Add a lot of primitive types

10 files changed, 974 insertions, 139 deletions

diff --git a/examples/booleans.bdl b/examples/booleans.bdl
index e38fb1f..0598450 100644
--- a/examples/booleans.bdl
+++ b/examples/booleans.bdl
@@ -44,7 +44,7 @@
 (print "(if (or false false) (+ 1 2 3) (+ 7 8 9)) -> ")     (if (or false false) (+ 1 2 3) (+ 7 8 9))
 (print "(if (or (+ 1 2 3) false) (+ 1 2 3) (+ 7 8 9)) -> ") (if (or (+ 1 2 3) false) (+ 1 2 3) (+ 7 8 9))
 (print "(if true 7) -> ")                                   (if true 7)
-(print "(if false 7) -> ")                                  (if false 7)
+(print "(if false 7) -> ")                                  (if false 7) (newline)
 
 ;; Cond.
 (print "(cond ((and true true true) 1) ((or true true false) 2) (else 3)) -> ")
@@ -61,7 +61,7 @@
       (else 3))
 (print "(cond ((and true true true) 1) ((or true true false) 2)) -> ")
 (cond ((and true true false) 1)
-      ((or false false false) 2))
+      ((or false false false) 2)) (newline)
 (print "(cond ((and true true true) (+ 1 2 3)) ((or true true false) 2) (else 3)) -> ")
 (cond ((and true true true) (+ 1 2 3))
       ((or true true false) 2)
diff --git a/examples/lists.bdl b/examples/lists.bdl
index 4a27005..36063d6 100644
--- a/examples/lists.bdl
+++ b/examples/lists.bdl
@@ -3,7 +3,7 @@
 ;;
 
 ;; List function.
-(print "(list) -> ") (list)
+(print "(list) -> ") (list) (newline)
 (print "(list 1) -> ") (list 1)
 (print "(list 1 2) -> ") (list 1 2)
 (print "(list 1 2 3) -> ") (list 1 2 3)
diff --git a/examples/types.bdl b/examples/types.bdl
index 43b7be9..36dab7a 100644
--- a/examples/types.bdl
+++ b/examples/types.bdl
@@ -12,14 +12,14 @@
 (print "(boolean? (not 1)) -> ")    (boolean? (not 1))
 
 ;; Empty list/null.
-(print "(null? true) -> ")       (null? true)
-(print "(null? false) -> ")      (null? false)
-(print "(null? 1) -> ")          (null? 1)
-(print "(null? 5) -> ")          (null? 5)
-(print "(null? \"string\") -> ") (null? "string")
-(print "(null? (+ 1 2 3)) -> ")  (null? (+ 1 2 3))
-(print "(null? (not 1)) -> ")    (null? (not 1))
-(print "(null? ()) -> ")         (null? ())
+(print "(nil? true) -> ")       (nil? true)
+(print "(nil? false) -> ")      (nil? false)
+(print "(nil? 1) -> ")          (nil? 1)
+(print "(nil? 5) -> ")          (nil? 5)
+(print "(nil? \"string\") -> ") (nil? "string")
+(print "(nil? (+ 1 2 3)) -> ")  (nil? (+ 1 2 3))
+(print "(nil? (not 1)) -> ")    (nil? (not 1))
+(print "(nil? ()) -> ")         (nil? ())
 
 ;; String.
 (print "(string? true) -> ")       (string? true)
@@ -40,7 +40,6 @@
 (print "(fixnum? (not 1)) -> ")    (fixnum? (not 1))
 
 ;; Symbol
-;; TODO: We need quotation to test for symbols.
 (print "(symbol? true) -> ")       (symbol? true)
 (print "(symbol? false) -> ")      (symbol? false)
 (print "(symbol? 1) -> ")          (symbol? 1)
@@ -48,8 +47,8 @@
 (print "(symbol? \"string\") -> ") (symbol? "string")
 (print "(symbol? (+ 1 2 3)) -> ")  (symbol? (+ 1 2 3))
 (print "(symbol? (not 1)) -> ")    (symbol? (not 1))
-; (print "(symbol? 'a) -> ")         (symbol? 'a)
-; (print "(symbol? 'c) -> ")         (symbol? 'c)
+(print "(symbol? 'a) -> ")         (symbol? 'a)
+(print "(symbol? 'c) -> ")         (symbol? 'c)
 
 ;; Pair.
 (print "(pair? false) -> ")        (pair? false)
diff --git a/src/bootstrap/errors.c b/src/bootstrap/errors.c
index c9d9c97..13a2f3c 100644
--- a/src/bootstrap/errors.c
+++ b/src/bootstrap/errors.c
@@ -17,6 +17,7 @@ typedef enum ErrorValue {
     ERR_OBJ_NOT_CALLABLE,
     ERR_NOT_ENOUGH_ARGS,
     ERR_WRONG_ARG_TYPE,
+    ERR_DIVISION_BY_ZERO,
 } ErrorValue;
 
 typedef struct Error {
@@ -39,6 +40,7 @@ static const char* error_msgs[] = {
     [ERR_OBJ_NOT_CALLABLE] = "error: object is not callable",
     [ERR_NOT_ENOUGH_ARGS] = "error: not enough arguments",
     [ERR_WRONG_ARG_TYPE] = "error: wrong argument type",
+    [ERR_DIVISION_BY_ZERO] = "error: division by zero",
 };
 
 #define ERR_MAX_NUMBER 16
diff --git a/src/bootstrap/main.c b/src/bootstrap/main.c
index 2aa3038..8092bbd 100755
--- a/src/bootstrap/main.c
+++ b/src/bootstrap/main.c
@@ -11,8 +11,14 @@
 #include "lexer.c"
 #include "objects.c"
 #include "parser.c"
+#include "environment.c"
+#include "primitives.c"
 
 #define MAKE_SYM(STR) make_symbol((StringView){(STR), sizeof(STR) - 1})
+#define MAKE_ENV_VAR(ENV,STR,VAR) \
+    (env_add_symbol((ENV), MAKE_SYM(STR), (VAR)))
+#define MAKE_ENV_PROC(ENV,STR,FUN) \
+    (env_add_symbol((ENV), MAKE_SYM(STR), make_procedure(FUN)))
 
 void
 init(void) {
@@ -24,8 +30,42 @@ init(void) {
 
     // Global environment.
     global_env = env_create(NULL);
-    env_add_symbol(global_env, MAKE_SYM("quote"), make_procedure(proc_quote));
-    env_add_symbol(global_env, MAKE_SYM("+"), make_procedure(proc_sum));
+
+    // Primitive symbols.
+    MAKE_ENV_VAR(global_env, "else", obj_true);
+    MAKE_ENV_VAR(global_env, "nil", obj_nil);
+
+    // Primitive procedures.
+    MAKE_ENV_PROC(global_env, "quote", proc_quote);
+    MAKE_ENV_PROC(global_env, "car", proc_car);
+    MAKE_ENV_PROC(global_env, "cdr", proc_cdr);
+    MAKE_ENV_PROC(global_env, "cons", proc_cons);
+    MAKE_ENV_PROC(global_env, "list", proc_list);
+    MAKE_ENV_PROC(global_env, "+", proc_sum);
+    MAKE_ENV_PROC(global_env, "-", proc_sub);
+    MAKE_ENV_PROC(global_env, "*", proc_mul);
+    MAKE_ENV_PROC(global_env, "/", proc_div);
+    MAKE_ENV_PROC(global_env, "%", proc_mod);
+    MAKE_ENV_PROC(global_env, "print", proc_print);
+    MAKE_ENV_PROC(global_env, "display", proc_display);
+    MAKE_ENV_PROC(global_env, "newline", proc_newline);
+    MAKE_ENV_PROC(global_env, "boolean?", proc_is_boolean);
+    MAKE_ENV_PROC(global_env, "nil?", proc_is_nil);
+    MAKE_ENV_PROC(global_env, "symbol?", proc_is_symbol);
+    MAKE_ENV_PROC(global_env, "string?", proc_is_string);
+    MAKE_ENV_PROC(global_env, "fixnum?", proc_is_fixnum);
+    MAKE_ENV_PROC(global_env, "pair?", proc_is_pair);
+    MAKE_ENV_PROC(global_env, "procedure?", proc_is_procedure);
+    MAKE_ENV_PROC(global_env, "not", proc_not);
+    MAKE_ENV_PROC(global_env, "and", proc_and);
+    MAKE_ENV_PROC(global_env, "or", proc_or);
+    MAKE_ENV_PROC(global_env, "if", proc_if);
+    MAKE_ENV_PROC(global_env, "cond", proc_cond);
+    MAKE_ENV_PROC(global_env, "<", proc_num_less_than);
+    MAKE_ENV_PROC(global_env, "<=", proc_num_lesseq_than);
+    MAKE_ENV_PROC(global_env, ">", proc_num_greater_than);
+    MAKE_ENV_PROC(global_env, ">=", proc_num_greatereq_than);
+    MAKE_ENV_PROC(global_env, "=", proc_num_equal);
 }
 
 void
@@ -51,12 +91,10 @@ process_source(const StringView *source) {
 
         // FIXME: Not freeing result or intermediate objects, can leak memory.
         Object *result = eval(global_env, root);
-        printf("AST: ");
-        display(root);
-        printf("\n");
-        printf("EVAL: ");
-        display(result);
-        printf("\n");
+        if (result != obj_nil) {
+            display(result);
+            printf("\n");
+        }
         free_objects(root);
     }
 
@@ -94,9 +132,6 @@ run_repl(void) {
             errors_n = 0;
             continue;
         }
-        if (sv.n != 0) {
-            printf("\n");
-        }
     }
 }
 
diff --git a/src/bootstrap/objects.c b/src/bootstrap/objects.c
index 30827f1..0361ae8 100644
--- a/src/bootstrap/objects.c
+++ b/src/bootstrap/objects.c
@@ -197,106 +197,3 @@ display(Object *root) {
     }
     return;
 }
-
-#include "environment.c"
-
-Object *
-eval(Environment* env, Object *root) {
-    switch (root->type) {
-        case OBJ_TYPE_FIXNUM:
-        case OBJ_TYPE_BOOL:
-        case OBJ_TYPE_NIL:
-        case OBJ_TYPE_STRING: {
-            return root;
-        } break;
-        case OBJ_TYPE_SYMBOL: {
-            Object *val = env_lookup(env, root);
-            if (val == obj_err) {
-                error_push((Error){
-                    .type = ERR_TYPE_RUNTIME,
-                    .value = ERR_SYMBOL_NOT_FOUND,
-                });
-                return obj_err;
-            }
-            return val;
-        } break;
-        case OBJ_TYPE_PAIR: {
-            if (root->car->type == OBJ_TYPE_SYMBOL) {
-                Object *val = env_lookup(env, root->car);
-                if (val == obj_err) {
-                    error_push((Error){
-                        .type = ERR_TYPE_RUNTIME,
-                        .value = ERR_SYMBOL_NOT_FOUND,
-                    });
-                    return obj_err;
-                }
-                if (val->type == OBJ_TYPE_PROCEDURE) {
-                    return val->proc(env, root->cdr);
-                }
-                error_push((Error){
-                    .type = ERR_TYPE_RUNTIME,
-                    .value = ERR_OBJ_NOT_CALLABLE,
-                });
-                return obj_err;
-            }
-        } break;
-        default: {
-            break;
-        } break;
-    }
-
-    error_push((Error){
-        .type = ERR_TYPE_RUNTIME,
-        .value = ERR_UNKNOWN_OBJ_TYPE,
-    });
-    return obj_err;
-}
-
-Object *
-proc_quote(Environment *env, Object *obj) {
-    (void)env;
-    return obj->car;
-}
-
-Object *
-proc_sum(Environment *env, Object *obj) {
-    // First argument.
-    if (obj == obj_nil) {
-        error_push((Error){
-            .type = ERR_TYPE_RUNTIME,
-            .value = ERR_NOT_ENOUGH_ARGS,
-        });
-        return obj_err;
-    }
-    Object *car = eval(env, obj->car);
-    if (car == obj_err) {
-        return obj_err;
-    }
-    if (car->type != OBJ_TYPE_FIXNUM) {
-        error_push((Error){
-            .type = ERR_TYPE_RUNTIME,
-            .value = ERR_WRONG_ARG_TYPE,
-        });
-        return obj_err;
-    }
-
-    // Traverse the list.
-    obj = obj->cdr;
-    ssize_t tot = car->fixnum;
-    while (obj->type == OBJ_TYPE_PAIR) {
-        Object *car = eval(env, obj->car);
-        if (car == obj_err) {
-            return obj_err;
-        }
-        if (car->type != OBJ_TYPE_FIXNUM) {
-            error_push((Error){
-                .type = ERR_TYPE_RUNTIME,
-                .value = ERR_WRONG_ARG_TYPE,
-            });
-            return obj_err;
-        }
-        tot += car->fixnum;
-        obj = obj->cdr;
-    }
-    return make_fixnum(tot);
-}
diff --git a/src/bootstrap/primitives.c b/src/bootstrap/primitives.c
new file mode 100644
index 0000000..8369fa8
--- /dev/null
+++ b/src/bootstrap/primitives.c
@@ -0,0 +1,900 @@
+Object *
+eval(Environment* env, Object *root) {
+    switch (root->type) {
+        case OBJ_TYPE_FIXNUM:
+        case OBJ_TYPE_BOOL:
+        case OBJ_TYPE_NIL:
+        case OBJ_TYPE_STRING: {
+            return root;
+        } break;
+        case OBJ_TYPE_SYMBOL: {
+            Object *val = env_lookup(env, root);
+            if (val == obj_err) {
+                error_push((Error){
+                    .type = ERR_TYPE_RUNTIME,
+                    .value = ERR_SYMBOL_NOT_FOUND,
+                });
+                return obj_err;
+            }
+            return val;
+        } break;
+        case OBJ_TYPE_PAIR: {
+            if (root->car->type == OBJ_TYPE_SYMBOL) {
+                Object *val = env_lookup(env, root->car);
+                if (val == obj_err) {
+                    error_push((Error){
+                        .type = ERR_TYPE_RUNTIME,
+                        .value = ERR_SYMBOL_NOT_FOUND,
+                    });
+                    return obj_err;
+                }
+                if (val->type == OBJ_TYPE_PROCEDURE) {
+                    return val->proc(env, root->cdr);
+                }
+                error_push((Error){
+                    .type = ERR_TYPE_RUNTIME,
+                    .value = ERR_OBJ_NOT_CALLABLE,
+                });
+                return obj_err;
+            }
+        } break;
+        default: {
+            break;
+        } break;
+    }
+
+    printf("DING\n");
+    display(root);
+    printf("\nTYPE: %d\n", root->type);
+
+    error_push((Error){
+        .type = ERR_TYPE_RUNTIME,
+        .value = ERR_UNKNOWN_OBJ_TYPE,
+    });
+    return obj_err;
+}
+
+Object *
+proc_quote(Environment *env, Object *obj) {
+    (void)env;
+    return obj->car;
+}
+
+//
+// Arithmetic procedures.
+//
+
+Object *
+proc_sum(Environment *env, Object *obj) {
+    // First argument.
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    Object *car = eval(env, obj->car);
+    if (car == obj_err) {
+        return obj_err;
+    }
+    if (car->type != OBJ_TYPE_FIXNUM) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+
+    // Traverse the list.
+    obj = obj->cdr;
+    ssize_t tot = car->fixnum;
+    while (obj != obj_nil) {
+        car = eval(env, obj->car);
+        if (car == obj_err) {
+            return obj_err;
+        }
+        if (car->type != OBJ_TYPE_FIXNUM) {
+            error_push((Error){
+                .type = ERR_TYPE_RUNTIME,
+                .value = ERR_WRONG_ARG_TYPE,
+            });
+            return obj_err;
+        }
+        tot += car->fixnum;
+        obj = obj->cdr;
+    }
+    return make_fixnum(tot);
+}
+
+Object *
+proc_sub(Environment *env, Object *obj) {
+    // First argument.
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    Object *car = eval(env, obj->car);
+    if (car == obj_err) {
+        return obj_err;
+    }
+    if (car->type != OBJ_TYPE_FIXNUM) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+
+    // Traverse the list.
+    obj = obj->cdr;
+    ssize_t tot = car->fixnum;
+    while (obj != obj_nil) {
+        car = eval(env, obj->car);
+        if (car == obj_err) {
+            return obj_err;
+        }
+        if (car->type != OBJ_TYPE_FIXNUM) {
+            error_push((Error){
+                .type = ERR_TYPE_RUNTIME,
+                .value = ERR_WRONG_ARG_TYPE,
+            });
+            return obj_err;
+        }
+        tot -= car->fixnum;
+        obj = obj->cdr;
+    }
+    return make_fixnum(tot);
+}
+
+Object *
+proc_mul(Environment *env, Object *obj) {
+    // First argument.
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    Object *car = eval(env, obj->car);
+    if (car == obj_err) {
+        return obj_err;
+    }
+    if (car->type != OBJ_TYPE_FIXNUM) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+
+    // Traverse the list.
+    obj = obj->cdr;
+    ssize_t tot = car->fixnum;
+    while (obj != obj_nil) {
+        Object *car = eval(env, obj->car);
+        if (car == obj_err) {
+            return obj_err;
+        }
+        if (car->type != OBJ_TYPE_FIXNUM) {
+            error_push((Error){
+                .type = ERR_TYPE_RUNTIME,
+                .value = ERR_WRONG_ARG_TYPE,
+            });
+            return obj_err;
+        }
+        tot *= car->fixnum;
+        obj = obj->cdr;
+    }
+    return make_fixnum(tot);
+}
+
+Object *
+proc_div(Environment *env, Object *obj) {
+    // First argument.
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    Object *car = eval(env, obj->car);
+    if (car == obj_err) {
+        return obj_err;
+    }
+    if (car->type != OBJ_TYPE_FIXNUM) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+
+    // Traverse the list.
+    obj = obj->cdr;
+    ssize_t tot = car->fixnum;
+    while (obj != obj_nil) {
+        Object *car = eval(env, obj->car);
+        if (car == obj_err) {
+            return obj_err;
+        }
+        if (car->type != OBJ_TYPE_FIXNUM) {
+            error_push((Error){
+                .type = ERR_TYPE_RUNTIME,
+                .value = ERR_WRONG_ARG_TYPE,
+            });
+            return obj_err;
+        }
+        if (car->fixnum == 0) {
+            error_push((Error){
+                .type = ERR_TYPE_RUNTIME,
+                .value = ERR_DIVISION_BY_ZERO,
+            });
+            return obj_err;
+        }
+        tot /= car->fixnum;
+        obj = obj->cdr;
+    }
+    return make_fixnum(tot);
+}
+
+Object *
+proc_mod(Environment *env, Object *obj) {
+    // First argument.
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    Object *car = eval(env, obj->car);
+    if (car == obj_err) {
+        return obj_err;
+    }
+    if (car->type != OBJ_TYPE_FIXNUM) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+
+    // Traverse the list.
+    obj = obj->cdr;
+    ssize_t tot = car->fixnum;
+    while (obj != obj_nil) {
+        Object *car = eval(env, obj->car);
+        if (car == obj_err) {
+            return obj_err;
+        }
+        if (car->type != OBJ_TYPE_FIXNUM) {
+            error_push((Error){
+                .type = ERR_TYPE_RUNTIME,
+                .value = ERR_WRONG_ARG_TYPE,
+            });
+            return obj_err;
+        }
+        if (car->fixnum == 0) {
+            error_push((Error){
+                .type = ERR_TYPE_RUNTIME,
+                .value = ERR_DIVISION_BY_ZERO,
+            });
+            return obj_err;
+        }
+        tot %= car->fixnum;
+        obj = obj->cdr;
+    }
+    return make_fixnum(tot);
+}
+
+//
+// Display/Evaluation procedues.
+//
+
+Object *
+proc_display(Environment *env, Object *obj) {
+    display(eval(env, obj->car));
+    return obj_nil;
+}
+
+Object *
+proc_print(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    Object *car = eval(env, obj->car);
+    if (car == obj_err) {
+        return obj_err;
+    }
+    if (car->type != OBJ_TYPE_STRING) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+
+    StringView scanner = (StringView) {
+        .start = car->string,
+        .n = car->string_n,
+    };
+    while (scanner.n != 0) {
+        char c = sv_next(&scanner);
+        if (c == '\\' && sv_peek(&scanner) == 'n') {
+            putchar('\n');
+            sv_next(&scanner);
+            continue;
+        }
+        if (c == '\\' && sv_peek(&scanner) == '"') {
+            putchar('"');
+            sv_next(&scanner);
+            continue;
+        }
+        putchar(c);
+    }
+    return obj_nil;
+}
+
+Object *
+proc_newline(Environment *env, Object *obj) {
+    printf("\n");
+    (void)env;
+    (void)obj;
+    return obj_nil;
+}
+
+//
+// Type info procedures.
+//
+
+Object *
+proc_is_boolean(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    obj = eval(env, obj->car);
+    if (obj == obj_err) {
+        return obj_err;
+    }
+    return (obj == obj_true || obj == obj_false) ? obj_true : obj_false;
+}
+
+Object *
+proc_is_nil(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    obj = eval(env, obj->car);
+    if (obj == obj_err) {
+        return obj_err;
+    }
+    return obj == obj_nil ? obj_true : obj_false;
+}
+
+Object *
+proc_is_symbol(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    obj = eval(env, obj->car);
+    if (obj == obj_err) {
+        return obj_err;
+    }
+    return obj->type == OBJ_TYPE_SYMBOL ? obj_true : obj_false;
+}
+
+Object *
+proc_is_string(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    obj = eval(env, obj->car);
+    if (obj == obj_err) {
+        return obj_err;
+    }
+    return obj->type == OBJ_TYPE_STRING ? obj_true : obj_false;
+}
+
+Object *
+proc_is_fixnum(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    obj = eval(env, obj->car);
+    if (obj == obj_err) {
+        return obj_err;
+    }
+    return obj->type == OBJ_TYPE_FIXNUM ? obj_true : obj_false;
+}
+
+Object *
+proc_is_pair(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    obj = eval(env, obj->car);
+    if (obj == obj_err) {
+        return obj_err;
+    }
+    return obj->type == OBJ_TYPE_PAIR ? obj_true : obj_false;
+}
+
+Object *
+proc_is_procedure(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    obj = eval(env, obj->car);
+    if (obj == obj_err) {
+        return obj_err;
+    }
+    return obj->type == OBJ_TYPE_PROCEDURE ? obj_true : obj_false;
+}
+
+//
+// Boolean/conditional procedures.
+//
+
+Object *
+proc_not(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    obj = eval(env, obj->car);
+    if (obj == obj_err) {
+        return obj_err;
+    }
+    return obj == obj_false ? obj_true : obj_false;
+}
+
+Object *
+proc_and(Environment *env, Object *obj) {
+    while (obj != obj_nil) {
+        if (proc_not(env, obj) == obj_true) {
+            return obj_false;
+        }
+        obj = obj->cdr;
+    }
+    return obj_true;
+}
+
+Object *
+proc_or(Environment *env, Object *obj) {
+    while (obj != obj_nil) {
+        if (proc_not(env, obj) == obj_false) {
+            return obj_true;
+        }
+        obj = obj->cdr;
+    }
+    return obj_false;
+}
+
+Object *
+proc_if(Environment *env, Object *obj) {
+    if (obj == obj_nil || obj->cdr == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    Object *car = obj->car;
+    Object *cdr = obj->cdr;
+    Object *clause = eval(env, car);
+    if (obj == obj_err) {
+        return obj_err;
+    }
+    if (clause == obj_true) {
+        return eval(env, cdr->car);
+    }
+    if (obj->cdr->cdr != obj_nil) {
+        return eval(env, cdr->cdr->car);
+    }
+
+    return obj_nil;
+}
+
+Object *
+proc_cond(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+
+    if (obj->car->type != OBJ_TYPE_PAIR) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+
+    // TODO: review this, the cdr->car could cause issues?
+    while (obj != obj_nil) {
+        Object *clause = obj->car;
+        Object *result = eval(env, clause->car);
+        if (result == obj_err) {
+            return obj_err;
+        }
+        if (result == obj_true) {
+            return eval(env, clause->cdr->car);
+        }
+        obj = obj->cdr;
+    }
+    return obj_nil;
+}
+
+Object *
+proc_num_less_than(Environment *env, Object *obj) {
+    // First argument.
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    Object *car = eval(env, obj->car);
+    if (car == obj_err) {
+        return obj_err;
+    }
+    if (car->type != OBJ_TYPE_FIXNUM) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+
+    // Traverse the list.
+    obj = obj->cdr;
+    ssize_t prev = car->fixnum;
+    while (obj != obj_nil) {
+        car = eval(env, obj->car);
+        if (car == obj_err) {
+            return obj_err;
+        }
+        if (car->type != OBJ_TYPE_FIXNUM) {
+            error_push((Error){
+                .type = ERR_TYPE_RUNTIME,
+                .value = ERR_WRONG_ARG_TYPE,
+            });
+            return obj_err;
+        }
+        if (prev >= car->fixnum) {
+            return obj_false;
+        }
+        prev = car->fixnum;
+        obj = obj->cdr;
+    }
+    return obj_true;
+}
+
+Object *
+proc_num_greater_than(Environment *env, Object *obj) {
+    // First argument.
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    Object *car = eval(env, obj->car);
+    if (car == obj_err) {
+        return obj_err;
+    }
+    if (car->type != OBJ_TYPE_FIXNUM) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+
+    // Traverse the list.
+    obj = obj->cdr;
+    ssize_t prev = car->fixnum;
+    while (obj != obj_nil) {
+        car = eval(env, obj->car);
+        if (car == obj_err) {
+            return obj_err;
+        }
+        if (car->type != OBJ_TYPE_FIXNUM) {
+            error_push((Error){
+                .type = ERR_TYPE_RUNTIME,
+                .value = ERR_WRONG_ARG_TYPE,
+            });
+            return obj_err;
+        }
+        if (prev <= car->fixnum) {
+            return obj_false;
+        }
+        prev = car->fixnum;
+        obj = obj->cdr;
+    }
+    return obj_true;
+}
+
+Object *
+proc_num_lesseq_than(Environment *env, Object *obj) {
+    // First argument.
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    Object *car = eval(env, obj->car);
+    if (car == obj_err) {
+        return obj_err;
+    }
+    if (car->type != OBJ_TYPE_FIXNUM) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+
+    // Traverse the list.
+    obj = obj->cdr;
+    ssize_t prev = car->fixnum;
+    while (obj != obj_nil) {
+        car = eval(env, obj->car);
+        if (car == obj_err) {
+            return obj_err;
+        }
+        if (car->type != OBJ_TYPE_FIXNUM) {
+            error_push((Error){
+                .type = ERR_TYPE_RUNTIME,
+                .value = ERR_WRONG_ARG_TYPE,
+            });
+            return obj_err;
+        }
+        if (prev > car->fixnum) {
+            return obj_false;
+        }
+        prev = car->fixnum;
+        obj = obj->cdr;
+    }
+    return obj_true;
+}
+
+Object *
+proc_num_greatereq_than(Environment *env, Object *obj) {
+    // First argument.
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    Object *car = eval(env, obj->car);
+    if (car == obj_err) {
+        return obj_err;
+    }
+    if (car->type != OBJ_TYPE_FIXNUM) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+
+    // Traverse the list.
+    obj = obj->cdr;
+    ssize_t prev = car->fixnum;
+    while (obj != obj_nil) {
+        car = eval(env, obj->car);
+        if (car == obj_err) {
+            return obj_err;
+        }
+        if (car->type != OBJ_TYPE_FIXNUM) {
+            error_push((Error){
+                .type = ERR_TYPE_RUNTIME,
+                .value = ERR_WRONG_ARG_TYPE,
+            });
+            return obj_err;
+        }
+        if (prev < car->fixnum) {
+            return obj_false;
+        }
+        prev = car->fixnum;
+        obj = obj->cdr;
+    }
+    return obj_true;
+}
+
+Object *
+proc_num_equal(Environment *env, Object *obj) {
+    // First argument.
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    Object *car = eval(env, obj->car);
+    if (car == obj_err) {
+        return obj_err;
+    }
+    if (car->type != OBJ_TYPE_FIXNUM) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+
+    // Traverse the list.
+    obj = obj->cdr;
+    ssize_t prev = car->fixnum;
+    while (obj != obj_nil) {
+        car = eval(env, obj->car);
+        if (car == obj_err) {
+            return obj_err;
+        }
+        if (car->type != OBJ_TYPE_FIXNUM) {
+            error_push((Error){
+                .type = ERR_TYPE_RUNTIME,
+                .value = ERR_WRONG_ARG_TYPE,
+            });
+            return obj_err;
+        }
+        if (prev != car->fixnum) {
+            return obj_false;
+        }
+        prev = car->fixnum;
+        obj = obj->cdr;
+    }
+    return obj_true;
+}
+
+//
+// List operation procedures.
+//
+
+Object *
+proc_car(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    obj = eval(env, obj->car);
+    if (obj == obj_err) {
+        return obj_err;
+    }
+    if (obj->type != OBJ_TYPE_PAIR) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+    return obj->car;
+}
+
+Object *
+proc_cdr(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_NOT_ENOUGH_ARGS,
+        });
+        return obj_err;
+    }
+    obj = eval(env, obj->car);
+    if (obj == obj_err) {
+        return obj_err;
+    }
+    if (obj->type != OBJ_TYPE_PAIR) {
+        error_push((Error){
+            .type = ERR_TYPE_RUNTIME,
+            .value = ERR_WRONG_ARG_TYPE,
+        });
+        return obj_err;
+    }
+    return obj->cdr;
+}
+
+Object *
+proc_cons(Environment *env, Object *obj) {
+    if (obj == obj_nil || obj->cdr == obj_nil) {
+        fprintf(stderr, "error: not enough arguments\n");
+        return obj_nil;
+    }
+    Object *a = eval(env, obj->car);
+    Object *b = eval(env, obj->cdr->car);
+    return make_pair(a, b);
+}
+
+Object *
+proc_list(Environment *env, Object *obj) {
+    if (obj == obj_nil) {
+        return obj_nil;
+    }
+    Object *head = make_pair(eval(env, obj->car), obj_nil);
+    Object *curr = head;
+    obj = obj->cdr;
+    while (obj != obj_nil) {
+        curr->cdr = make_pair(eval(env, obj->car), obj_nil);
+        curr = curr->cdr;
+        obj = obj->cdr;
+    }
+    return head;
+}
+
+//
+// Polymorphic procedures.
+//
+
+//Object *
+//proc_equal(Object *args) {
+//    // TODO: stub
+//    (void) args;
+//    return NULL;
+//}
+
+//// TODO: fixnum left/right shift, mask, invert
+//// TODO: implement and test missing procedures
+//// TODO: add primitives for type transforms: string->symbol, symbol->string, etc
+//// TODO: properly implement nested environments
+//// TODO: implement support for quotes and semi-quotes
+//// TODO: LAMBDA
+//// TODO: let
+//// TODO: better error handling?
+//// TODO: Revise all instances where we are returning an object, since currently
+//// we may be returning a pointer to an object instead of a new one. Check also
+//// on eval function and everytime we do make_xxx(obj).
diff --git a/tests/booleans_expected.txt b/tests/booleans_expected.txt
index f47d32f..43f67e5 100644
--- a/tests/booleans_expected.txt
+++ b/tests/booleans_expected.txt
@@ -33,11 +33,11 @@
 (if (or false false) (+ 1 2 3) (+ 7 8 9)) -> 24
 (if (or (+ 1 2 3) false) (+ 1 2 3) (+ 7 8 9)) -> 6
 (if true 7) -> 7
-(if false 7) -> ()
+(if false 7) -> 
 (cond ((and true true true) 1) ((or true true false) 2) (else 3)) -> 1
 (cond ((and true true false) 1) ((or true true false) 2) (else 3)) -> 2
 (cond ((and true true false) 1) ((or false false false) 2) (else 3)) -> 3
-(cond ((and true true true) 1) ((or true true false) 2)) -> ()
+(cond ((and true true true) 1) ((or true true false) 2)) -> 
 (cond ((and true true true) (+ 1 2 3)) ((or true true false) 2) (else 3)) -> 6
 (< 1 2 3) -> true
 (< 3 2 1) -> false
diff --git a/tests/lists_expected.txt b/tests/lists_expected.txt
index 9030886..6ddb71b 100644
--- a/tests/lists_expected.txt
+++ b/tests/lists_expected.txt
@@ -1,4 +1,4 @@
-(list) -> ()
+(list) -> 
 (list 1) -> (1)
 (list 1 2) -> (1 2)
 (list 1 2 3) -> (1 2 3)
diff --git a/tests/types_expected.txt b/tests/types_expected.txt
index 3a5a2de..58eaa7f 100644
--- a/tests/types_expected.txt
+++ b/tests/types_expected.txt
@@ -5,14 +5,14 @@
 (boolean? "string") -> false
 (boolean? (+ 1 2 3)) -> false
 (boolean? (not 1)) -> true
-(null? true) -> false
-(null? false) -> false
-(null? 1) -> false
-(null? 5) -> false
-(null? "string") -> false
-(null? (+ 1 2 3)) -> false
-(null? (not 1)) -> false
-(null? ()) -> true
+(nil? true) -> false
+(nil? false) -> false
+(nil? 1) -> false
+(nil? 5) -> false
+(nil? "string") -> false
+(nil? (+ 1 2 3)) -> false
+(nil? (not 1)) -> false
+(nil? ()) -> true
 (string? true) -> false
 (string? false) -> false
 (string? 1) -> false
@@ -34,6 +34,8 @@
 (symbol? "string") -> false
 (symbol? (+ 1 2 3)) -> false
 (symbol? (not 1)) -> false
+(symbol? 'a) -> true
+(symbol? 'c) -> true
 (pair? false) -> false
 (pair? 1) -> false
 (pair? 5) -> false