User-friendly error message for missing main function

self_hosted/main.jou

@@ -90,6 +90,11 @@ def parse_args(argc: int, argv: byte**) -> CommandLineArgs:
 
     return result
 
+def find_file(files: FileState*, nfiles: int, path: byte*) -> FileState*:
+    for i = 0; i < nfiles; i++:
+        if strcmp(files[i].ast.path, path) == 0:
+            return &files[i]
+    return NULL
 
 # C:\Users\myname\.foo-bar.jou --> "_foo_bar"
 # Result never contains "-", so you can add "-" separated suffixes without conflicts.
@@ -117,6 +122,14 @@ def get_sane_filename(path: byte*) -> byte[50]:
             name[i] = '_'
     return name
 
+
+def check_main_function(ast: AstFile*) -> bool:
+    for i = 0; i < ast->body.nstatements; i++:
+        s = &ast->body.statements[i]
+        if s->kind == AstStatementKind::Function and strcmp(s->function.signature.name, "main") == 0:
+            return True
+    return False
+
 def check_ast_and_import_conflicts(ast: AstFile*, symbol: ExportSymbol*) -> None:
     for i = 0; i < ast->body.nstatements; i++:
         ts = &ast->body.statements[i]
@@ -210,6 +223,10 @@ class Compiler:
 
             evaluate_compile_time_if_statements_in_body(&ast.body)
 
+            if item.is_imported and check_main_function(&ast):
+                assert item.import_location.path != NULL
+                fail(item.import_location, "imported file should not have `main` function")
+
             self->files = realloc(self->files, sizeof self->files[0] * (self->nfiles + 1))
             self->files[self->nfiles++] = FileState{ast = ast}
 
@@ -467,6 +484,13 @@ def main(argc: int, argv: byte**) -> int:
         compiler.process_imports_and_exports()
         compiler.typecheck_stage3_all_files()
 
+        mainfile = find_file(compiler.files, compiler.nfiles, args.main_path)
+        assert mainfile != NULL
+
+        if not check_main_function(&mainfile->ast):
+            l = Location{path=mainfile->ast.path, lineno=0}
+            fail(l, "missing `main` function to execute the program")
+
         object_files = compiler.create_object_files()
         executable = compiler.link(object_files)
         for i = 0; object_files[i] != NULL; i++:

src/main.c

@@ -196,6 +196,16 @@ static FILE *open_the_file(const char *path, const Location *import_location)
     return f;
 }
 
+static bool defines_main(const AstFile *ast)
+{
+    for (int i = 0; i < ast->body.nstatements; i++) {
+        const AstStatement *s = &ast->body.statements[i];
+        if (s->kind == AST_STMT_FUNCTION && !strcmp(s->data.function.signature.name, "main"))
+            return true;
+    }
+    return false;
+}
+
 static void parse_file(struct CompileState *compst, const char *filename, const Location *import_location)
 {
     if (find_file(compst, filename))
@@ -224,6 +234,16 @@ static void parse_file(struct CompileState *compst, const char *filename, const
     if(command_line_args.verbosity >= 2)
         print_ast(&fs.ast);
 
+    // If it's not the file passed on command line, it shouldn't define main()
+    if (strcmp(filename, command_line_args.infile) && defines_main(&fs.ast)) {
+        /*
+        Set error location to import, so user immediately knows which file
+        imports something that defines main().
+        */
+        assert(import_location);
+        fail(*import_location, "imported file should not have `main` function");
+    }
+
     for (const AstImport *imp = fs.ast.imports.ptr; imp < End(fs.ast.imports); imp++) {
         Append(&compst->parse_queue, (struct ParseQueueItem){
             .filename = imp->resolved_path,
@@ -492,6 +512,15 @@ int main(int argc, char **argv)
     for (struct FileState *fs = compst.files.ptr; fs < End(compst.files); fs++)
         objpaths[fs - compst.files.ptr] = compile_ast_to_object_file(fs);
 
+    /*
+    Check for missing main() as late as possible, so that other errors come first.
+    This way Jou users can work on other functions before main() function is written.
+    */
+    struct FileState *mainfile = find_file(&compst, command_line_args.infile);
+    assert(mainfile);
+    if (!defines_main(&mainfile->ast))
+        fail((Location){.filename=mainfile->path, .lineno=0}, "missing `main` function to execute the program");
+
     for (struct FileState *fs = compst.files.ptr; fs < End(compst.files); fs++) {
         free_ast(&fs->ast);
         free(fs->path);

tests/other_errors/import_file_with_main.jou

@@ -0,0 +1 @@
+import "../../examples/hello.jou"  # Error: imported file should not have `main` function

tests/other_errors/missing_main_function.jou

@@ -0,0 +1,3 @@
+# Output: compiler error in file "tests/other_errors/missing_main_function.jou": missing `main` function to execute the program
+def test() -> None:
+    _ = "lol no main func"