Mario Arias RedDotRuby Conference 2024 Write an interpreter in Ruby 

About me • Not a Ruby expert • Ruby-curious? • A Kotlin nerd • Almost 20 years of experience • OSS Contributor 2 

Why do you want to write an interpreter? … is everything ok at home? • Is fun • To refresh or to a cquire new knowledge a bout computer science. • Useful c a ses: • Expression l a ngu a ges • Business rules • Extern a l DSLs 3 

The Monkey Language … it doesn’t exists, sort of, in a way • Cre a ted by Thorsten B a ll • It exists when you write it • C-like-JS-like synt a x • First-cl a ss a nd high-order functions • Integers, boole a ns, a rr a ys a nd h a shes • It h a s a reference implement a tion in Go, with unit tests 4 

A f ibonacci function Recursive let fibonacci = fn(x) { if (x < 2) { return x; } else { fibonacci(x - 1) + fibonacci(x - 2); } }; fibonacci(35); 5 

Pepa … like the character from the movie Encanto • A Ruby implement a tion of the Monkey L a ngu a ge • Ruby 3.1 a nd onw a rds • Fully tested • RBS • https://github.com/M a rioAri a sC/pep a 6 

The interpreter f low …a simpli fi ed version of it Source code Lexer Tokens P a rser AST Ev a lu a tor Execution 7 

Tokens … not the LLM ones https://github.com/M a rioAri a sC/pep a /blob/m a in/lib/tokens.rb let fibonacci = fn(x) { if (x < 2) { return x; } else { fibonacci(x - 1) + fibonacci(x - 2); } }; fibonacci(35); LET ^ “let” FUNCTION ^ “fn” RETURN ^ “return” ELSE ^ “else” IF ^ “if” IDENT ^ “ f ibon a cci” IDENT ^ “ f ibon a cci” IDENT ^ “ f ibon a cci” IDENT ^ “ f ibon a cci” IDENT ^ “x” IDENT ^ “x” ASSIGN ^ “=” LPAREN ^ “(” RPAREN ^ “)” LBRACE ^ “(” 8 

Lexer One character at the time https://github.com/M a rioAri a sC/pep a /blob/m a in/lib/lexers.rb We need to m a n a ge sever a l scen a rios • Single ch a r a cter tokens: =, (, ), {, } a nd others • Double ch a r a cter tokens: !=, == • Strings • Integers • One a lph a betic ch a r a cter + one or more a lph a numeric ch a r a cters • Reserved word • Identi f ier 9 

AST let fibonacci = fn(x) { if (x < 2) { return x; } else { fibonacci(x - 1) + fibonacci(x - 2); } }; fibonacci(35); https://github.com/M a rioAri a sC/pep a /blob/m a in/lib/ a st.rb Progr a m • st a tements • let st a tement • function liter a l • p a r a meters • block st a tement • if expression • condition • in f ix expression • identi f ier • oper a tor • integer liter a l … 10 

AST https://github.com/M a rioAri a sC/pep a /blob/m a in/lib/ a st.rb Progr a m: List of St a tements St a tement LetSt a tement: let foo = 1; ExpressionSt a tement: A st a tement th a t cont a ins a n expression ReturnSt a tement: return foo; BlockSt a tement: List of St a tements inside a block Expression Pre f ixExpression: !true; In f ixExpression: 1 + 1; C a llExpression: myFunction(1); IndexExpression: myHash[“foo”] IfExpression: if (x > 2) {“x”} else {“y”} Identi f ier IntegerLiter a l: 1; Boole a nLiter a l: true; Arr a yLiter a l: [1, 2, 3]; FunctionLiter a l fn(x) {x + 2} StringLiter a l: “Hello” H a shLiter a l: {“foo”: 1, true:2, 3:3} 11 

Parser One token at the time https://github.com/M a rioAri a sC/pep a /blob/m a in/lib/p a rsers.rb P a rsers Recursive descent p a rsers Top-down oper a tor precedence p a rser Top-down oper a tor precedence p a rser a .k. a “Pr a tt P a rser”, cre a ted by V a ugh a n Pr a tt • E a sy to implement a nd underst a nd • Slowish […] is very simple to understand, trivial to implement, easy to use, extremely ef f icient in practice if not in theory, yet f lexible enough to meet most reasonable syntactic needs of users […] 12 

Parser One token at the time P a rse a progr a m, i.e., a list of st a tements Every method uses parse_expression 13 

Parser One token at the time • Is there a method to p a rse the current token? • INT, TRUE, FALSE, STRING, FUNCTION • IDENT, • BANG, MINUS, • LPAREN, LBRACKET, LBRACE • IF • Is there a nother token on the right, a nd does it h a ve the correct precedence? • Is there a method to p a rse the next token? • PLUS, MINUS, SLASH, ASTERISK, LT, GT, EQ, NOT_EQ • LPAREN e.g., myFuction ( … • LBRACKET e.g., myArray [ … • 1 + 2 + 3 => ((1 + 2) + 3) 14 

Evaluator …now we’re talking To ev a lu a te, we need three things: • A w a y to communic a te with our host l a ngu a ge. We will c a ll it the Object System • https://github.com/M a rioAri a sC/pep a /blob/m a in/lib/objects.rb • Where to store v a lues a nd their scopes, e.g. Identi f iers, p a r a meters. We will c a ll it the Environment • The ev a lu a tor, in our c a se, is a Tree-W a lking ev a lu a tor • https://github.com/M a rioAri a sC/pep a /blob/m a in/lib/ev a lu a tor.rb 15 

Evaluator …now we’re talking Ev a lu a te a ll st a tements, return the l a st one Recursively w a lks the AST 16 

Evaluator …now we’re talking Re a d from the environment or the BUILTINS Re a d from itself or its outer Environment 17 

Executing …Gotta go fast!! hyperfine - w 3 './benchmark' Benchmark 1: ./benchmark Time (mean ± σ): 149.411 s ± 1.317 s [User: 149.215 s, System: 0.193 s] Range (min … max): 146.906 s … 150.827 s 10 runs Ruby 3.3.3 - 20.5 MB 18 

Executing …Gotta go fast!! Python 3.12 - 11.37 MB hyperfine - w 1 - M 3 'python benchmarks.py' Benchmark 1: python benchmarks.py Time (mean ± σ): 682.299 s ± 3.465 s [User: 468.764 s, System: 204.651 s] Range (min … max): 678.526 s … 685.340 s 3 runs 0 s 175 s 350 s 525 s 700 s Ruby 3.3.3 Python 3.12 682.299 149.411 https://github.com/M a rioAri a sC/bruno 19 

Executing …Gotta go fast!! Lu a - 2.75 MB hyperfine - w 3 'lua benchmarks.lua' Benchmark 1: lua benchmarks.lua Time (mean ± σ): 140.583 s ± 2.741 s [User: 140.575 s, System: 0.004 s] Range (min … max): 138.264 s … 147.545 s 10 runs 0 s 175 s 350 s 525 s 700 s Ruby 3.3.3 Python 3.12 Lu a 140.583 682.299 149.411 https://github.com/M a rioAri a sC/juliet a 20 

The Lua code in this post is c**p. Get rid of the silly OO emulation and it would be tons faster. - Some Reddit user. 21 

Executing …Gotta go fast!! Lu a JIT - 2.62 MB hyperfine - w 3 'luajit benchmarks.lua' Benchmark 1: luajit benchmarks.lua Time (mean ± σ): 82.749 s ± 1.416 s [User: 82.740 s, System: 0.002 s] Range (min … max): 80.846 s … 85.703 s 10 runs 175 s 350 s 525 s 700 s Ruby 3.3.3 Python 3.12 Lu a Lu a JIT 82.749 140.583 682.299 149.411 22 

Executing …Gotta go fast!! Ruby YJIT - 21.25 MB hyperfine - w 3 './benchmark - linux - yjit' Benchmark 1: ./benchmark - linux - yjit Time (mean ± σ): 52.484 s ± 0.681 s [User: 52.279 s, System: 0.197 s] Range (min … max): 51.637 s … 54.159 s 10 runs 175 s 350 s 525 s 700 s Ruby 3.3.3 Python 3.12 Lu a Lu a JIT Ruby YJIT 52.484 82.749 140.583 682.299 149.411 Good Job M a xime! 23 

Executing …Gotta go fast!! JRuby - 751.07 MB hyperfine - w 3 './benchmark - linux - jruby' -- export - json jruby.json Benchmark 1: ./benchmark - linux - jruby Time (mean ± σ): 186.716 s ± 23.751 s [User: 195.723 s, System: 1.971 s] Range (min … max): 119.831 s … 199.868 s 10 runs Warning: Statistical outliers were detected. 175 s 350 s 525 s 700 s Ruby 3.3.3 Python 3.12 Lu a Lu a JIT Ruby YJIT JRuby 186.716 52.484 82.749 140.583 682.299 149.411 - Xcompile.invokedynamic 24 

Executing …Gotta go fast!! Tru ff leRuby - 1946.62 MB hyperfine - w 3 './benchmark - truffleruby' Benchmark 1: ./benchmark - truffleruby Time (mean ± σ): 8.173 s ± 0.177 s [User: 11.883 s, System: 0.716 s] Range (min … max): 7.898 s … 8.460 s 10 runs 175 s 350 s 525 s 700 s Ruby 3.3.3 Python 3.12 Lu a Lu a JIT Ruby YJIT JRuby Tru ff leRuby 8.173 186.716 52.484 82.749 140.583 682.299 149.411 25 

Executing …Gotta go fast!! Go 1.22.4 - 8 MB hyperfine - w 3 './fibonacci -- engine=eval' Benchmark 1: ./fibonacci -- engine=eval Time (mean ± σ): 10.546 s ± 0.042 s [User: 11.254 s, System: 0.353 s] Range (min … max): 10.503 s … 10.650 s 10 runs 175 s 350 s 525 s 700 s Ruby 3.3.3 Python 3.12 Lu a Lu a JIT Ruby YJIT JRuby Tru ff leRuby G o 1.22.4 10.546 8.173 186.716 52.484 82.749 140.583 682.299 149.411 https://github.com/M a rioAri a sC/monkey 26 

Executing …Gotta go fast!! C - 1.75 MB hyperfine - w 3 './bin/benchmark eval' Benchmark 1: ./bin/benchmark eval Time (mean ± σ): 10.583 s ± 0.211 s [User: 10.581 s, System: 0.001 s] Range (min … max): 10.407 s … 11.075 s 10 runs 175 s 350 s 525 s 700 s Ruby 3.3.3 Python 3.12 Lu a Lu a JIT Ruby YJIT JRuby Tru ff leRuby G o 1.22.4 C 10.583 10.546 8.173 186.716 52.484 82.749 140.583 682.299 149.411 https://github.com/ a bhin a v-up a dhy a y/cmonkey 27 

Executing …Gotta go fast!! Cryst a l 1.12.2 - 4.25 MB hyperfine - w 3 './benchmarks.sh -- eval - fast' Benchmark 1: ./benchmarks.sh -- eval - fast Time (mean ± σ): 4.942 s ± 0.102 s [User: 4.935 s, System: 0.006 s] Range (min … max): 4.829 s … 5.189 s 10 runs 175 s 350 s 525 s 700 s Ruby 3.3.3 Python 3.12 Lu a Lu a JIT Ruby YJIT JRuby Tru ff leRuby G o 1.22.4 C ryst a l 1.12.2 4.942 10.583 10.546 8.173 186.716 52.484 82.749 140.583 682.299 149.411 https://github.com/M a rioAri a sC/monyet 28 

Ruby Cryst a l 29 

Even faster?! Is it possible? We need a di ff erent a ppro a ch. A new kind of interpreter: • Production-re a dy • VM a nd m a ybe a JIT • Written by sm a rt people. 30 

Of course, Ruby!! D’oh! But Ruby c a nnot interpret Monkey code, therefore we need … 31 

Mario Arias RedDotRuby 2024 Write an interpreter in Ruby Transpiler 32 

Our new f low We have 80% of it Source code Lexer Tokens P a rser AST Ev a lu a tor Execution Tr a nspil a tion Kernel.eval 33 

Transpilation Everyone knows what to do Invoke to_rb in a ll the st a tements M a n a ge the synt a ctic a l di ff erences 34 

Executing …Gotta go fast!! Ruby Tr a nspiled - 20.38 MB hyperfine - w 3 './exe/benchmark - linux - tr - yjit' Benchmark 1: ./exe/benchmark - linux - tr - yjit Time (mean ± σ): 150.3 ms ± 1.2 ms [User: 133.1 ms, System: 16.9 ms] Range (min … max): 148.3 ms … 152.6 ms 19 runs 175 s 350 s 525 s 700 s Ruby 3.3.3 Python 3.12 Lu a Lu a JIT Ruby YJIT JRuby Tru ff leRuby G o 1.22.4 C ryst a l 1.12.2 Tr a nspiled 0.15 4.942 10.583 10.546 8.173 186.716 52.484 82.749 140.583 682.299 149.411 35 

The best way to write an interpreter is not to write an interpreter - Me 36 

Thanks!! 37