From f2e90efc16268bc2191f9ef61ec258de52d6fbd6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jose=20Luis=20Monta=C3=B1es=20Ojados?= <joseluis@zubi.group>
Date: Wed, 18 Feb 2026 03:16:54 +0100
Subject: [PATCH] Add user-defined functions with call frames

Implement fn/return across the full pipeline:
- Lexer: TOK_FN, TOK_RETURN keywords
- Parser: NODE_FN_DEF, NODE_RETURN AST nodes
- Compiler: FunctionEntry table, inline compilation with jump-over and backpatching
- VM: CallFrame stack with variable snapshot for scoped calls and OP_RETURN
---
 projects/custom_fn.j            |   4 +
 src/backend/bytecode/compiler.h | 460 +++++++++++++++++++-------------
 src/backend/bytecode/opcodes.h  |   1 +
 src/backend/bytecode/vm.h       |  62 ++++-
 src/frontend/lexer.h            |   6 +
 src/frontend/parser.h           |  79 +++++-
 6 files changed, 421 insertions(+), 191 deletions(-)
 create mode 100644 projects/custom_fn.j

diff --git a/projects/custom_fn.j b/projects/custom_fn.j
new file mode 100644
index 0000000..74c80dc
--- /dev/null
+++ b/projects/custom_fn.j
@@ -0,0 +1,4 @@
+fn greet(name):
+    println("Hola, " + name)
+
+greet("mundo!")
\ No newline at end of file
diff --git a/src/backend/bytecode/compiler.h b/src/backend/bytecode/compiler.h
index 06e9aa0..e1ee711 100644
--- a/src/backend/bytecode/compiler.h
+++ b/src/backend/bytecode/compiler.h
@@ -1,226 +1,310 @@
 #ifndef JLANG_COMPILER_H
 #define JLANG_COMPILER_H
 
+#include "../../frontend/parser.h"
 #include "opcodes.h"
 #include <string.h>
-#include "../../frontend/parser.h"
+
 
 typedef struct {
-    Instruction code[4096]; // bytecodes
-    int code_count;
-    char *constants[256];   // pool de strings literales
-    int const_count;
-    char *names[256];       // tabla de nombres (variables + funciones)
-    int name_count;
+  char *name;
+  int entry_point; // indice de la primera instruccion
+  int param_count;
+  char **param_names; // nombres de parametros
+} FunctionEntry;
+
+typedef struct {
+  Instruction code[4096]; // bytecodes
+  int code_count;
+  char *constants[256]; // pool de strings literales
+  int const_count;
+  char *names[256]; // tabla de nombres (variables + funciones)
+  int name_count;
+  FunctionEntry functions[64];
+  int func_count;
 } Chunk;
 
-int emit(Chunk* chunk, Instruction instr) {
-    chunk->code[chunk->code_count++] = instr;
-    return chunk->code_count -1;
+int emit(Chunk *chunk, Instruction instr) {
+  chunk->code[chunk->code_count++] = instr;
+  return chunk->code_count - 1;
 }
 
-int add_constant(Chunk* chunk, char* str) {
-    for (int i=0;i<chunk->const_count; i++){
-        if (strcmp(chunk->constants[i], str) == 0) {
-            return i;
-        }
+int add_constant(Chunk *chunk, char *str) {
+  for (int i = 0; i < chunk->const_count; i++) {
+    if (strcmp(chunk->constants[i], str) == 0) {
+      return i;
     }
+  }
 
-    chunk->constants[chunk->const_count++] = str;
-    return chunk->const_count - 1;
+  chunk->constants[chunk->const_count++] = str;
+  return chunk->const_count - 1;
 }
 
 Instruction make_instruction(OpCode op) {
-    Instruction instr;
-    instr.op = op;
-    return instr;
+  Instruction instr;
+  instr.op = op;
+  return instr;
 }
 
-int add_name(Chunk* chunk, char* name) {
-    for (int i=0;i<chunk->name_count; i++){
-        if (strcmp(chunk->names[i], name) == 0) {
-            return i;
-        }
+int add_name(Chunk *chunk, char *name) {
+  for (int i = 0; i < chunk->name_count; i++) {
+    if (strcmp(chunk->names[i], name) == 0) {
+      return i;
+    }
+  }
+
+  chunk->names[chunk->name_count++] = name;
+  return chunk->name_count - 1;
+}
+
+int compile_node(Chunk *chunk, ASTNode *node) {
+  switch (node->type) {
+  case NODE_INT_LIT: {
+    Instruction instr = make_instruction(OP_CONST_INT);
+    instr.operand.int_val = node->data.int_val;
+    return emit(chunk, instr);
+  }
+  case NODE_STRING_LIT: {
+    Instruction instr = make_instruction(OP_CONST_STRING);
+    instr.operand.str_index = add_constant(chunk, node->data.string_val);
+    return emit(chunk, instr);
+  }
+  case NODE_VAR: {
+    Instruction instr = make_instruction(OP_LOAD_VAR);
+    instr.operand.var_index = add_name(chunk, node->data.string_val);
+    return emit(chunk, instr);
+  }
+  case NODE_ASSIGN: {
+    compile_node(chunk, node->data.assign.value);
+    Instruction instr = make_instruction(OP_STORE_VAR);
+    instr.operand.var_index = add_name(chunk, node->data.assign.name);
+    return emit(chunk, instr);
+  }
+  case NODE_CALL: {
+    // Compilar cada argumento y pushear al stack
+    for (int i = 0; i < node->data.call.arg_count; i++) {
+      compile_node(chunk, node->data.call.args[i]);
     }
 
-    chunk->names[chunk->name_count++] = name;
-    return chunk->name_count - 1;
-}
+    // Registrar el nombre de la funcion
+    Instruction instr = make_instruction(OP_CALL);
+    instr.operand.call.arg_count = node->data.call.arg_count;
+    instr.operand.call.name_index = add_name(chunk, node->data.call.name);
+    return emit(chunk, instr);
+  }
+  case NODE_BLOCK: {
+    int n = node->data.block.count;
 
-int compile_node(Chunk *chunk, ASTNode* node) {
-    switch (node->type) {
-        case NODE_INT_LIT: {
-            Instruction instr = make_instruction(OP_CONST_INT);
-            instr.operand.int_val = node->data.int_val;
-            return emit(chunk, instr);
-        }
-        case NODE_STRING_LIT: {
-            Instruction instr = make_instruction(OP_CONST_STRING);
-            instr.operand.str_index = add_constant(chunk, node->data.string_val);
-            return emit(chunk, instr);
-        }
-        case NODE_VAR: {
-            Instruction instr = make_instruction(OP_LOAD_VAR);
-            instr.operand.var_index = add_name(chunk, node->data.string_val);
-            return emit(chunk, instr);
-        }
-        case NODE_ASSIGN: {
-            compile_node(chunk, node->data.assign.value);
-            Instruction instr = make_instruction(OP_STORE_VAR);
-            instr.operand.var_index = add_name(chunk, node->data.assign.name);
-            return emit(chunk, instr);
-        }
-        case NODE_CALL: {
-            // Compilar cada argumento y pushear al stack
-            for (int i=0; i<node->data.call.arg_count; i++){
-                compile_node(chunk, node->data.call.args[i]);
-            }
+    // NOP for gc
+    emit(chunk, make_instruction(OP_NOP));
 
-            // Registrar el nombre de la funcion
-            Instruction instr = make_instruction(OP_CALL);
-            instr.operand.call.arg_count = node->data.call.arg_count;
-            instr.operand.call.name_index = add_name(chunk, node->data.call.name);
-            return emit(chunk, instr);
-        }
-        case NODE_BLOCK: {
-            int n = node->data.block.count;
-            
-            // NOP for gc
-            emit(chunk, make_instruction(OP_NOP));
-
-            for (int i=0; i<n; i++){
-                compile_node(chunk, node->data.block.stmts[i]);
-            }
-
-            return 0;
-        }
-        case NODE_BINOP: {
-            int leftOffset = compile_node(chunk, node->data.binop.left);
-            int rightOffset = compile_node(chunk, node->data.binop.right);
-
-            OpCode opCode;
-            switch (node->data.binop.op) {
-                case '+':
-                    opCode = OP_ADD;
-                    break;
-                case '-':
-                    opCode = OP_SUB;
-                    break;
-                case '*':
-                    opCode = OP_MUL;
-                    break;
-                case '/':
-                    opCode = OP_DIV;
-                    break;
-                case '>':
-                    opCode = OP_CMP_GT;
-                    break;
-                case '<':
-                    opCode = OP_CMP_LT;
-                    break;
-                default:
-                    break;
-            }
-            emit(chunk, make_instruction(opCode));
-            return 0;
-        }
-        case NODE_WHILE: { 
-            int loop_start = chunk->code_count;
-            compile_node(chunk, node->data.while_loop.cond);
-            // jump if zero, zero = false
-            Instruction instr = make_instruction(OP_JUMP_IF_ZERO);
-            instr.operand.jump_target = -1;
-            int jump_offset = emit(chunk, instr);
-
-            // compile body
-            compile_node(chunk, node->data.while_loop.body);
-
-            instr = make_instruction(OP_JUMP);
-            instr.operand.jump_target = loop_start;
-            emit(chunk, instr);
-
-            // Bachpatching
-            chunk->code[jump_offset].operand.jump_target = chunk->code_count;
-            break; 
-        }
-
-        case NODE_IF: {
-            // compile condition
-            compile_node(chunk, node->data.if_statement.cond);
-
-            // add jump if zero
-            Instruction instr = make_instruction(OP_JUMP_IF_ZERO);
-            instr.operand.jump_target = -1;
-            int jump_offset = emit(chunk, instr);
-
-            // compile body
-            compile_node(chunk, node->data.if_statement.body);
-
-            chunk->code[jump_offset].operand.jump_target = chunk->code_count;
-            break;
-        }
-        
-        default:
-            break;
+    for (int i = 0; i < n; i++) {
+      compile_node(chunk, node->data.block.stmts[i]);
     }
 
     return 0;
-}
+  }
+  case NODE_BINOP: {
+    int leftOffset = compile_node(chunk, node->data.binop.left);
+    int rightOffset = compile_node(chunk, node->data.binop.right);
 
-Chunk* compile(ASTNode* root) {
-    // Create chunk
-    Chunk* chunk = (Chunk*) malloc(sizeof(Chunk));
+    OpCode opCode;
+    switch (node->data.binop.op) {
+    case '+':
+      opCode = OP_ADD;
+      break;
+    case '-':
+      opCode = OP_SUB;
+      break;
+    case '*':
+      opCode = OP_MUL;
+      break;
+    case '/':
+      opCode = OP_DIV;
+      break;
+    case '>':
+      opCode = OP_CMP_GT;
+      break;
+    case '<':
+      opCode = OP_CMP_LT;
+      break;
+    default:
+      break;
+    }
+    emit(chunk, make_instruction(opCode));
+    return 0;
+  }
+  case NODE_WHILE: {
+    int loop_start = chunk->code_count;
+    compile_node(chunk, node->data.while_loop.cond);
+    // jump if zero, zero = false
+    Instruction instr = make_instruction(OP_JUMP_IF_ZERO);
+    instr.operand.jump_target = -1;
+    int jump_offset = emit(chunk, instr);
 
-    // Set arrays to 0
-    memset(chunk, 0, sizeof(Chunk));
+    // compile body
+    compile_node(chunk, node->data.while_loop.body);
 
-    compile_node(chunk, root);
-
-    Instruction instr;
-    instr.op = OP_HALT;
+    instr = make_instruction(OP_JUMP);
+    instr.operand.jump_target = loop_start;
     emit(chunk, instr);
 
-    return chunk;
+    // Bachpatching
+    chunk->code[jump_offset].operand.jump_target = chunk->code_count;
+    break;
+  }
+
+  case NODE_IF: {
+    // compile condition
+    compile_node(chunk, node->data.if_statement.cond);
+
+    // add jump if zero
+    Instruction instr = make_instruction(OP_JUMP_IF_ZERO);
+    instr.operand.jump_target = -1;
+    int jump_offset = emit(chunk, instr);
+
+    // compile body
+    compile_node(chunk, node->data.if_statement.body);
+
+    chunk->code[jump_offset].operand.jump_target = chunk->code_count;
+    break;
+  }
+
+  case NODE_FN_DEF: {
+    // emitir jmp para ignorar la funcion por defecto
+    Instruction jump = make_instruction(OP_JUMP);
+    jump.operand.jump_target = -1; // backpatch despues
+    int jump_idx = emit(chunk, jump);
+
+    // registrar entrypoint de la funcion
+    int entry = chunk->code_count;
+    FunctionEntry *fn = &chunk->functions[chunk->func_count++];
+    fn->name = node->data.fn_def.name;
+    fn->entry_point = entry;
+    fn->param_count = node->data.fn_def.param_count;
+    fn->param_names = node->data.fn_def.params;
+    
+    // emitir store_var para cada parametro (orden inverso al stack)
+    for (int i = node->data.fn_def.param_count - 1; i >= 0; i--) {
+        Instruction store = make_instruction(OP_STORE_VAR);
+        store.operand.var_index = add_name(chunk, node->data.fn_def.params[i]);
+        emit(chunk, store);
+    }
+    
+    // compilar el cuerpo
+    compile_node(chunk, node->data.fn_def.body);
+
+    // emitir el return implicito (por si no hay return explicito)
+    emit(chunk, make_instruction(OP_RETURN));
+
+    // backpatch jump
+    chunk->code[jump_idx].operand.jump_target = chunk->code_count;
+    break;
+  }
+  default:
+    break;
+  }
+
+  return 0;
 }
 
-void print_chunk(Chunk* chunk) {
-    printf("=== Names (%d) ===\n", chunk->name_count);
-    for (int i = 0; i < chunk->name_count; i++) {
-        printf("  [%d] %s\n", i, chunk->names[i]);
-    }
+Chunk *compile(ASTNode *root) {
+  // Create chunk
+  Chunk *chunk = (Chunk *)malloc(sizeof(Chunk));
 
-    printf("=== Constants (%d) ===\n", chunk->const_count);
-    for (int i = 0; i < chunk->const_count; i++) {
-        printf("  [%d] \"%s\"\n", i, chunk->constants[i]);
-    }
+  // Set arrays to 0
+  memset(chunk, 0, sizeof(Chunk));
 
-    printf("=== Bytecode (%d instructions) ===\n", chunk->code_count);
-    for (int i = 0; i < chunk->code_count; i++) {
-        Instruction instr = chunk->code[i];
-        printf("%04d  ", i);
-        switch (instr.op) {
-        case OP_CONST_INT:    printf("CONST_INT %d", instr.operand.int_val); break;
-        case OP_CONST_STRING: printf("CONST_STRING [%d] \"%s\"", instr.operand.str_index, chunk->constants[instr.operand.str_index]); break;
-        case OP_POP:          printf("POP"); break;
-        case OP_ADD:          printf("ADD"); break;
-        case OP_SUB:          printf("SUB"); break;
-        case OP_MUL:          printf("MUL"); break;
-        case OP_DIV:          printf("DIV"); break;
-        case OP_NEG:          printf("NEG"); break;
-        case OP_CMP_LT:      printf("CMP_LT"); break;
-        case OP_CMP_GT:      printf("CMP_GT"); break;
-        case OP_LOAD_VAR:    printf("LOAD_VAR [%d] %s", instr.operand.var_index, chunk->names[instr.operand.var_index]); break;
-        case OP_STORE_VAR:   printf("STORE_VAR [%d] %s", instr.operand.var_index, chunk->names[instr.operand.var_index]); break;
-        case OP_JUMP:        printf("JUMP -> %04d", instr.operand.jump_target); break;
-        case OP_JUMP_IF_ZERO: printf("JUMP_IF_ZERO -> %04d", instr.operand.jump_target); break;
-        case OP_CALL:        printf("CALL %s(%d args)", chunk->names[instr.operand.call.name_index], instr.operand.call.arg_count); break;
-        case OP_NOP:         printf("NOP"); break;
-        case OP_HALT:        printf("HALT"); break;
-        default:             printf("UNKNOWN op=%d", instr.op); break;
-        }
-        printf("\n");
+  compile_node(chunk, root);
+
+  Instruction instr;
+  instr.op = OP_HALT;
+  emit(chunk, instr);
+
+  return chunk;
+}
+
+void print_chunk(Chunk *chunk) {
+  printf("=== Names (%d) ===\n", chunk->name_count);
+  for (int i = 0; i < chunk->name_count; i++) {
+    printf("  [%d] %s\n", i, chunk->names[i]);
+  }
+
+  printf("=== Constants (%d) ===\n", chunk->const_count);
+  for (int i = 0; i < chunk->const_count; i++) {
+    printf("  [%d] \"%s\"\n", i, chunk->constants[i]);
+  }
+
+  printf("=== Bytecode (%d instructions) ===\n", chunk->code_count);
+  for (int i = 0; i < chunk->code_count; i++) {
+    Instruction instr = chunk->code[i];
+    printf("%04d  ", i);
+    switch (instr.op) {
+    case OP_CONST_INT:
+      printf("CONST_INT %d", instr.operand.int_val);
+      break;
+    case OP_CONST_STRING:
+      printf("CONST_STRING [%d] \"%s\"", instr.operand.str_index,
+             chunk->constants[instr.operand.str_index]);
+      break;
+    case OP_POP:
+      printf("POP");
+      break;
+    case OP_ADD:
+      printf("ADD");
+      break;
+    case OP_SUB:
+      printf("SUB");
+      break;
+    case OP_MUL:
+      printf("MUL");
+      break;
+    case OP_DIV:
+      printf("DIV");
+      break;
+    case OP_NEG:
+      printf("NEG");
+      break;
+    case OP_CMP_LT:
+      printf("CMP_LT");
+      break;
+    case OP_CMP_GT:
+      printf("CMP_GT");
+      break;
+    case OP_LOAD_VAR:
+      printf("LOAD_VAR [%d] %s", instr.operand.var_index,
+             chunk->names[instr.operand.var_index]);
+      break;
+    case OP_STORE_VAR:
+      printf("STORE_VAR [%d] %s", instr.operand.var_index,
+             chunk->names[instr.operand.var_index]);
+      break;
+    case OP_JUMP:
+      printf("JUMP -> %04d", instr.operand.jump_target);
+      break;
+    case OP_JUMP_IF_ZERO:
+      printf("JUMP_IF_ZERO -> %04d", instr.operand.jump_target);
+      break;
+    case OP_CALL:
+      printf("CALL %s(%d args)", chunk->names[instr.operand.call.name_index],
+             instr.operand.call.arg_count);
+      break;
+    case OP_RETURN:
+      printf("RETURN");
+      break;
+    case OP_NOP:
+      printf("NOP");
+      break;
+    case OP_HALT:
+      printf("HALT");
+      break;
+    default:
+      printf("UNKNOWN op=%d", instr.op);
+      break;
     }
-    printf("=== End ===\n");
+    printf("\n");
+  }
+  printf("=== End ===\n");
 }
 
 #endif
\ No newline at end of file
diff --git a/src/backend/bytecode/opcodes.h b/src/backend/bytecode/opcodes.h
index 22efe42..b2b62ff 100644
--- a/src/backend/bytecode/opcodes.h
+++ b/src/backend/bytecode/opcodes.h
@@ -13,6 +13,7 @@ typedef enum {
     OP_JUMP,                            // salto incondicional
     OP_JUMP_IF_ZERO,                    // pop -> si false, saltar
     OP_CALL,                            // llamar built-in por indice de nombre
+    OP_RETURN,                          // retornar de funcion (pop call frame)
     OP_NOP, OP_HALT,
 } OpCode;
 
diff --git a/src/backend/bytecode/vm.h b/src/backend/bytecode/vm.h
index 2a54002..5726236 100644
--- a/src/backend/bytecode/vm.h
+++ b/src/backend/bytecode/vm.h
@@ -5,6 +5,13 @@
 #include "compiler.h"
 #include "value.h"
 
+typedef struct {
+  int return_ip;         // a donde volver
+  int saved_sp;          // base del stack
+  Value saved_vars[256]; // variables del caller (snapshot)
+  int saved_var_set[256];
+} CallFrame;
+
 typedef struct {
   Chunk *chunk;
   int ip; // instruction pointer
@@ -12,6 +19,10 @@ typedef struct {
   int sp;           // stack pointer
   Value vars[256];  // variables por indice
   int var_set[256]; // 0=no definida, 1=definida
+
+  CallFrame frames[64];
+  int frame_count;
+
   JLANG_memory_allocator *allocator;
 } VM;
 
@@ -88,8 +99,30 @@ void run_vm(VM *vm) {
 
     case OP_CALL: {
       int nameIdx = instr.operand.call.name_index;
-
       char *name = vm->chunk->names[nameIdx];
+
+      // check if is an user function
+      FunctionEntry *fn = NULL;
+      for (int i = 0; i < vm->chunk->func_count; i++) {
+        if (strcmp(vm->chunk->functions[i].name, name) == 0) {
+          fn = &vm->chunk->functions[i];
+          break;
+        }
+      }
+
+      if (fn != NULL) {
+        // Guardar estado actual en call frame
+        CallFrame *frame = &vm->frames[vm->frame_count++];
+        frame->return_ip = vm->ip + 1; // volver a la siguiente instruccion
+        frame->saved_sp = vm->sp - fn->param_count;
+        memcpy(frame->saved_vars, vm->vars, sizeof(vm->vars));
+        memcpy(frame->saved_var_set, vm->var_set, sizeof(vm->var_set));
+
+        // Saltar al entrypoint
+        vm->ip = fn->entry_point;
+        continue; // no hacer ip++
+      }
+
       if (strcmp(name, "print") == 0 || strcmp(name, "println") == 0) {
 
         int nParams = instr.operand.call.arg_count;
@@ -141,6 +174,30 @@ void run_vm(VM *vm) {
       break;
     }
 
+    case OP_RETURN: {
+      // Captrurar valor de retorno si hay alguno en el stack
+      Value return_val = {0};
+      int has_return = 0;
+      if (vm->sp > vm->frames[vm->frame_count-1].saved_sp) {
+        return_val = vm->stack[--vm->sp];
+        has_return = 1;
+      }
+
+      // Restaurar call frame
+      CallFrame *frame = &vm->frames[--vm->frame_count];
+      vm->ip = frame->return_ip;
+      vm->sp = frame->saved_sp;
+      memcpy(vm->vars, frame->saved_vars, sizeof(vm->vars));
+      memcpy(vm->var_set, frame->saved_var_set, sizeof(vm->var_set));
+
+      // Push return value
+      if (has_return) {
+        vm->stack[vm->sp++] = return_val;
+      }
+
+      continue;
+    }
+
     case OP_ADD: {
       // Pop from stack
       Value var2 = vm->stack[--vm->sp];
@@ -275,9 +332,10 @@ void run_vm(VM *vm) {
       }
 
       gc_collect(vm->allocator, roots, root_count);
-      
+
       break;
     }
+
     default:
       break;
     }
diff --git a/src/frontend/lexer.h b/src/frontend/lexer.h
index 9a7598e..d1300ad 100644
--- a/src/frontend/lexer.h
+++ b/src/frontend/lexer.h
@@ -17,6 +17,8 @@ typedef enum {
   TOK_ID,      // x, foo, mi_var
   TOK_IF,      // if
   TOK_WHILE,   // while
+  TOK_FN,      // fn
+  TOK_RETURN,  // return
 
   // Operadores
   TOK_ASSIGN, // =
@@ -153,6 +155,10 @@ Token *tokenize(const char *source, int *token_count) {
         tokens[count++] = make_token(TOK_IF, word);
       } else if (strcmp(word, "while") == 0) {
         tokens[count++] = make_token(TOK_WHILE, word);
+      } else if (strcmp(word, "fn") == 0) {
+        tokens[count++] = make_token(TOK_FN, word);
+      } else if (strcmp(word, "return") == 0) {
+        tokens[count++] = make_token(TOK_RETURN, word);
       } else {
         tokens[count++] = make_token(TOK_ID, word);
       }
diff --git a/src/frontend/parser.h b/src/frontend/parser.h
index f9fc03c..cf200aa 100644
--- a/src/frontend/parser.h
+++ b/src/frontend/parser.h
@@ -17,6 +17,8 @@ typedef enum {
   NODE_WHILE,      // while cond: bloque
   NODE_BLOCK,      // secuencia de statements
   NODE_CALL,
+  NODE_FN_DEF,    // definicion de funcion
+  NODE_RETURN, // return expr
 } NodeType;
 
 typedef struct ASTNode {
@@ -53,6 +55,15 @@ typedef struct ASTNode {
       struct ASTNode **args;
       int arg_count;
     } call;
+    struct {
+      char *name;
+      char **params;
+      int param_count;
+      struct ASTNode *body;
+    } fn_def; // NODE_FN_DEF
+    struct {
+      struct ASTNode *value; // expresion de retorno
+    } ret; // NODE_RETURN
   } data;
 } ASTNode;
 
@@ -68,7 +79,6 @@ ASTNode *parse_expr(Token *tokens);
 ASTNode *parse_term(Token *tokens);
 ASTNode *parse_factor(Token *tokens);
 
-
 ASTNode *parse_factor(Token *tokens) {
   if (tokens[pos].type == TOK_INT) {
     ASTNode *node = make_node(NODE_INT_LIT);
@@ -274,6 +284,57 @@ ASTNode *parse_statement(Token *tokens) {
     return node;
   }
 
+  if (tokens[pos].type == TOK_FN) {
+    pos++; // consumir "fn"
+    char* name = tokens[pos].value;
+    pos++; // consumir name
+    pos++; // consumir "("
+
+    // Parsear parametros (max 16)
+    char **params = malloc(sizeof(char *) * 16);
+    int param_count = 0;
+    if (tokens[pos].type != TOK_RPAREN) {
+      params[param_count++] = tokens[pos].value;
+      pos++;
+      while (tokens[pos].type == TOK_COMMA) {
+        pos++; // consumir ","
+        params[param_count++] = tokens[pos].value;
+        pos++;
+      }
+    }
+    pos++; // consumir ")"
+    pos++; // consumir ":"
+    pos++; // consumir NEWLINE
+    pos++; // consumir INDENT
+
+    // Parsear bloque de statements hasta DEDENT
+    ASTNode *body = make_node(NODE_BLOCK);
+    body->data.block.stmts = (ASTNode **)malloc(sizeof(ASTNode *) * 256);
+    body->data.block.count = 0;
+    while (tokens[pos].type != TOK_DEDENT) {
+      body->data.block.stmts[body->data.block.count++] =
+          parse_statement(tokens);
+      if (tokens[pos].type == TOK_NEWLINE) {
+        pos++;
+      }
+    }
+    pos++; // Consumir DEDENT
+
+    ASTNode*node = make_node(NODE_FN_DEF);
+    node->data.fn_def.name = name;
+    node->data.fn_def.params = params;
+    node->data.fn_def.param_count = param_count;
+    node->data.fn_def.body = body;
+    return node;
+  }
+
+  if (tokens[pos].type == TOK_RETURN) {
+    pos++;
+    ASTNode *node = make_node(NODE_RETURN);
+    node->data.ret.value = parse_expr(tokens);
+    return node;
+  }
+  
   printf("ERROR: statement inesperado\n");
   exit(1);
 }
@@ -366,6 +427,22 @@ void ast_print(ASTNode *node, const char *prefix, int is_last) {
     }
     break;
 
+  case NODE_FN_DEF:
+    printf("NODE_FN_DEF(\"%s\"", node->data.fn_def.name);
+    for (int i = 0; i < node->data.fn_def.param_count; i++) {
+      printf(", %s", node->data.fn_def.params[i]);
+    }
+    printf(")\n");
+    ast_print(node->data.fn_def.body, new_prefix, 1);
+    break;
+
+  case NODE_RETURN:
+    printf("NODE_RETURN\n");
+    if (node->data.ret.value) {
+      ast_print(node->data.ret.value, new_prefix, 1);
+    }
+    break;
+
   default:
     printf("UNKNOWN\n");
     break;