From 2ca7824bedade35a08da5c037b8dc999a54e84f1 Mon Sep 17 00:00:00 2001
From: Mustafa Quraish <mustafaq9@gmail.com>
Date: Sat, 5 Feb 2022 05:47:52 -0500
Subject: Allow function declarations without a definition

---
 src/ast.h       |  2 ++
 src/generator.c |  3 +++
 src/parser.c    | 50 ++++++++++++++++++++++++++++++++++++--------------
 3 files changed, 41 insertions(+), 14 deletions(-)

(limited to 'src')

diff --git a/src/ast.h b/src/ast.h
index 074228c..70896ac 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -95,6 +95,8 @@ typedef struct ast_node {
             // TODO: Arguments / etc?
             Variable *args;
             int num_args;
+
+            bool is_defined;
         } func;
 
         // Block of statements
diff --git a/src/generator.c b/src/generator.c
index 82c492b..d380f36 100644
--- a/src/generator.c
+++ b/src/generator.c
@@ -444,6 +444,9 @@ void generate_function_header(Node *func, FILE *out)
 void generate_function(Node *func, FILE *out)
 {
     assert(func->type == AST_FUNC);
+    // This will happen for declarations.
+    if (func->func.body == NULL)
+        return;
     current_function = func;
     generate_function_header(func, out);
     generate_block(func->func.body, out);
diff --git a/src/parser.c b/src/parser.c
index 9f66060..0294dc2 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -896,18 +896,34 @@ void parse_func_args(Lexer *lexer, Node *func)
 
 Node *parse_func(Lexer *lexer)
 {
-    Token token;
-    token = assert_token(Lexer_next(lexer), TOKEN_FN);
-
-    token = assert_token(Lexer_next(lexer), TOKEN_IDENTIFIER);
-    if (identifier_exists(&token))
-        die_location(token.loc, "Function name already exists as an identifier");
+    assert_token(Lexer_next(lexer), TOKEN_FN);
+    Token token = assert_token(Lexer_next(lexer), TOKEN_IDENTIFIER);
 
     Node *func = Node_new(AST_FUNC);
-    push_new_function(func);
+    Node *dfunc = func; // The pointer to a declaration node
+    func->func.name = token.value.as_string;
 
+    // If the identifier exists, there's 3 possible cases:
+    //  1. It's another variable / struct, which is an error.
+    //  2. It's a function that's been defined, which is an error.
+    //  3. It's a function that's been declared (but not defined), which is OK
+    if (identifier_exists(&token)) {
+        dfunc = find_function_definition(&token);
+        // Case 1
+        if (dfunc == NULL)
+            die_location(token.loc, "Function name already exists as an identifier");
+        // Case 2
+        if (dfunc->func.is_defined)
+            die_location(token.loc, "Function already defined earlier");
+
+        // Case 3 (No error, just set the current function correctly)
+        current_function = func;
+    } else {
+        // We don't have a declaration yet, push this.
+        push_new_function(func);
+    }
 
-    func->func.name = token.value.as_string;
+    // TODO: IMPORTANT: Make sure that args match the declaration, if any.
     parse_func_args(lexer, func);
 
     token = Lexer_peek(lexer);
@@ -920,12 +936,18 @@ Node *parse_func(Lexer *lexer)
         func->func.return_type = type_new(TYPE_VOID);
     }
 
-    // Make sure there's no funny business with the stack offset
-    assert(cur_stack_offset == 0);
-    assert(block_stack_count == 0);
-    func->func.body = parse_block(lexer);
-    assert(block_stack_count == 0);
-    assert(cur_stack_offset == 0);
+    token = Lexer_peek(lexer);
+    if (token.type != TOKEN_SEMICOLON) {
+        // Make sure there's no funny business with the stack offset
+        assert(cur_stack_offset == 0);
+        assert(block_stack_count == 0);
+        func->func.body = parse_block(lexer);
+        assert(block_stack_count == 0);
+        assert(cur_stack_offset == 0);
+        
+        // Mark the declaration as defined.
+        dfunc->func.is_defined = true;
+    }
 
     // Reset current function
     current_function = NULL;
-- 
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