreter # ...

def initialize # ... self.env = Environment.new end

# ...end




Scope every callclass UbyInterpreter < SexpInterpreter # ...



if recv then recv.send(msg, *args) # less of a hack else self.env.scope do decls.rest.zip(args).each do |name, val| self.env[name] = val end

process_block s(:block, *body) end end endend




Finally# Running tests:

......






While Loops




Runtime

Environment

Variables

Functions

Parser

Test

Dri

ven

ConditionalsLoops




Refactor Fibonnaciclass TestUbyInterpreter < MiniTest::Unit::TestCase # ...

def define_fib assert_eval nil, <<-END def fib n if n <= 2 then 1 else fib(n-2) + fib(n-1) end end END endend




Add :while testsclass TestUbyInterpreter < MiniTest::Unit::TestCase # ...

def test_while_sum_of_fibs define_fib

assert_eval 1+1+2+3+5+8+13+21+34+55, <<-EOM n = 1 sum = 0 while n <= 10 sum += fib(n) n += 1 end sum EOM endend




Unknown :while 1) Error:TestUbyInterpreter#test_while_fib:UnknownNodeError: Bug! Unknown node-type :while to UbyInterpreter ./lib/ruby_interpreter.rb:29:in `process_block' ./lib/ruby_interpreter.rb:28:in èach' ./lib/ruby_interpreter.rb:28:in `process_block' ./lib/ruby_interpreter.rb:19:in èval' ./test/test_ruby_interpreter.rb:15:in àssert_eval' ./test/test_ruby_interpreter.rb:86:in `test_while_fib'




Define :while genericallyclass UbyInterpreter < SexpInterpreter # ...

def process_while s raise "Boom: #{s.inspect} endend




See sub-sexps 1) Error:TestUbyInterpreter#test_while:RuntimeError: Boom: s(:while, s(:call, s(:lvar, :n), :<, s(:lit, 41)), s(:lasgn, :n, s(:call, s(:lvar, :n), :+, s(:lit, 1))), true) ./lib/ruby_interpreter.rb:104:in `process_while' ./lib/ruby_interpreter.rb:29:in `process_block' ./lib/ruby_interpreter.rb:28:in èach' ./lib/ruby_interpreter.rb:28:in `process_block' ./lib/ruby_interpreter.rb:19:in èval' ./test/test_ruby_interpreter.rb:15:in àssert_eval' ./test/test_ruby_interpreter.rb:62:in `test_while'




Define :whileclass UbyInterpreter < SexpInterpreter # ...

def process_while s _, cond, *body = s body.pop # pre vs post condition -- ignore for now

while process cond process_block s(:block, *body) end endend




Voilà# Running tests:

.......






What Have We Done?



Ryan Davis, Seattle.rbLet’s Write an Interpreter!require "ruby_parser"require "sexp_processor"

class UbyInterpreter < SexpInterpreter VERSION = "1.0.0"

attr_accessor :parser attr_accessor :env

def initialize super

self.parser = Ruby19Parser. new self.env = Environment.new end


def parse src self.parser.process src end

def process_block s result = nil s.rest.each do |sub| result = process sub end result end



if recv then recv.send(msg, *args) else d, body = self.env[msg]

self.env.scope do d.rest.zip(args). each do |k, v| self.env[k] = v end

process_block s(:block, *body) end end end

def process_defn s _, name, args, *body = s

self.env[name] = [args, body]

nil end

def process_false s false end

def process_if s _, c, t, f = s

c = process c

if c then process t else process f end end

def process_lasgn s _, name, val = s

self.env[name] = process val end

def process_lit s s.last end

def process_lvar s _, name = s

self.env[name] end

def process_nil s nil end

def process_true s true end

def process_while s _, cond, *body = s body.pop

while process cond process_block s(:block, *body) end end

class Environment def [] k self.all[k] end

def []= k, v @env.last[k] = v end

def all @env.inject(&:merge) end

def scope @env.push({})

yield ensure @env.pop end

def initialize @env = [{}] end endend




class TestUbyInterpreter < MiniTest::Unit::TestCase attr_accessor :int


def assert_eval exp, src, msg = nil assert_equal exp, int.eval(src), msg end

def define_fib assert_eval nil, <<-END def fib n if n <= 2 then 1 else fib(n-2) + fib(n-1) end end END end

def test_sanity assert_eval 3, "3" assert_eval 7, "3 + 4" end

def test_defn assert_eval nil, <<-EOM n = 24 def double n 2 * n end EOM

assert_eval 42, "double(21)" end

def test_fib define_fib

assert_eval 8, "fib(6)" end

def test_if assert_eval 42, "if true then 42 else 24 end" end

def test_if_falsey assert_eval 24, "if nil then 42 else 24 end" assert_eval 24, "if false then 42 else 24 end" end

def test_lvar assert_eval 42, "x = 42; x" end

def test_while_fib define_fib

assert_eval 1+1+2+3+5+8+13+21+34+55, <<-EOM n = 1 sum = 0 while n <= 10 sum += fib(n) n += 1 end sum EOM endend




“Uby” Language

• Basic numeric types, true, false, nil.• Conditional branching and looping.• Primitive & user defined functions.• Local variables & variable scoping.• Test-driven, extensible, patterns-based design.• ~2 hours, ~130 LOC impl, ~70 LOC test.• Fits in one head.




What Else Can We Do?• Add richer types: strings, arrays, hashes, etc.

• Enforce different scoping rules (eg. ruby's opaque def vs scheme's transparent lambdas).

• Implement recursive tail-calls or closures.

• Add an object system.

• Change the way functions are called.




Further Study




Fantastic Books




No time for questions.

Please grab me at the afterparty.




"ank You…




This is the last talk!




I get to correct an injustice…




…from twitter.




He who made it…




…defines it.




“Sea!le S"le”




Warning Free




No extra syntax




Not in 'def' lines




Not in calls




Nowhere




This:remove_parens while warning_free and passes_tests




Not this:remove_parens() while (warning_free() and passes_tests())




Not Seattle Styledef method() call()end

def method(arg) call(arg)end

def method(arg1, arg2) call(arg1, arg2)end

call1(call2(call3()))




Seattle Styledef method callend

def method arg call argend

def method arg1, arg2 call arg1, arg2end

call1 call2 call3

call(/regexp/) # ambiguous w/o parens




"ank You!


let’s wr e an interpreter!ryan/presentations/2013_gogaruco_lets… · gogaruco 2013, san...

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