Lexer: use the first text found for a token in order to unparse it

Author: pmderodat

langkit/lexer/__init__.py

@@ -620,7 +620,11 @@ def add_rules(self,
                 # representation.
                 self.literals_map[m.to_match] = a
 
-                a.matcher = m
+                # Keep track of a canonical representation of this token, to be
+                # used by default in unparsers. The first one found is the
+                # canonical one.
+                if a.matcher is None:
+                    a.matcher = m
 
     def add_spacing(
         self,

testsuite/python_support/lexer_example.lkt

@@ -24,7 +24,7 @@ lexer foo_lexer {
         r_par <- ")"
         l_brace <- "{"
         r_brace <- "}"
-        equal <- "="
+        equal <- or("=" | ":=")
         plus <- "+"
     }