parson

otstrap/blob/master/peg.md (from a peg grammar with semantic actions to javascript, 66 lines of code) - https://github.com/kragen/peg-bootstrap/blob/master/handaxew... (tangles a literate program in virtually any plain-text format, such as markdown, html, restructuredtext, or tex, into virtually any plain-text programming language, 208 lines of code; used to tangle both itself and the peg compiler compiler above) - http://canonical.org/~kragen/sw/dev3/meta5ix.m5 (from meta5ix, a language for writing parsers derived from val schorre's meta-ii, to meta5ix assembly, 18 lines of code; it is a top-down parsing language with limited backtracking, like pegs, but less expressive and implementable without heap allocation or the ability to backtrack past the beginning of a line; see http://canonical.org/~kragen/sw/dev3/meta5ixrun.py for an m5asm interpreter) - http://canonical.org/~kragen/sw/dev3/meta5ix2c2.m5 (from a slightly different variation of meta5ix to c, 133 lines of code) most of these do take the same ast-free approach as the linked blog post and crenshaw, just emitting code as they finish parsing a production (or, in the case of stoneknifeforth, a token), but ur-scheme in particular does not; it uses scheme s-expressions as its syntax tree, because some kind of syntax tree is necessary to be able to allocate some local variables on the stack while also supporting closures. peg-bootstrap can certainly handle code that builds an ast with its semantic actions, but its implementation of itself just glues together snippets of javascript text also worth mentioning in this connection: - darius bacon's work, especially parson (for example, https://github.com/darius/parson/blob/master/eg_calc_compile...) - the oberon compiler in wirth and gutknecht's oberon book - the later chapters of sicp

articularly awkward and inefficient way to output that 1 or 0. A compiler for Brainf*** or SK-combinators or the untyped λ-calculus would be "more real" in that sense. More practical, and maybe also fitting into 150 lines, might be a language for something like a programmable calculator (untested sketch): program ::= stmt | stmt ";" program. stmt ::= "print" numexpr | variable ":=" numexpr | "while" numexpr comparator numexpr "{" program "}". comparator ::= "<" | ">" | "==" | "!=" | "<=" | ">=". numexpr ::= term | numexpr "+" term | numexpr "-" term. term ::= factor | term "*" factor | term "/" factor | term "%" factor. factor ::= atom | "-" atom | "+" atom | atom "^" factor. atom ::= "(" numexpr ")" | numeric_constant | variable. That's powerful enough to conveniently write, for example, a numerical root finding program for an arbitrary arithmetic expression. Maybe for pedagogical purposes (which, as I said, I am totally ignorant of) it would be useful to leave out one of the levels of precedence and some of the operators and have the students add them as an exercise. But I think that within a complexity budget of 150 lines of code you can maybe be even more ambitious than that. Darius Bacon's example compiler in https://github.com/darius/parson/blob/master/eg_calc_compile... is a bit more stripped down than that, but in its 32 lines of code it compiles arithmetic assignment statements to a three-address RISC-like code (though using an unbounded number of registers). https://github.com/darius/parson/blob/master/eg_calc_to_rpn.... is a 16-line version that compiles the same language to a stack machine like your tutorial example: from parson import Grammar, alter g = Grammar(r""" stmt* :end. stmt : ident '=' exp0 ';' :assign. exp0 : exp1 ('+' exp1 :'add')*. exp1 : exp2 ('*' exp2 :'mul')*. exp2 : '(' exp0 ')' | /(\d+)/ | ident :'fetch'. ident : /([A-Za-z]+)/. FNORD ~: /\s*/. """)(assign=alter(lambda name, *rpn: rpn + (name, 'store'))) ## print ' '.join(g('v = 42 * (5+3) + 2*2; v = v + 1;')) #. 42 5 3 add mul 2 2 mul add v store v fetch 1 add v store Of course that still contains no control flow, so it's still not a very convincing example of why you'd take the extra trouble to write a compiler rather than an interpreter. In 66 lines of code in https://github.com/kragen/peg-bootstrap/blob/master/peg.md I wrote an example compiler which compiles a PEG grammar into a JavaScript parser for that grammar. Admittedly those 66 lines do not include an implementation of JavaScript to run the code on. It compiles the language it's written in. In 132 lines of code in https://github.com/kragen/stoneknifeforth/blob/master/tinybo... I wrote an example compiler which compiles a crippled Forth dialect into i386 machine code, including an ELF header so you can run the result. It also compiles the language it's written in. It also doesn't include an i386 emulator to run it on. In 83 lines of code in http://canonical.org/~kragen/sw/dev3/neelcompiler.ml Neel Krishnaswami wrote a compiler from the untyped λ-calculus to a simple assembly language for a register machine. It also doesn't include an implementation of the assembly language. In 18 lines of code in http://canonical.org/~kragen/sw/dev3/meta5ix.m5, a simplification of Val Schorre's META-II, I wrote a compiler from grammar descriptions to an assembly code for a parsing-oriented virtual machine. It compiles the language it's written in. A Python interpreter for the machine is in http://canonical.org/~kragen/sw/dev3/meta5ixrun.py (109 lines of code) and a precompiled version of the compiler-compiler for bootstrapping is in http://canonical.org/~kragen/sw/dev3/meta5ix.generated.m5asm. It does recursive descent with only a single token of backtracking; a minimal summary of the most eccentric aspects of its syntax is that "" encloses text to expect on input, {} encloses text to output, <<>> encloses a token to parse from the input that might be copied to the output with $it (or unindented with @it), commas separate alternatives, and [x] is zero or more repetitions of x. - program: ["-" name @it ":" terms {return}, "#" [:notin ""]] - terms: term ["," {continue $choice} term] @choice - term: (factor {else $seq}, output) [factor {assert}, output] @seq - factor: string {literal $it} , "(" terms ")" , "[" @many terms {continue $many} "]" , name {call $it} , "<<" {begin} terms ">>" {end} , ":fnord" {fnord} factor , ":notin" string {notin $it} , ":between" string {between $it} - output: (quasiquote, "@" {dedent} (var, quasiquote)) {writeline} - quasiquote: "{" [(<<ch [ch]>>, "\" <<:notin "">>) {say "$it"}, "$" var] "}" - ch: :notin "$}\" - var: "it" {copyinput}, name {gen $it} - string: :fnord <<'"' [:notin '"'] '"', "'" [:notin "'"] "'">> - name: :fnord <<letter [letter, :between "09"]>> - letter: :between "az", :between "AZ", :between "__" A slightly incompatible variant of Meta5ix which instead compiles itself to C is in http://canonical.org/~kragen/sw/dev3/meta5ix2c.m5 (133 lines of code, depending on how you count). (No C compiler is included.) The precompiled C output for bootstrapping is in http://canonical.org/~kragen/sw/dev3/meta5ix2c.c. Meta5ix is extremely weak and limited, really only enough for a compiler front-end. Dave Long described META-II as "a field-improvised lever", which I think is still apt for Meta5ix. Now, again, I am totally ignorant about pedagogy, so I could be wrong about this, but I think that probably self-compiling compiler-compilers like most of my examples here are not the best tutorial examples, because they refer to themselves in complex and confusing ways. But I feel like they represent pretty good evidence that you can do something a lot more interesting in a 150-line-of-code compiler than evaluating a Boolean circuit.

-- just something I've been using in my own projects.) Of course, another reason to support left recursion is to let you use a grammar someone else has already written with left-recursion baked in, without having to mess with it.