feat: implement functions

include/compiler/fzl_parser.h

@@ -1,6 +1,7 @@
 #ifndef FZL_PARSER_H
 #define FZL_PARSER_H
 
+#include "common/fzl_types.h"
 #include <compiler/fzl_reader.h>
 #include <compiler/fzl_token.h>
 #include <stddef.h>
@@ -13,6 +14,8 @@ typedef enum
   FZL_NODETYPE_LITERAL,
   FZL_NODETYPE_IDENTIFIER,
   FZL_NODETYPE_CONDITIONAL,
+  FZL_NODETYPE_FUNCTION,
+  FZL_NODETYPE_RETURN,
 
   FZL_NODETYPE_ADD,
   FZL_NODETYPE_SUB,
@@ -93,6 +96,23 @@ typedef struct fzl_node
       struct fzl_node *true_block;
       struct fzl_node *else_block;
     } cond;
+
+    // Function declaration
+    struct
+    {
+      struct fzl_node **args;
+      size_t args_count;
+
+      struct fzl_node *body;
+      fzl_token_t *name;
+      fzl_token_t *type;
+    } fn;
+
+    // Return value
+    struct
+    {
+      struct fzl_node *value;
+    } ret;
   };
 } fzl_node_t;