lexer: introduce StateFn and the basic state machine loop

v2: make the error break more explicit

Signed-off-by: Alejandro Mery <amery@jpi.io>

lexer/lexer.go

@@ -1,2 +1,22 @@
 // Package lexer provides basic helpers to implement parsers
 package lexer
+
+// StateFn is a State Function of the parser
+type StateFn func() (StateFn, error)
+
+// Run runs a state machine until the state function either
+// returns nil or an error
+func Run(fn StateFn) error {
+	for fn != nil {
+		var err error
+
+		fn, err = fn()
+		if err != nil {
+			// failed
+			return err
+		}
+	}
+
+	// ended
+	return nil
+}