Make main() optional (#388)

Author: vtereshkov

doc/api.md

@@ -38,7 +38,7 @@ Compiles the Umka program into bytecode. Here, `umka` is the interpreter instanc
 UMKA_API int umkaRun(void *umka);
 ```
 
-Runs the Umka program previously compiled to bytecode, i. e., calls its `main()` function. Here, `umka` is the interpreter instance handle. Returns `0` if the program execution finishes successfully and no run-time errors were detected, otherwise return the error code.
+Runs the Umka program previously compiled to bytecode, i.e., calls its `main()` function, if it exists, and gracefully deallocates heap memory referenced by global variables. Here, `umka` is the interpreter instance handle. Returns `0` if the program execution finishes successfully and no run-time errors were detected, otherwise return the error code.
 
 ```
 UMKA_API void umkaFree(void *umka);

doc/lang.md

@@ -1422,7 +1422,7 @@ fn p(a: int): (int, bool) {
 
 ## Modules
 
-An Umka program consists of one or more source files, or modules. The main module should have the `main()` function with no parameters and no return values, which is the entry point of the program.
+An Umka program consists of one or more source files, or modules. If the main module has a `main()` function with no parameters and no return values, this function becomes the entry point of the program.
 
 Modules can be imported from other modules using the `import` clause in the beginning of the importing module. In this case, all the identifiers declared as exported in the module scope of the imported module become valid in the importing module, if qualified with the imported module name.
 

examples/3dcam/3dcam.um

@@ -45,7 +45,3 @@ fn drawBodies() {
         drawCube(cube.position, 2, cube.height, 2, cube.color)
     }
 }
-
-fn main() {}
-    
-    

playground/umka.js

@@ -921,14 +921,12 @@ void parseProgram(Compiler *comp)
     // Entry point
     genEntryPoint(&comp->gen, 0);
 
-    Ident *mainFn = identAssertFind(&comp->idents, &comp->modules, &comp->blocks, mainModule, "main", NULL);
-    if (!identIsMain(mainFn))
-        comp->error.handler(comp->error.context, "Illegal main() function");
-
-    // Dummy upvalue
-    genPushZero(&comp->gen, sizeof(Interface) / sizeof(Slot));
-
-    genCall(&comp->gen, mainFn->offset);
+    Ident *mainFn = identFind(&comp->idents, &comp->modules, &comp->blocks, mainModule, "main", NULL, false);
+    if (mainFn && identIsMain(mainFn))
+    {
+        genPushZero(&comp->gen, sizeof(Interface) / sizeof(Slot));  // Dummy upvalue
+        genCall(&comp->gen, mainFn->offset);
+    }
 
     doGarbageCollection(comp, 0);
     genHalt(&comp->gen);