Implement cons, car, cdr, null?, pair? resolves #1

src/environment.rs

@@ -21,6 +21,12 @@ pub fn get_global_environment<'a>() -> Env<'a> {
             (make_symbol("-"), Datum::Procedure(primitives::sub)),
             (make_symbol("="), Datum::Procedure(primitives::eq)),
             (make_symbol("<"), Datum::Procedure(primitives::lt)),
+            (make_symbol("list"), Datum::Procedure(primitives::list)),
+            (make_symbol("cons"), Datum::Procedure(primitives::cons)),
+            (make_symbol("car"), Datum::Procedure(primitives::car)),
+            (make_symbol("cdr"), Datum::Procedure(primitives::cdr)),
+            (make_symbol("pair?"), Datum::Procedure(primitives::pair)),
+            (make_symbol("null?"), Datum::Procedure(primitives::null)),
             (
                 make_symbol("display"),
                 Datum::Procedure(primitives::display),

src/interpreter.rs

@@ -13,7 +13,7 @@ fn is_true(exp: &Datum) -> bool {
     }
 }
 
-fn has_tag(vec: &Vec<Datum>, tag: &str) -> bool {
+pub fn has_tag(vec: &Vec<Datum>, tag: &str) -> bool {
     if vec.len() == 0 {
         return false;
     }
@@ -127,6 +127,7 @@ fn interpret_application(vec: &Vec<Datum>, env: &mut Env) -> Datum {
                 for i in 0..(seq.len() - 1) {
                     interpret(&seq[i], &mut new_env);
                 }
+                // XXX: not tail-call optimized
                 interpret(&seq[seq.len() - 1], &mut new_env)
             } else {
                 panic!("INTERPRET-APPLICATION -- no-list {:?}", lambda_body)

src/lexer.rs

@@ -171,4 +171,4 @@ const SPECIAL_INITIAL: &[char] = &[
     '!', '$', '%', '&', '*', '/', ':', '<', '=', '>', '?', '^', '_', '~', '+', '-',
 ];
 
-const SPECIAL_SUBSEQUENT: &[char] = &['+', '-', '.', '@', '!'];
+const SPECIAL_SUBSEQUENT: &[char] = &['+', '-', '.', '@', '!', '?'];

src/main.rs

@@ -54,11 +54,11 @@ fn main() {
     // let scm_code = "(begin (define (f n) (if (= n 1) 1 (* n (f (- n 1))))) (f 10))";
     let scm_code = "";
 
-    if scm_code.len() > 0 {
-        interpret_code(scm_code);
-    } else if args.len() == 2 {
+    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

@@ -1,6 +1,7 @@
 use crate::lexer::Token;
 use crate::lexer::Tokens;
 use std::cmp::Ordering;
+use std::fmt;
 
 #[derive(Debug, Clone)]
 pub enum Datum {
@@ -8,6 +9,7 @@ pub enum Datum {
     Number(i64),
     String(String),
     Symbol(String),
+    Pair(Box<(Datum, Datum)>),
     List(Vec<Datum>),
     Procedure(fn(Vec<Datum>) -> Datum),
     Unspecified,
@@ -33,6 +35,32 @@ impl PartialEq for Datum {
     }
 }
 
+impl fmt::Display for Datum {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Datum::Boolean(b) => write!(f, "{}", b),
+            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) {

src/primitives.rs

@@ -1,3 +1,5 @@
+use crate::interpreter::has_tag;
+use crate::parser::make_symbol;
 use crate::parser::Datum;
 use crate::parser::Datum::*;
 use std::io;
@@ -89,14 +91,91 @@ pub fn lt(args: Vec<Datum>) -> Datum {
 
 pub fn display(args: Vec<Datum>) -> Datum {
     for i in 1..args.len() {
-        match &args[i] {
-            Boolean(b) => print!("{}", b),
-            Number(n) => print!("{}", n),
-            String(s) => print!("{}", s),
-            List(v) => print!("{:?}", v),
-            _ => panic!("DISPLAY -- cannot-print {:?}", args[i]),
-        };
+        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),
+    }
+}