Fix env model resolves #11 - I call this good enough :)

Fix commit message

README.md

@@ -1,3 +1,9 @@
 # schemers
 
 A crude Scheme interpreter implemented for SICP exercise 5.51.
+
+```bash
+cargo build
+./target/debug/schemers test.scm
+```
+

src/environment.rs

@@ -0,0 +1,118 @@
+use crate::parser::make_symbol;
+use crate::parser::Datum;
+use std::cell::RefCell;
+use std::mem;
+use std::rc::Rc;
+
+type Mapping = (Datum, Datum);
+
+#[derive(Debug)]
+pub struct Frame {
+    mappings: Vec<Mapping>,
+    outer: Option<Rc<RefCell<Frame>>>,
+}
+
+pub type Env = Rc<RefCell<Frame>>;
+
+pub fn get_global_environment() -> Env {
+    use crate::parser::Datum::Primitive;
+    use crate::primitives::*;
+    let f = Frame {
+        mappings: vec![
+            (make_symbol("+"), Primitive(add)),
+            (make_symbol("*"), Primitive(mul)),
+            (make_symbol("-"), Primitive(sub)),
+            (make_symbol("="), Primitive(eq)),
+            (make_symbol("<"), Primitive(lt)),
+            (make_symbol("or"), Primitive(or)),
+            (make_symbol("and"), Primitive(and)),
+            (make_symbol("not"), Primitive(not)),
+            (make_symbol("list"), Primitive(list)),
+            (make_symbol("cons"), Primitive(cons)),
+            (make_symbol("car"), Primitive(car)),
+            (make_symbol("cdr"), Primitive(cdr)),
+            (make_symbol("pair?"), Primitive(pair)),
+            (make_symbol("null?"), Primitive(null)),
+            (make_symbol("display"), Primitive(display)),
+        ],
+        outer: None,
+    };
+    Rc::new(RefCell::new(f))
+}
+
+pub fn lookup_variable_value(var: &Datum, env: &Env) -> Datum {
+    let symbol_name = Datum::as_str(var).expect("LOOKUP-VARIABLE-VALUE -- not a symbol");
+    let frame = env.borrow();
+
+    for mapping in &frame.mappings {
+        if let (Datum::Symbol(current_name), datum) = mapping {
+            if symbol_name == *current_name {
+                return datum.clone();
+            }
+        }
+    }
+
+    match &frame.outer {
+        Some(outer_env) => lookup_variable_value(var, &outer_env),
+        _ => panic!("LOOKUP-VARIABLE-VALUE -- does not exist {}", var),
+    }
+}
+
+pub fn define_variable(var: &Datum, val: Datum, env: &mut Env) -> Datum {
+    let symbol_name = Datum::as_str(var).expect("DEFINE-VARIABLE -- not a symbol");
+    let mut frame = env.borrow_mut();
+
+    // Assign to variable if it already exits
+    for mapping in &mut frame.mappings {
+        if let (Datum::Symbol(current_name), _) = mapping {
+            if symbol_name == *current_name {
+                mapping.1 = val;
+                return var.clone();
+            }
+        }
+    }
+
+    // Create variable because it does not exist yet
+    let pair = (var.clone(), val);
+    frame.mappings.push(pair);
+    var.clone()
+}
+
+pub fn set_variable(var: &Datum, val: Datum, env: &Env) -> Datum {
+    let symbol_name = Datum::as_str(var).expect("SET-VARIABLE -- not a symbol");
+    let mut frame = env.borrow_mut();
+    for mapping in &mut frame.mappings {
+        if let (Datum::Symbol(current_name), _) = mapping {
+            if symbol_name == *current_name {
+                mapping.1 = val;
+                return var.clone();
+            }
+        }
+    }
+
+    match &frame.outer {
+        Some(outer_env) => set_variable(var, val, &outer_env),
+        _ => panic!("LOOKUP-VARIABLE-VALUE -- does not exist {}", var),
+    }
+}
+
+pub fn extend(vars: Datum, vals: Datum, env: &mut Env) -> Env {
+    let mut mappings = vec![];
+    if let (Datum::List(mut vars), Datum::List(mut vals)) = (vars, vals) {
+        if vars.len() != vals.len() {
+            panic!("EXTEND-ENVIRONMENT -- length of vars and vals does not match")
+        }
+        for i in 0..vars.len() {
+            let pair = (mem::take(&mut vars[i]), mem::take(&mut vals[i]));
+            mappings.push(pair);
+        }
+    } else {
+        panic!("EXTEND-ENVIRONMENT -- vars or vals not lists")
+    }
+
+    let f = Frame {
+        mappings,
+        outer: Some(env.clone()),
+    };
+    Rc::new(RefCell::new(f))
+}

src/interpreter.rs

@@ -0,0 +1,206 @@
+use crate::environment;
+use crate::environment::Env;
+use crate::parser::Datum;
+use crate::parser::Datum::*;
+
+pub fn is_true(exp: &Datum) -> bool {
+    match exp {
+        Boolean(b) => *b,
+        Number(_) => true,
+        List(v) if has_tag(v, "list") => true,
+        _ => panic!("IS-TRUE -- unknown-truth-value {:?}", exp),
+    }
+}
+
+pub fn has_tag(vec: &Vec<Datum>, tag: &str) -> bool {
+    if vec.len() == 0 {
+        return false;
+    }
+    if let Symbol(s) = &vec[0] {
+        if s == tag {
+            return true;
+        }
+    }
+    false
+}
+
+fn interpret_if(args: &Vec<Datum>, env: &mut Env) -> Datum {
+    let predicate = &args[1];
+    let alternative = &args[2];
+
+    let predicate_interpreted = interpret(predicate, env);
+    if is_true(&predicate_interpreted) {
+        interpret(alternative, env)
+    } else {
+        if args.len() == 4 {
+            let consequent = &args[3];
+            interpret(consequent, env)
+        } else {
+            Unspecified
+        }
+    }
+}
+
+fn interpret_set(args: &Vec<Datum>, env: &mut Env) -> Datum {
+    match &args[1] {
+        Symbol(_) => {
+            let var = &args[1];
+            let val = interpret(&args[2], env);
+            environment::set_variable(var, val, env)
+        }
+        _ => panic!("INTERPRET-SET -- cannot set {:?}", args),
+    }
+}
+
+fn interpret_define(args: &Vec<Datum>, env: &mut Env) -> Datum {
+    match &args[1] {
+        Symbol(_) => {
+            // regular definition
+            let var = &args[1];
+            let val = interpret(&args[2], env);
+            environment::define_variable(var, val, env)
+        }
+        List(v) => {
+            // procedure definition
+            let definition_var = &v[0];
+            let mut parameters = vec![];
+            for p in &v[1..] {
+                if let Symbol(_) = &p {
+                    parameters.push(p.clone());
+                } else {
+                    panic!("INTERPRET-DEFINE -- parameter not a symbol {:?}", p);
+                }
+            }
+
+            let body = args[2..].to_vec();
+            let proc = Procedure {
+                parameters: parameters,
+                body: body,
+                env: env.clone(),
+            };
+            environment::define_variable(definition_var, proc, env)
+        }
+        _ => panic!("INTERPRET-DEFINE -- cannot define {:?}", args),
+    }
+}
+
+fn interpret_cond(vec: &Vec<Datum>, env: &mut Env) -> Datum {
+    for i in 1..vec.len() {
+        if let List(v) = &vec[i] {
+            if has_tag(v, "else") || is_true(&interpret(&v[0], env)) {
+                return interpret_begin(v, env);
+            }
+        } else {
+            panic!("INTERPRET-COND -- not a list {:?}", vec[i])
+        }
+    }
+    panic!("INTERPRET-COND -- missing else statement? {:?}", vec)
+}
+
+fn interpret_lambda(vec: &Vec<Datum>, _env: &mut Env) -> Datum {
+    let mut parameters = vec![];
+    if let List(params) = &vec[1] {
+        for p in params {
+            if let Symbol(_) = p {
+                parameters.push(p.clone());
+            } else {
+                panic!("INTERPRET-LAMBDA - not a symbol {:?}", p);
+            }
+        }
+    }
+    let body = vec[2..].to_vec();
+    Lambda { parameters, body }
+}
+
+fn transform_let_to_lambda(vec: &Vec<Datum>) -> Datum {
+    let mut args = vec![];
+    let mut parameters = vec![];
+    let body = vec[2..].to_vec();
+
+    if let List(bindings) = &vec[1] {
+        for binding in bindings {
+            if let List(binding) = binding {
+                parameters.push(binding[0].clone());
+                args.push(binding[1].clone());
+            } else {
+                panic!("TRANSFORM-LET-TO-LAMBDA -- invalid binding {:?}", binding);
+            }
+        }
+        let lambda = Lambda { parameters, body };
+        let mut result = vec![lambda];
+        result.append(&mut args);
+        List(result)
+    } else {
+        panic!("TRANSFORM-LET-TO-LAMBDA -- invalid bindings {:?}", vec[1]);
+    }
+}
+
+fn interpret_begin(vec: &Vec<Datum>, env: &mut Env) -> Datum {
+    let mut r = Datum::Unspecified;
+    for i in 1..vec.len() {
+        r = interpret(&vec[i], env);
+    }
+    r
+}
+
+fn interpret_application(vec: &Vec<Datum>, env: &mut Env) -> Datum {
+    if vec.len() == 0 {
+        panic!("INTERPRET-APPLICATION -- no-procedure")
+    }
+
+    let mut args = vec![];
+    for v in vec {
+        let a = interpret(v, env);
+        args.push(a)
+    }
+
+    match &args[0] {
+        Primitive(proc) => proc(args),
+        Lambda { parameters, body } => {
+            let lambda_args = List(args[1..].to_vec());
+            let lambda_params = List(parameters.to_vec());
+            let mut new_env = environment::extend(lambda_params, lambda_args, env);
+            for i in 0..(body.len() - 1) {
+                interpret(&body[i], &mut new_env);
+            }
+            interpret(&body[body.len() - 1], &mut new_env)
+        }
+        Procedure {
+            parameters,
+            body,
+            env,
+        } => {
+            let lambda_args = List(args[1..].to_vec());
+            let lambda_params = List(parameters.to_vec());
+            let mut proc_env = env.clone();
+            let mut new_env = environment::extend(lambda_params, lambda_args, &mut proc_env);
+            for i in 0..(body.len() - 1) {
+                interpret(&body[i], &mut new_env);
+            }
+            // This is my attempt to implement tail-call optimiation.
+            interpret(&body[body.len() - 1], &mut new_env)
+        }
+        _ => panic!("INTERPRET-APPLICATION -- not-aplicable {:?}", args[0]),
+    }
+}
+
+pub fn interpret(exp: &Datum, env: &mut Env) -> Datum {
+    match exp {
+        Boolean(_) => exp.clone(),
+        Number(_) => exp.clone(),
+        String(_) => exp.clone(),
+        Symbol(_) => environment::lookup_variable_value(exp, env),
+        List(v) if has_tag(v, "if") => interpret_if(v, env),
+        List(v) if has_tag(v, "set!") => interpret_set(v, env),
+        List(v) if has_tag(v, "define") => interpret_define(v, env),
+        List(v) if has_tag(v, "cond") => interpret_cond(v, env),
+        List(v) if has_tag(v, "lambda") => interpret_lambda(v, env),
+        List(v) if has_tag(v, "let") => interpret(&transform_let_to_lambda(v), env),
+        List(v) if has_tag(v, "begin") => interpret_begin(v, env),
+        List(v) if has_tag(v, "quote") => v[1].clone(),
+        List(v) => interpret_application(v, env),
+        Lambda { .. } => exp.clone(),
+        Unspecified => Unspecified,
+        _ => panic!("unknown-expression {:?}", exp),
+    }
+}

src/lexer.rs

@@ -3,6 +3,7 @@ pub enum Token {
     Identifier(String),
     Boolean(bool),
     Number(i64),
+    String(String),
     LeftRoundBracket,
     RightRoundBracket,
     Quote,
@@ -33,6 +34,8 @@ fn scan(code: &str, mut ix: usize, mut tokens: Tokens) -> Tokens {
         return scan_boolean(code, ix, tokens);
     } else if c == ';' {
         return scan_comment(code, ix, tokens);
+    } else if c == '"' {
+        return scan_string(code, ix, tokens);
     } else if c.is_ascii_digit() {
         return scan_number(code, ix, tokens);
     } else if c.is_ascii_alphabetic() || SPECIAL_INITIAL.contains(&c) {
@@ -70,12 +73,12 @@ fn scan_boolean(code: &str, ix: usize, mut tokens: Tokens) -> Tokens {
         "#t" => {
             let token = Token::Boolean(true);
             tokens.push(token);
-        },
+        }
         "#f" => {
             let token = Token::Boolean(false);
             tokens.push(token);
         }
-        _ => panic!("Expected boolean but got {}", c)
+        _ => panic!("Expected boolean but got {}", c),
     }
     return scan(code, ix + 2, tokens);
 }
@@ -95,6 +98,39 @@ fn scan_comment(code: &str, mut ix: usize, tokens: Tokens) -> Tokens {
     scan(code, ix, tokens)
 }
 
+fn scan_string(code: &str, mut ix: usize, mut tokens: Tokens) -> Tokens {
+    let mut s = String::new();
+    let mut chars = code[ix..].chars();
+    let c = chars.next().unwrap();
+    assert!(c == '"');
+
+    ix += 1;
+    while let Some(c) = chars.next() {
+        ix += 1;
+        if c == '"' {
+            break;
+        } else if c == '\\' {
+            ix += 1;
+            if let Some(next_c) = chars.next() {
+                let escaped_c = match next_c {
+                    'n' => '\n',
+                    '"' => '"',
+                    '\\' => '\\',
+                    _ => panic!("SCAN-STRING -- cannot escape {}", next_c),
+                };
+                s.push(escaped_c);
+            } else {
+                panic!("SCAN-STRING -- found escape at eof")
+            }
+        } else {
+            s.push(c);
+        }
+    }
+    let token = Token::String(s);
+    tokens.push(token);
+    scan(code, ix, tokens)
+}
+
 fn scan_number(code: &str, mut ix: usize, mut tokens: Tokens) -> Tokens {
     let start_ix = ix;
     let mut chars = code[ix..].chars();
@@ -135,6 +171,4 @@ const SPECIAL_INITIAL: &[char] = &[
     '!', '$', '%', '&', '*', '/', ':', '<', '=', '>', '?', '^', '_', '~', '+', '-',
 ];
 
-const SPECIAL_SUBSEQUENT: &[char] = &[
+const SPECIAL_SUBSEQUENT: &[char] = &['+', '-', '.', '@', '!', '?'];
-    '+', '-', '.', '@',
-];

src/main.rs

@@ -1,9 +1,64 @@
+mod environment;
+mod interpreter;
 mod lexer;
 mod parser;
+mod primitives;
 
-fn main() {
+use std::env;
-    let scm_code = "(+ a (* 32 b) c #t #f)";
+use std::fs;
+use std::io;
+use std::io::prelude::*;
+use std::path::Path;
+
+fn repl() {
+    let mut env = environment::get_global_environment();
+    loop {
+        let mut input = String::new();
+        print!(" ]=> ");
+        io::stdout().flush().ok().expect("Could not flush stdout");
+        match io::stdin().read_line(&mut input) {
+            Ok(_) => {
+                let tokens = lexer::read(&input);
+                let datum = parser::parse(tokens);
+                let result = interpreter::interpret(&datum, &mut env);
+                println!("{:?}", result);
+            }
+            Err(error) => println!("error: {}", error),
+        }
+    }
+}
+
+fn interpret_code(scm_code: &str) {
     let tokens = lexer::read(scm_code);
     let datum = parser::parse(tokens);
-    println!("{:?}", datum);
+    let mut env = environment::get_global_environment();
+    let result = interpreter::interpret(&datum, &mut env);
+    println!("{:?}", result);
+}
+
+fn interpret_file(filename: &Path) {
+    match fs::read_to_string(filename) {
+        Ok(scm_code) => {
+            let tokens = lexer::read(scm_code.as_str());
+            let datum = parser::parse(tokens);
+            let mut env = environment::get_global_environment();
+            let _result = interpreter::interpret(&datum, &mut env);
+            // println!("{:?}", result);
+        }
+        Err(error) => println!("error: {} {:?}", error, filename),
+    }
+}
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+    let scm_code = "";
+
+    if args.len() == 2 {
+        let arg_path = Path::new(&args[1]);
+        interpret_file(arg_path);
+    } else if scm_code.len() > 0 {
+        interpret_code(scm_code);
+    } else {
+        repl();
+    }
 }

src/parser.rs

@@ -1,13 +1,94 @@
+use crate::environment::Env;
 use crate::lexer::Token;
 use crate::lexer::Tokens;
+use std::cmp::Ordering;
+use std::fmt;
 
-
+#[derive(Debug, Clone)]
-#[derive(Debug)]
 pub enum Datum {
     Boolean(bool),
     Number(i64),
+    String(String),
     Symbol(String),
+    Pair(Box<(Datum, Datum)>),
     List(Vec<Datum>),
+    Primitive(fn(Vec<Datum>) -> Datum),
+    Lambda {
+        parameters: Vec<Datum>,
+        body: Vec<Datum>,
+    },
+    Procedure {
+        parameters: Vec<Datum>,
+        body: Vec<Datum>,
+        env: Env,
+    },
+    Unspecified,
+}
+
+pub fn make_symbol(s: &str) -> Datum {
+    Datum::Symbol(s.to_string())
+}
+
+impl Default for Datum {
+    fn default() -> Self {
+        Datum::Unspecified
+    }
+}
+
+impl PartialEq for Datum {
+    fn eq(&self, other: &Self) -> bool {
+        match (self, other) {
+            (Datum::Boolean(b1), Datum::Boolean(b2)) => b1 == b2,
+            (Datum::Number(n1), Datum::Number(n2)) => n1 == n2,
+            _ => false,
+        }
+    }
+}
+
+impl fmt::Display for Datum {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Datum::Boolean(true) => write!(f, "#t"),
+            Datum::Boolean(false) => write!(f, "#f"),
+            Datum::Number(n) => write!(f, "{}", n),
+            Datum::Symbol(s) => write!(f, "{}", s),
+            Datum::String(s) => write!(f, "{}", s),
+            Datum::Pair(p) => write!(f, "({} . {})", p.0, p.1),
+            Datum::List(v) => {
+                let mut s = String::new();
+                s.push_str("(");
+                for e in v {
+                    let es = format!("{} ", e);
+                    if es != "list " {
+                        s.push_str(&es);
+                    }
+                }
+                s.pop(); // remove last space
+                s.push_str(")");
+                write!(f, "{}", s)
+            }
+            _ => panic!("DISPLAY -- cannot print {:?}", self),
+        }
+    }
+}
+
+impl PartialOrd for Datum {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        match (self, other) {
+            (Datum::Boolean(b1), Datum::Boolean(b2)) => Some(b1.cmp(b2)),
+            (Datum::Number(n1), Datum::Number(n2)) => Some(n1.cmp(n2)),
+            _ => None,
+        }
+    }
+}
+
+impl Datum {
+    pub fn as_str(&self) -> Option<std::string::String> {
+        match self {
+            Datum::Symbol(s) => Some(s.to_string()),
+            _ => None,
+        }
+    }
 }
 
 pub fn parse(tokens: Tokens) -> Datum {
@@ -20,20 +101,31 @@ fn parse_datum(tokens: &Tokens, ix: usize) -> (Datum, usize) {
         Token::Identifier(s) => (Datum::Symbol(s.to_string()), ix + 1),
         Token::Boolean(b) => (Datum::Boolean(*b), ix + 1),
         Token::Number(n) => (Datum::Number(*n), ix + 1),
+        Token::String(s) => (Datum::String(s.to_string()), ix + 1),
+        Token::Quote => parse_quote(tokens, ix + 1),
         Token::LeftRoundBracket => parse_list(tokens, ix + 1),
-        _ => panic!("Unexpected token {:?}", tokens[ix])
+        _ => panic!("Unexpected token {:?}", tokens[ix]),
     }
 }
 
+pub fn parse_quote(tokens: &Tokens, ix: usize) -> (Datum, usize) {
+    let mut datums = vec![make_symbol("quote")];
+    let (datum, ix) = parse_datum(tokens, ix);
+    datums.push(datum);
+    (Datum::List(datums), ix)
+}
+
 pub fn parse_list(tokens: &Tokens, mut ix: usize) -> (Datum, usize) {
     let mut datums = vec![];
 
-    // FIXME: will crash when RightRoundBracket is missing 
+    while ix < tokens.len() && tokens[ix] != Token::RightRoundBracket {
-    while tokens[ix] != Token::RightRoundBracket {
         let (datum, new_ix) = parse_datum(tokens, ix);
         datums.push(datum);
         ix = new_ix;
     }
+    if ix == tokens.len() {
+        panic!("PARSE_LIST -- missing closing bracket {:?}", tokens)
+    }
 
     (Datum::List(datums), ix + 1)
 }

src/primitives.rs

@@ -0,0 +1,220 @@
+use crate::interpreter::has_tag;
+use crate::interpreter::is_true;
+use crate::parser::make_symbol;
+use crate::parser::Datum;
+use crate::parser::Datum::*;
+use std::io;
+use std::io::prelude::*;
+use std::mem;
+
+pub fn add(mut args: Vec<Datum>) -> Datum {
+    let mut r = 0;
+    for i in 1..args.len() {
+        let datum = mem::take(&mut args[i]);
+        if let Number(n) = datum {
+            r += n;
+        } else {
+            panic!("ADD -- not-all-args-are-numbers {:?}", args)
+        }
+    }
+    Number(r)
+}
+
+pub fn mul(mut args: Vec<Datum>) -> Datum {
+    let mut r = 1;
+    for i in 1..args.len() {
+        let datum = mem::take(&mut args[i]);
+        if let Number(n) = datum {
+            r *= n;
+        } else {
+            panic!("MUL -- not-all-args-are-numbers {:?}", args)
+        }
+    }
+    Number(r)
+}
+
+pub fn sub(mut args: Vec<Datum>) -> Datum {
+    if args.len() == 2 {
+        let datum = mem::take(&mut args[1]);
+        if let Number(n) = datum {
+            return Number(-n);
+        } else {
+            panic!("SUB -- not-all-args-are-numbers {:?}", args)
+        }
+    }
+
+    let mut r;
+    let datum = mem::take(&mut args[1]);
+    if let Number(n) = datum {
+        r = n;
+    } else {
+        panic!("SUB -- not-all-args-are-numbers {:?}", args)
+    }
+
+    for i in 2..args.len() {
+        let datum = mem::take(&mut args[i]);
+        if let Number(n) = datum {
+            r -= n;
+        } else {
+            panic!("SUB -- not-all-args-are-numbers {:?}", args)
+        }
+    }
+    Number(r)
+}
+
+pub fn eq(args: Vec<Datum>) -> Datum {
+    if args.len() == 1 {
+        Boolean(true)
+    } else {
+        let first = &args[1];
+        for i in 2..args.len() {
+            let current = &args[i];
+            if first != current {
+                return Boolean(false);
+            }
+        }
+        Boolean(true)
+    }
+}
+
+pub fn lt(args: Vec<Datum>) -> Datum {
+    if args.len() == 1 {
+        Boolean(true)
+    } else {
+        for i in 1..(args.len() - 1) {
+            if &args[i] >= &args[i + 1] {
+                return Boolean(false);
+            }
+        }
+        Boolean(true)
+    }
+}
+
+pub fn or(args: Vec<Datum>) -> Datum {
+    if args.len() == 1 {
+        Boolean(false)
+    } else {
+        for i in 1..args.len() {
+            if is_true(&args[i]) {
+                return args[i].clone();
+            }
+        }
+        Boolean(false)
+    }
+}
+
+pub fn and(args: Vec<Datum>) -> Datum {
+    if args.len() == 1 {
+        Boolean(true)
+    } else {
+        for i in 1..args.len() {
+            if !is_true(&args[i]) {
+                return Boolean(false);
+            }
+        }
+        args[args.len() - 1].clone()
+    }
+}
+
+pub fn not(args: Vec<Datum>) -> Datum {
+    if args.len() != 2 {
+        panic!("NOT -- takes one argument");
+    } else {
+        if is_true(&args[1]) {
+            Boolean(false)
+        } else {
+            Boolean(true)
+        }
+    }
+}
+
+pub fn display(args: Vec<Datum>) -> Datum {
+    for i in 1..args.len() {
+        print!("{}", args[i]);
+    }
+    io::stdout().flush().ok().expect("Could not flush stdout");
+    Unspecified
+}
+
+pub fn list(mut args: Vec<Datum>) -> Datum {
+    args[0] = make_symbol("list");
+    Datum::List(args)
+}
+
+pub fn cons(mut args: Vec<Datum>) -> Datum {
+    if args.len() != 3 {
+        panic!("CONS -- two args expected {:?}", args);
+    }
+    let a = std::mem::take(&mut args[1]);
+    let b = std::mem::take(&mut args[2]);
+    match b {
+        List(b_vec) if has_tag(&b_vec, "list") => {
+            let mut r_vec = vec![make_symbol("list"), a];
+            r_vec.append(&mut b_vec[1..].to_vec());
+            Datum::List(r_vec)
+        }
+        _ => Datum::Pair(Box::new((a, b))),
+    }
+}
+
+pub fn car(mut args: Vec<Datum>) -> Datum {
+    if args.len() != 2 {
+        panic!("CAR -- expected a single arg {:?}", args);
+    }
+    let a = std::mem::take(&mut args[1]);
+    match a {
+        List(mut v) if has_tag(&v, "list") => std::mem::take(&mut v[1]),
+        Pair(p) => p.0,
+        _ => panic!("CAR -- could not get car {:?}", a),
+    }
+}
+
+pub fn cdr(mut args: Vec<Datum>) -> Datum {
+    if args.len() != 2 {
+        panic!("CDR -- expected a single arg {:?}", args);
+    }
+    let a = std::mem::take(&mut args[1]);
+    match a {
+        List(v) if has_tag(&v, "list") => {
+            let mut v_cdr = v[1..].to_vec();
+            v_cdr[0] = make_symbol("list");
+            List(v_cdr)
+        }
+        Pair(p) => p.1,
+        _ => panic!("CAR -- could not get car {:?}", a),
+    }
+}
+
+pub fn pair(args: Vec<Datum>) -> Datum {
+    if args.len() != 2 {
+        panic!("PAIR? -- expected a single arg {:?}", args);
+    }
+    match &args[1] {
+        List(v) if has_tag(&v, "list") => {
+            if v.len() == 1 {
+                Boolean(false)
+            } else {
+                Boolean(true)
+            }
+        }
+        Pair(_) => Boolean(true),
+        _ => Boolean(false),
+    }
+}
+
+pub fn null(args: Vec<Datum>) -> Datum {
+    if args.len() != 2 {
+        panic!("PAIR? -- expected a single arg {:?}", args);
+    }
+    match &args[1] {
+        List(v) if has_tag(&v, "list") => {
+            if v.len() == 1 {
+                Boolean(true)
+            } else {
+                Boolean(false)
+            }
+        }
+        Pair(_) => Boolean(false),
+        _ => Boolean(false),
+    }
+}

test.scm

@@ -0,0 +1,43 @@
+(begin
+  (define (newline) (display "\n"))
+  (display (or #t #f)) (newline)
+  (display (and #t #f)) (newline)
+  (display (not #f)) (newline)
+  (define a 5)
+  (define (fib n)
+    (cond
+      ((< n 0) (display "WARNING: n smaller than 0"))
+      ((< n 3) 1)
+      (else (+ (fib (- n 2)) (fib (- n 1))))))
+  (define (fac n) (if (= n 1) 1 (* n (fac (- n 1)))))
+  (set! a (+ a a))
+  (define l (list a (* 3 3) (+ a a)))
+  (begin
+    (display "(fac ") (display a)
+    (display ") -> ") (display (fac a)) (display "\n"))
+  (begin
+    (display "(fib ") (display a)
+    (display ") -> ") (display (fib a)) (display "\n"))
+  (define l (cons 'a l))
+  (display l) (newline)
+  (display (cons 'a 'b)) (newline)
+  (display (car l)) (newline)
+  (display (cdr (cdr l))) (newline)
+  (display (pair? l)) (newline)
+  (display (pair? 'a)) (newline)
+  (display ((lambda (x y) (* x a)) 3 5)) (newline)
+  (display
+    (let ((a (+ a a))
+          (b (* a a))
+          (x 42))
+      (+ a b x)))
+  (newline)
+  (define (p)
+    (define a 42)
+    (define (f) a)
+    f)
+  (display ((p))) ; should return 42
+  (newline)
+  'done
+  )
+