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

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),
+    }
+}