lang: ast, parser, interfaces: Implementation of nested class sugar

This implements a new type of syntactic sugar for the common pattern of
a base class which returns a child class, and so on. Instead of needing
to repeatedly indent the child classes, we can instead prefix them at
the definition site (where created with the class keyword) with the name
of the parent class, followed by a colon, to get the desired embedded
sugar.

For example, instead of writing:

class base() {
	class inner() {
		class deepest() {
		}
	}
}

You can instead write:

class base() {
}
class base:inner() {
}
class base:inner:deepest() {
}

Of course, you can only access any of the inner classes by first
including (with the include keyword) a parent class, and then
subsequently including the inner one.

lang/ast/structs.go

@@ -2921,7 +2921,67 @@ func (obj *StmtProg) Init(data *interfaces.Data) error {
 // Interpolate returns a new node (aka a copy) once it has been expanded. This
 // generally increases the size of the AST when it is used. It calls Interpolate
 // on any child elements and builds the new node with those new node contents.
+// This particular implementation can currently modify the source AST in-place,
+// and then finally return a copy. This isn't ideal, but it is much more optimal
+// as it avoids a lot of copying, and the code is simpler. If we need our AST to
+// be static, then we can improve this.
 func (obj *StmtProg) Interpolate() (interfaces.Stmt, error) {
+	// First, make a list of class name to class pointer.
+	classes := make(map[string]*StmtClass)
+	for _, x := range obj.Body {
+		stmt, ok := x.(*StmtClass)
+		if !ok {
+			continue
+		}
+		if _, exists := classes[stmt.Name]; exists {
+			return nil, fmt.Errorf("duplicate class name of: `%s`", stmt.Name)
+		}
+		// if it contains a colon we could skip it (perf busy work)
+		//if strings.Contains(stmt.Name, interfaces.ClassSep) {
+		//	continue
+		//}
+		classes[stmt.Name] = stmt
+	}
+
+	// Now, loop through (in reverse so that remove will work without
+	// breaking the index offset) the body and pull any colon prefixed class
+	// into the base class that it belongs inside. We also rename it to pop
+	// off the front prefix name once it's inside the new base class. This
+	// is all syntactic sugar to implement the class child nesting.
+	for i := len(obj.Body) - 1; i >= 0; i-- { // reverse order for remove
+		stmt, ok := obj.Body[i].(*StmtClass)
+		if !ok || stmt.Name == "" {
+			continue
+		}
+
+		// equivalent to: strings.Contains(stmt.Name, interfaces.ClassSep)
+		split := strings.Split(stmt.Name, interfaces.ClassSep)
+		if len(split) == 0 || len(split) == 1 {
+			continue
+		}
+		if split[0] == "" { // prefix, eg: `:foo:bar`
+			return nil, fmt.Errorf("class name prefix is empty")
+		}
+
+		class, exists := classes[split[0]]
+		if !exists {
+			continue
+		}
+		prog, ok := class.Body.(*StmtProg) // probably a *StmtProg
+		if !ok {
+			// TODO: print warning or error?
+			continue
+		}
+
+		// It's not ideal to modify things here, but we do since it's so
+		// much easier and faster to do it like this. We can use copies
+		// if it turns out we need to preserve the original input AST.
+		stmt.Name = strings.Join(split[1:], interfaces.ClassSep) // new name w/o prefix
+		prog.Body = append(prog.Body, stmt)                      // append it to child body
+		obj.Body = append(obj.Body[:i], obj.Body[i+1:]...)       // remove it (from the end)
+	}
+
+	// Now perform the normal recursive interpolation calls.
 	body := []interfaces.Stmt{}
 	for _, x := range obj.Body {
 		interpolated, err := x.Interpolate()