From d3f90a5ed13e9e0313fa4cf64776834f87b3f069 Mon Sep 17 00:00:00 2001
From: bog <bog@gozmail.net>
Date: Mon, 11 Dec 2023 18:01:22 +0100
Subject: [PATCH] :sparkles: assert expression.

---
 doc/roza.bnf         |  5 +++-
 lib/compiler.c       | 20 +++++++++-----
 lib/err.c            | 34 +++++++++++++++++++----
 lib/err.h            | 12 ++++++--
 lib/lexer.c          | 66 ++++++++++++++++++++++++++++----------------
 lib/lexer.h          |  1 +
 lib/loader.c         | 39 ++++++++++++++++++++------
 lib/mod.c            | 18 +++++++++---
 lib/node.c           | 10 ++++++-
 lib/node.h           |  8 +++---
 lib/opcodes.h        |  2 +-
 lib/parser.c         | 29 +++++++++++++++++--
 lib/parser.h         |  1 +
 lib/prepass.c        | 40 +++++++++++++++++++++++++++
 lib/prepass.h        | 17 ++++++++++++
 lib/tysy.c           | 12 ++++----
 lib/tysy.h           |  6 ++--
 lib/value.c          |  3 +-
 lib/value.h          |  3 +-
 lib/vm.c             | 58 +++++++++++++++++++++++++++++++++++---
 lib/vm.h             |  9 ++++--
 meson.build          |  1 +
 tests/units/lexer.c  |  4 +++
 tests/units/parser.c |  5 ++++
 24 files changed, 325 insertions(+), 78 deletions(-)
 create mode 100644 lib/prepass.c
 create mode 100644 lib/prepass.h

diff --git a/doc/roza.bnf b/doc/roza.bnf
index 7015dad..ac9fcd6 100644
--- a/doc/roza.bnf
+++ b/doc/roza.bnf
@@ -1,3 +1,6 @@
 MOD ::= EXPR*
-EXPR ::= BUILTIN
+EXPR ::=
+| ASSERT
+| BUILTIN
+ASSERT ::= assert EXPR
 BUILTIN ::= num | bool | str
diff --git a/lib/compiler.c b/lib/compiler.c
index 33de0fa..42ee972 100644
--- a/lib/compiler.c
+++ b/lib/compiler.c
@@ -32,7 +32,7 @@ void compiler_run(compiler_t* compiler, node_t* node)
 
     case NODE_NUM: {
       double value = atof(node->value.data);
-      value_t* val = tysy_new_num(compiler->tysy, value);
+      value_t* val = tysy_new_num(compiler->tysy, value, node->line);
       Opcode op = OP_PUSH;
       param_t param = (param_t) mod_push_new_value(compiler->mod, val);
 
@@ -42,7 +42,7 @@ void compiler_run(compiler_t* compiler, node_t* node)
 
     case NODE_BOOL: {
       int value = strcmp(node->value.data, "true") == 0;
-      value_t* val = tysy_new_bool(compiler->tysy, value);
+      value_t* val = tysy_new_bool(compiler->tysy, value, node->line);
       Opcode op = OP_PUSH;
       param_t param = (param_t) mod_push_new_value(compiler->mod, val);
 
@@ -51,18 +51,24 @@ void compiler_run(compiler_t* compiler, node_t* node)
 
     case NODE_STR: {
       char* value = node->value.data;
-      value_t* val = tysy_new_str(compiler->tysy, value);
+      value_t* val = tysy_new_str(compiler->tysy, value, node->line);
       Opcode op = OP_PUSH;
       param_t param = (param_t) mod_push_new_value(compiler->mod, val);
 
       mod_push_instr(compiler->mod, op, param);
     } break;
 
+    case NODE_ASSERT: {
+      assert(node->children.size == 1);
+      compiler_run(compiler, node_child(node, 0));
+      mod_push_instr(compiler->mod, OP_ASSERT, RZ_NO_PARAM);
+
+    } break;
+
     default: {
-      char msg[RZ_STR_LIMIT];
-      snprintf(msg, RZ_STR_LIMIT, "Unknown node '%s'",
-               NodeTypeStr[node->type]);
-      err_error(compiler->err, msg, node->line);
+      fprintf(stderr, "Cannot compile unknown node '%s'",
+              NodeTypeStr[node->type]);
+      abort();
     } break;
     }
 }
diff --git a/lib/err.c b/lib/err.c
index 9d3eb6e..3a70312 100644
--- a/lib/err.c
+++ b/lib/err.c
@@ -1,5 +1,7 @@
 #include "err.h"
 
+RZ_ENUM_C(ErrType, ERR_TYPES);
+
 void err_init(err_t* err)
 {
   assert(err);
@@ -19,7 +21,7 @@ void err_free(err_t* err)
   err->size = 0;
 }
 
-void err_error(err_t* err, char* what, int line)
+void err_of_type(err_t* err, char* what, int line, ErrType type)
 {
   assert(err);
   assert(err->size + 1 < RZ_ERROR_STACK_SIZE);
@@ -27,21 +29,41 @@ void err_error(err_t* err, char* what, int line)
   err->errors[err->size] = malloc(sizeof(err_msg_t));
   err->errors[err->size]->what = strdup(what);
   err->errors[err->size]->line = line;
+  err->errors[err->size]->type = type;
   err->size++;
 }
 
-void err_abort(err_t* err)
+void err_fatal(err_t* err, char* what, int line)
+{
+  err_of_type(err, what, line, ERR_FATAL);
+}
+
+int err_dump(err_t* err)
 {
   assert(err);
+  int is_fatal = 0;
 
   for (size_t i=0; i<err->size; i++)
     {
-      fprintf(stderr, "ERR(%d) %s\n",
-              err->errors[i]->line,
-              err->errors[i]->what);
+      if (err->errors[i]->type == ERR_WARNING)
+        {
+          fprintf(stderr, "\33[33mWARNING\33[0m[:%d] %s\n",
+                  err->errors[i]->line,
+                  err->errors[i]->what);
+        }
+
+      else if (err->errors[i]->type == ERR_FATAL)
+        {
+          fprintf(stderr, "\33[31mERROR\33[0m[:%d] %s\n",
+                  err->errors[i]->line,
+                  err->errors[i]->what);
+
+          is_fatal = 1;
+        }
     }
 
   err_free(err);
+  err_init(err);
 
-  abort();
+  return is_fatal;
 }
diff --git a/lib/err.h b/lib/err.h
index 09d2c4f..d8a2074 100644
--- a/lib/err.h
+++ b/lib/err.h
@@ -5,7 +5,13 @@
 
 #include "commons.h"
 
+#define ERR_TYPES(G) \
+  G(ERR_WARNING), G(ERR_FATAL)
+
+RZ_ENUM_H(ErrType, ERR_TYPES);
+
 typedef struct {
+  ErrType type;
   char* what;
   int line;
 } err_msg_t;
@@ -18,7 +24,9 @@ typedef struct {
 void err_init(err_t* err);
 void err_free(err_t* err);
 
-void err_error(err_t* err, char* what, int line);
-void err_abort(err_t* err);
+void err_of_type(err_t* err, char* what, int line, ErrType type);
+void err_fatal(err_t* err, char* what, int line);
+
+int err_dump(err_t* err);
 
 #endif
diff --git a/lib/lexer.c b/lib/lexer.c
index 883c08e..7bb5543 100644
--- a/lib/lexer.c
+++ b/lib/lexer.c
@@ -1,6 +1,12 @@
 #include "lexer.h"
 #include "lib/commons.h"
 
+#define RZ_KEYWORD(KW, NODE, VAL)                               \
+  {                                                             \
+    node_t* kw = lexer_try_new_keyword(lexer, KW, NODE, VAL);   \
+    if (kw) { lexer_skip_spaces(lexer); return kw; }            \
+  }
+
 void lexer_init(lexer_t* lexer, char const* source, err_t* err)
 {
   assert(lexer);
@@ -22,35 +28,23 @@ node_t* lexer_try_new_next(lexer_t* lexer)
   assert(lexer);
   size_t len = strlen(lexer->source);
 
-  // skip spaces
-  {
-    while (lexer->cursor < len
-           && isspace(lexer->source[lexer->cursor]))
-      {
-        if (lexer->source[lexer->cursor] == '\n')
-          {
-            lexer->line++;
-          }
-
-        lexer->cursor++;
-      }
-  }
+  lexer_skip_spaces(lexer);
 
   // Keywords
   // ========
-  {
-    node_t* kw = lexer_try_new_keyword(lexer, "true", NODE_BOOL, 1);
-    if (kw) { return kw; }
-  }
-  {
-    node_t* kw = lexer_try_new_keyword(lexer, "false", NODE_BOOL, 1);
-    if (kw) { return kw; }
-  }
+  RZ_KEYWORD("true", NODE_BOOL, 1);
+  RZ_KEYWORD("false", NODE_BOOL, 1);
+  RZ_KEYWORD("assert", NODE_ASSERT, 0);
 
   // scan str
   {
     node_t* node = lexer_try_new_str(lexer);
-    if (node) { return node; }
+
+    if (node)
+      {
+        lexer_skip_spaces(lexer);
+        return node;
+      }
   }
 
   // scan num
@@ -94,6 +88,8 @@ node_t* lexer_try_new_next(lexer_t* lexer)
         str_free(&res_str);
 
         lexer->cursor = cursor;
+
+        lexer_skip_spaces(lexer);
         return tok;
       }
 
@@ -105,7 +101,7 @@ node_t* lexer_try_new_next(lexer_t* lexer)
       size_t const SZ = RZ_STR_LIMIT;
       char msg[SZ];
       snprintf(msg, SZ, "unexpected symbol '%c'", lexer->source[lexer->cursor]);
-      err_error(lexer->err, msg, lexer->line);
+      err_fatal(lexer->err, msg, lexer->line);
     }
 
   return NULL;
@@ -117,7 +113,7 @@ NodeType lexer_peek(lexer_t* lexer, int lookahead)
   size_t cursor = lexer->cursor;
   int line = lexer->line;
 
-  NodeType type = 0;
+  NodeType type = -1;
 
   for (int i=0; i<lookahead; i++)
     {
@@ -129,6 +125,10 @@ NodeType lexer_peek(lexer_t* lexer, int lookahead)
           node_free(node);
           free(node);
         }
+      else
+        {
+          break;
+        }
     }
 
   lexer->cursor = cursor;
@@ -137,6 +137,24 @@ NodeType lexer_peek(lexer_t* lexer, int lookahead)
   return type;
 }
 
+void lexer_skip_spaces(lexer_t* lexer)
+{
+  assert(lexer);
+  size_t len = strlen(lexer->source);
+
+  while (lexer->cursor < len
+         && isspace(lexer->source[lexer->cursor]))
+    {
+      if (lexer->source[lexer->cursor] == '\n')
+        {
+          lexer->line++;
+        }
+
+      lexer->cursor++;
+    }
+
+}
+
 node_t* lexer_try_new_keyword(lexer_t* lexer, char* kw,
                               NodeType type, int has_value)
 {
diff --git a/lib/lexer.h b/lib/lexer.h
index 8a1f2d7..1ec7f29 100644
--- a/lib/lexer.h
+++ b/lib/lexer.h
@@ -17,6 +17,7 @@ void lexer_free(lexer_t* lexer);
 
 node_t* lexer_try_new_next(lexer_t* lexer);
 NodeType lexer_peek(lexer_t* lexer, int lookahead);
+void lexer_skip_spaces(lexer_t* lexer);
 
 node_t* lexer_try_new_keyword(lexer_t* lexer, char* kw,
                               NodeType type, int has_value);
diff --git a/lib/loader.c b/lib/loader.c
index a5109a5..068e228 100644
--- a/lib/loader.c
+++ b/lib/loader.c
@@ -2,6 +2,7 @@
 #include "parser.h"
 #include "loader.h"
 #include "compiler.h"
+#include "prepass.h"
 #include "mod.h"
 #include "vm.h"
 
@@ -67,20 +68,41 @@ void loader_ldfile(loader_t* loader, char const* path, int debug)
       mod_t mod;
       mod_init(&mod);
 
+      // prepass
+      {
+        prepass_t prepass;
+        prepass_init(&prepass, &err);
+        prepass_run(&prepass, node);
+        prepass_free(&prepass);
+      }
+
       // compile
       {
         compiler_t compiler;
         compiler_init(&compiler, &mod, &tysy, &err);
         compiler_run(&compiler, node);
         compiler_free(&compiler);
-        node_free(node);
-        free(node);
       }
 
+      node_free(node);
+      free(node);
+
+      // check compilation errors
+      if (err_dump(&err))
+        {
+          parser_free(&parser);
+          lexer_free(&lex);
+          str_free(&source);
+          tysy_free(&tysy);
+          err_free(&err);
+
+          abort();
+        }
+
       // execute
       {
         vm_t vm;
-        vm_init(&vm);
+        vm_init(&vm, &err);
 
         vm_exec_mod(&vm, &mod);
 
@@ -110,15 +132,14 @@ void loader_ldfile(loader_t* loader, char const* path, int debug)
   parser_free(&parser);
   lexer_free(&lex);
   str_free(&source);
+  tysy_free(&tysy);
 
-  if (err.size > 0)
-    {
-      err_abort(&err);
-    }
-  else
+  // check execution errors
+  if (err_dump(&err))
     {
       err_free(&err);
+      abort();
     }
 
-  tysy_free(&tysy);
+  err_free(&err);
 }
diff --git a/lib/mod.c b/lib/mod.c
index 9f5ca93..e361e22 100644
--- a/lib/mod.c
+++ b/lib/mod.c
@@ -1,4 +1,5 @@
 #include "mod.h"
+#include "lib/commons.h"
 
 void mod_init(mod_t* mod)
 {
@@ -74,10 +75,19 @@ size_t mod_str(mod_t* mod, char* buffer, size_t size)
   sz += snprintf(buffer + sz, size - sz, "\n======== PROGRAM ========\n");
   for (size_t i=0; i<mod->program.size; i++)
     {
-      sz += snprintf(buffer + sz, size - sz, "%d| %s %d\n",
-                     (int) i,
-                     OpcodeStr[mod->program.ops[i]] + strlen("OP_"),
-                     mod->program.params[i]);
+      if (mod->program.params[i] != RZ_NO_PARAM)
+        {
+          sz += snprintf(buffer + sz, size - sz, "%d| %s %d\n",
+                         (int) i,
+                         OpcodeStr[mod->program.ops[i]] + strlen("OP_"),
+                         mod->program.params[i]);
+        }
+      else
+        {
+          sz += snprintf(buffer + sz, size - sz, "%d| %s\n",
+                         (int) i,
+                         OpcodeStr[mod->program.ops[i]] + strlen("OP_"));
+        }
     }
 
   return sz;
diff --git a/lib/node.c b/lib/node.c
index f186cdc..2190479 100644
--- a/lib/node.c
+++ b/lib/node.c
@@ -34,7 +34,7 @@ void node_free(node_t* node)
   node->children.cap = 0;
 }
 
-void node_add_child(node_t* node, node_t* child)
+void node_add_new_child(node_t* node, node_t* child)
 {
   assert(node);
   assert(child);
@@ -57,6 +57,14 @@ void node_add_child(node_t* node, node_t* child)
   node->children.size++;
 }
 
+node_t* node_child(node_t* node, size_t index)
+{
+  assert(node);
+  assert(index < node->children.size);
+
+  return (node_t*) node->children.data[index];
+}
+
 size_t node_str(node_t* node, char* buffer, size_t size)
 {
   assert(node);
diff --git a/lib/node.h b/lib/node.h
index c0342b2..9ed6c9e 100644
--- a/lib/node.h
+++ b/lib/node.h
@@ -5,9 +5,8 @@
 
 #define NODE_TYPE(G)                            \
   G(NODE_MOD),                                  \
-    G(NODE_NUM),                                \
-    G(NODE_BOOL),                               \
-    G(NODE_STR)
+    G(NODE_NUM), G(NODE_BOOL), G(NODE_STR),     \
+    G(NODE_ASSERT)
 
 RZ_ENUM_H(NodeType, NODE_TYPE);
 
@@ -26,7 +25,8 @@ typedef struct {
 void node_init(node_t* node, NodeType type, char* value, int line);
 void node_free(node_t* node);
 
-void node_add_child(node_t* node, node_t* child);
+void node_add_new_child(node_t* node, node_t* child);
+node_t* node_child(node_t* node, size_t index);
 
 size_t node_str(node_t* node, char* buffer, size_t size);
 
diff --git a/lib/opcodes.h b/lib/opcodes.h
index 77c7040..cea0b73 100644
--- a/lib/opcodes.h
+++ b/lib/opcodes.h
@@ -4,7 +4,7 @@
 #include "commons.h"
 
 #define OPCODES(G)                              \
-  G(OP_HALT),                                   \
+  G(OP_ASSERT),                                 \
     G(OP_PUSH), G(OP_POP)
 
 RZ_ENUM_H(Opcode, OPCODES);
diff --git a/lib/parser.c b/lib/parser.c
index 0812e50..77a3541 100644
--- a/lib/parser.c
+++ b/lib/parser.c
@@ -33,7 +33,7 @@ node_t* parser_try_new_mod(parser_t* parser)
 
       if (expr)
         {
-          node_add_child(mod, expr);
+          node_add_new_child(mod, expr);
         }
       else
         {
@@ -49,9 +49,34 @@ node_t* parser_try_new_mod(parser_t* parser)
 node_t* parser_try_new_expr(parser_t* parser)
 {
   assert(parser);
+  NodeType type = lexer_peek(parser->lexer, 1);
+
+  if (type == -1)
+    {
+      return NULL;
+    }
+
+  if (type == NODE_ASSERT)
+    {
+      return parser_try_new_assert(parser);
+    }
+
   return parser_try_new_builtin(parser);
 }
 
+node_t* parser_try_new_assert(parser_t* parser)
+{
+  assert(parser);
+  node_t* node = parser_try_new_consume(parser, NODE_ASSERT);
+  assert(node);
+
+  node_t* expr = parser_try_new_expr(parser);
+  assert(expr);
+  node_add_new_child(node, expr);
+
+  return node;
+}
+
 node_t* parser_try_new_builtin(parser_t* parser)
 {
   assert(parser);
@@ -80,7 +105,7 @@ node_t* parser_try_new_consume(parser_t* parser, NodeType type)
       size_t const SZ = RZ_STR_LIMIT;
       char err_msg[SZ];
       snprintf(err_msg, SZ, "unexpected node '%s'", NodeTypeStr[next->type]);
-      err_error(parser->err, err_msg, next->line);
+      err_fatal(parser->err, err_msg, next->line);
       node_free(next);
       free(next);
       return NULL;
diff --git a/lib/parser.h b/lib/parser.h
index 894a393..f300764 100644
--- a/lib/parser.h
+++ b/lib/parser.h
@@ -16,6 +16,7 @@ void parser_free(parser_t* parser);
 node_t* parser_try_new_tree(parser_t* parser);
 node_t* parser_try_new_mod(parser_t* parser);
 node_t* parser_try_new_expr(parser_t* parser);
+node_t* parser_try_new_assert(parser_t* parser);
 node_t* parser_try_new_builtin(parser_t* parser);
 
 node_t* parser_try_new_consume(parser_t* parser, NodeType type);
diff --git a/lib/prepass.c b/lib/prepass.c
new file mode 100644
index 0000000..1159a4b
--- /dev/null
+++ b/lib/prepass.c
@@ -0,0 +1,40 @@
+#include "prepass.h"
+
+void prepass_init(prepass_t* prepass, err_t* err)
+{
+  assert(prepass);
+  assert(err);
+
+  prepass->err = err;
+}
+
+void prepass_free(prepass_t* prepass)
+{
+  assert(prepass);
+}
+
+void prepass_run(prepass_t* prepass, node_t* node)
+{
+  assert(prepass);
+  assert(node);
+
+  switch (node->type)
+    {
+    case NODE_MOD:
+    case NODE_NUM:
+    case NODE_STR:
+    case NODE_BOOL:
+    case NODE_ASSERT:{
+      for (size_t i=0; i<node->children.size; i++)
+        {
+          prepass_run(prepass, (node_t*) node->children.data[i]);
+        }
+    } break;
+
+    default: {
+      fprintf(stderr, "Cannot prepass unknown node '%s'.\n",
+              NodeTypeStr[node->type]);
+      abort();
+    } break;
+    }
+}
diff --git a/lib/prepass.h b/lib/prepass.h
new file mode 100644
index 0000000..5590c3a
--- /dev/null
+++ b/lib/prepass.h
@@ -0,0 +1,17 @@
+#ifndef RZ_PREPASS_H
+#define RZ_PREPASS_H
+
+#include "commons.h"
+#include "err.h"
+#include "node.h"
+
+typedef struct {
+  err_t* err;
+} prepass_t;
+
+void prepass_init(prepass_t* prepass, err_t* err);
+void prepass_free(prepass_t* prepass);
+
+void prepass_run(prepass_t* prepass, node_t* node);
+
+#endif
diff --git a/lib/tysy.c b/lib/tysy.c
index 85e3bc5..ebb44e6 100644
--- a/lib/tysy.c
+++ b/lib/tysy.c
@@ -69,34 +69,34 @@ type_t* tysy_try_find_type(tysy_t* tysy, char* name)
   return NULL;
 }
 
-value_t* tysy_new_num(tysy_t* tysy, double value)
+value_t* tysy_new_num(tysy_t* tysy, double value, int line)
 {
   assert(tysy);
 
   value_t* val = malloc(sizeof(value_t));
-  value_init(val, tysy_try_find_type(tysy, "num"));
+  value_init(val, tysy_try_find_type(tysy, "num"), line);
   val->value.num = value;
 
   return val;
 }
 
-value_t* tysy_new_bool(tysy_t* tysy, int value)
+value_t* tysy_new_bool(tysy_t* tysy, int value, int line)
 {
   assert(tysy);
 
   value_t* val = malloc(sizeof(value_t));
-  value_init(val, tysy_try_find_type(tysy, "bool"));
+  value_init(val, tysy_try_find_type(tysy, "bool"), line);
   val->value.bool = value;
 
   return val;
 }
 
-value_t* tysy_new_str(tysy_t* tysy, char* value)
+value_t* tysy_new_str(tysy_t* tysy, char* value, int line)
 {
   assert(tysy);
 
   value_t* val = malloc(sizeof(value_t));
-  value_init(val, tysy_try_find_type(tysy, "str"));
+  value_init(val, tysy_try_find_type(tysy, "str"), line);
   val->value.str = strdup(value);
 
   return val;
diff --git a/lib/tysy.h b/lib/tysy.h
index f4966ad..c75a88b 100644
--- a/lib/tysy.h
+++ b/lib/tysy.h
@@ -16,8 +16,8 @@ void tysy_free(tysy_t* tysy);
 void tysy_register_new_type(tysy_t* tysy, char* name, type_t* type);
 type_t* tysy_try_find_type(tysy_t* tysy, char* name);
 
-value_t* tysy_new_num(tysy_t* tysy, double value);
-value_t* tysy_new_bool(tysy_t* tysy, int value);
-value_t* tysy_new_str(tysy_t* tysy, char* value);
+value_t* tysy_new_num(tysy_t* tysy, double value, int line);
+value_t* tysy_new_bool(tysy_t* tysy, int value, int line);
+value_t* tysy_new_str(tysy_t* tysy, char* value, int line);
 
 #endif
diff --git a/lib/value.c b/lib/value.c
index b131ec2..e61eb82 100644
--- a/lib/value.c
+++ b/lib/value.c
@@ -1,11 +1,12 @@
 #include "value.h"
 
-void value_init(value_t* value, type_t* type)
+void value_init(value_t* value, type_t* type, int line)
 {
   assert(value);
   assert(type);
 
   value->type = type;
+  value->line = line;
 }
 
 void value_free(value_t* value)
diff --git a/lib/value.h b/lib/value.h
index 4416cc8..596da5e 100644
--- a/lib/value.h
+++ b/lib/value.h
@@ -12,9 +12,10 @@ typedef struct {
     char* str;
   } value;
 
+  int line;
 } value_t;
 
-void value_init(value_t* value, type_t* type);
+void value_init(value_t* value, type_t* type, int line);
 void value_free(value_t* value);
 
 int value_eq(value_t* value, value_t* rhs);
diff --git a/lib/vm.c b/lib/vm.c
index f977ca9..6c495cb 100644
--- a/lib/vm.c
+++ b/lib/vm.c
@@ -1,13 +1,16 @@
 #include "vm.h"
 #include "lib/commons.h"
+#include "lib/type.h"
 
-void vm_init(vm_t* vm)
+void vm_init(vm_t* vm, err_t* err)
 {
   assert(vm);
 
   vm->pc = 0;
   vm->bp = 0;
   vm->sp = 0;
+
+  vm->err = err;
 }
 
 void vm_free(vm_t* vm)
@@ -15,6 +18,16 @@ void vm_free(vm_t* vm)
   assert(vm);
 }
 
+param_t vm_pop(vm_t* vm)
+{
+  assert(vm);
+  assert(vm->sp > 0);
+  param_t param = vm->stack[vm->sp - 1];
+  vm->sp--;
+
+  return param;
+}
+
 size_t vm_stack_str(vm_t* vm, char* buffer, size_t size)
 {
   assert(vm);
@@ -37,12 +50,19 @@ void vm_exec_mod(vm_t* vm, mod_t* mod)
 
   while (vm->pc < mod->program.size)
     {
-      vm_exec_instr(vm, mod->program.ops[vm->pc],
-                    mod->program.params[vm->pc]);
+      int status = vm_exec_instr(vm,
+                                 mod,
+                                 mod->program.ops[vm->pc],
+                                 mod->program.params[vm->pc]);
+
+      if (!status)
+        {
+          return;
+        }
     }
 }
 
-void vm_exec_instr(vm_t* vm, Opcode op, param_t param)
+int vm_exec_instr(vm_t* vm, mod_t* mod, Opcode op, param_t param)
 {
   assert(vm);
 
@@ -61,10 +81,40 @@ void vm_exec_instr(vm_t* vm, Opcode op, param_t param)
       vm->pc++;
     } break;
 
+    case OP_ASSERT: {
+      param_t top = vm_pop(vm);
+      value_t* value = mod->values.data[top];
+
+      if (value->type->kind != TYPE_BOOL)
+        {
+          char msg[RZ_STR_LIMIT];
+          snprintf(msg, RZ_STR_LIMIT,
+                   "type mismatch: BOOL expected, got %s.",
+                   TypeKindStr[value->type->kind] + strlen("TYPE_"));
+
+          err_fatal(vm->err, msg, value->line);
+
+          vm->pc++;
+          break;
+        }
+
+      if (!value->value.bool)
+        {
+          fprintf(stderr, "\33[31mASSERT\33[0m[:%d] assertion failed\n",
+                  value->line);
+          return 0;
+        }
+
+      vm->pc++;
+
+    } break;
+
     default: {
       fprintf(stderr, "Cannot execute unknown opcode '%s'.\n",
               OpcodeStr[op]);
       abort();
     };
     }
+
+  return 1;
 }
diff --git a/lib/vm.h b/lib/vm.h
index ace7d1a..b93080f 100644
--- a/lib/vm.h
+++ b/lib/vm.h
@@ -4,20 +4,25 @@
 #include "commons.h"
 #include "opcodes.h"
 #include "mod.h"
+#include "err.h"
 
 typedef struct {
   param_t stack[RZ_STACK_LIMIT];
   size_t pc;
   size_t bp;
   size_t sp;
+
+  err_t* err;
 } vm_t;
 
-void vm_init(vm_t* vm);
+void vm_init(vm_t* vm, err_t* err);
 void vm_free(vm_t* vm);
 
+param_t vm_pop(vm_t* vm);
+
 size_t vm_stack_str(vm_t* vm, char* buffer, size_t size);
 
 void vm_exec_mod(vm_t* vm, mod_t* mod);
-void vm_exec_instr(vm_t* vm, Opcode op, param_t param);
+int vm_exec_instr(vm_t* vm, mod_t* mod, Opcode op, param_t param);
 
 #endif
diff --git a/meson.build b/meson.build
index 3f41da5..758f755 100644
--- a/meson.build
+++ b/meson.build
@@ -34,6 +34,7 @@ roza_lib = static_library(
     # comp
     'lib/opcodes.c',
     'lib/mod.c',
+    'lib/prepass.c',
     'lib/compiler.c',
 
     # exec
diff --git a/tests/units/lexer.c b/tests/units/lexer.c
index 6c402d7..855540f 100644
--- a/tests/units/lexer.c
+++ b/tests/units/lexer.c
@@ -105,3 +105,7 @@ Test(lexer, str) {
   test_lexer("\"the \\\"cat\\\"\"", 1, "STR[the \"cat\"]");
   test_lexer("\"hello\\n\\tworld\"", 1, "STR[hello\n\tworld]");
 }
+
+Test(lexer, assert) {
+  test_lexer("assert", 1, "ASSERT");
+}
diff --git a/tests/units/parser.c b/tests/units/parser.c
index 5b08530..3a34166 100644
--- a/tests/units/parser.c
+++ b/tests/units/parser.c
@@ -66,3 +66,8 @@ Test(parser, bool) {
 Test(parser, str) {
   test_parser_ok("MOD(STR[bim bam bum])", " \"bim bam bum\"");
 }
+
+Test(parser, assert) {
+  test_parser_ok("MOD(ASSERT(BOOL[false]))", " assert false ");
+  test_parser_ok("MOD(ASSERT(BOOL[true]))", " assert true ");
+}