parser: Introduce statement keywords

This commit introduces a few changes in pipalang syntax. Now, both
functions and variables requires keywords to be defined.

before:

  main(): i32 {
    a: i32 = 2;
    return a;
  }

now:

  fn main(): i32 {
    let a: i32 = 2;
    return a;
  }

Signed-off-by: Carlos Maniero <carlos@maniero.me>
Reviewed-by: Johnny Richard <johnny@johnnyrichard.com>

3 files changed, 26 insertions, 9 deletions

diff --git a/src/lexer.c b/src/lexer.c
index 3f6948a..56e24af 100644
--- a/src/lexer.c
+++ b/src/lexer.c
@@ -97,10 +97,18 @@ lexer_tokenize_name(lexer_t *lexer, token_t *token)
 static void
 lexer_token_process_keyword(token_t *token)
 {
+  if (string_view_eq(string_view_from_str("let"), token->value)) {
+    token->kind = TOKEN_KEYWORD_LET;
+    return;
+  }
   if (string_view_eq(string_view_from_str("return"), token->value)) {
     token->kind = TOKEN_KEYWORD_RETURN;
     return;
   }
+  if (string_view_eq(string_view_from_str("fn"), token->value)) {
+    token->kind = TOKEN_KEYWORD_FN;
+    return;
+  }
 }
 
 void
@@ -318,6 +326,10 @@ token_kind_to_str(token_kind_t kind)
       return "=";
     case TOKEN_KEYWORD_RETURN:
       return "return";
+    case TOKEN_KEYWORD_FN:
+      return "fn";
+    case TOKEN_KEYWORD_LET:
+      return "let";
     case TOKEN_EOF:
       return "TOKEN_EOF";
     case TOKEN_UNKNOWN:
diff --git a/src/lexer.h b/src/lexer.h
index 9aa8efe..6449a0a 100644
--- a/src/lexer.h
+++ b/src/lexer.h
@@ -38,6 +38,8 @@ typedef enum
   TOKEN_SLASH,
   TOKEN_EQUAL,
   TOKEN_KEYWORD_RETURN,
+  TOKEN_KEYWORD_FN,
+  TOKEN_KEYWORD_LET,
   TOKEN_EOF,
   TOKEN_UNKNOWN
 } token_kind_t;
diff --git a/src/parser.c b/src/parser.c
index 82d25f5..49803eb 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -296,9 +296,14 @@ parser_parse_variable_assignment(parser_t *parser)
 }
 
 static ast_node_t *
-parser_parse_variable_definition(parser_t *parser)
+parser_parse_variable_declaration(parser_t *parser)
 {
   token_t variable_name;
+
+  if (!drop_expected_token(parser, TOKEN_KEYWORD_LET)) {
+    return NULL;
+  }
+
   if (!expected_token(&variable_name, parser, TOKEN_NAME)) {
     return NULL;
   }
@@ -340,13 +345,7 @@ is_next_statement_a_variable_declaration(parser_t *parser)
   token_t token;
   lexer_peek_next_token(parser->lexer, &token);
 
-  if (token.kind != TOKEN_NAME) {
-    return false;
-  }
-
-  lexer_lookahead(parser->lexer, &token, 2);
-
-  return token.kind == TOKEN_COLON;
+  return token.kind == TOKEN_KEYWORD_LET;
 }
 
 static bool
@@ -440,7 +439,7 @@ parser_parse_block_declarations(parser_t *parser)
     }
 
     if (is_next_statement_a_variable_declaration(parser)) {
-      ast_node_t *variable_node = parser_parse_variable_definition(parser);
+      ast_node_t *variable_node = parser_parse_variable_declaration(parser);
 
       if (variable_node == NULL) {
         scope_leave(parser->scope);
@@ -493,6 +492,10 @@ parser_parse_function_declaration(parser_t *parser)
 {
   token_t func_name_token;
 
+  if (!drop_expected_token(parser, TOKEN_KEYWORD_FN)) {
+    return NULL;
+  }
+
   if (!expected_token(&func_name_token, parser, TOKEN_NAME)) {
     return NULL;
   }