From ce29e3d72ded432b9ac9354ac92c588142de9b89 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Fri, 20 Mar 2020 08:40:59 +0200
Subject: Lexer support for default value assignment (?=)

Note: not yet supported in the parser.
---
 libbuild2/lexer+normal.test.testscript | 36 ++++++++++++++++++++++++++++++++++
 libbuild2/lexer.cxx                    | 23 ++++++++++++++++------
 libbuild2/parser.cxx                   | 10 ++++++++++
 libbuild2/token.cxx                    |  1 +
 libbuild2/token.hxx                    |  1 +
 5 files changed, 65 insertions(+), 6 deletions(-)
 create mode 100644 libbuild2/lexer+normal.test.testscript

diff --git a/libbuild2/lexer+normal.test.testscript b/libbuild2/lexer+normal.test.testscript
new file mode 100644
index 0000000..c9448c3
--- /dev/null
+++ b/libbuild2/lexer+normal.test.testscript
@@ -0,0 +1,36 @@
+# file      : libbuild2/lexer+normal.test.testscript
+# license   : MIT; see accompanying LICENSE file
+
+test.arguments = normal
+
+: assign
+:
+$* <:'x=y' >>EOO
+'x'
+=
+'y'
+EOO
+
+: append
+:
+$* <:'x+=y' >>EOO
+'x'
++=
+'y'
+EOO
+
+: prepend
+:
+$* <:'x=+y' >>EOO
+'x'
+=+
+'y'
+EOO
+
+: default-assign
+:
+$* <:'x?=y' >>EOO
+'x'
+?=
+'y'
+EOO
diff --git a/libbuild2/lexer.cxx b/libbuild2/lexer.cxx
index 9989d1e..5ffac54 100644
--- a/libbuild2/lexer.cxx
+++ b/libbuild2/lexer.cxx
@@ -43,8 +43,8 @@ namespace build2
     case lexer_mode::normal:
       {
         a  = true;
-        s1 = ":<>=+ $(){}#\t\n";
-        s2 = "    =         ";
+        s1 = ":<>=+? $(){}#\t\n";
+        s2 = "    ==         ";
         break;
       }
     case lexer_mode::value:
@@ -182,6 +182,8 @@ namespace build2
     // NOTE: remember to update mode(), next_eval() if adding any new special
     // characters.
 
+    // These are special in all the modes handled by this function.
+    //
     switch (c)
     {
     case '\n':
@@ -248,11 +250,10 @@ namespace build2
       }
     }
 
-    // The following characters are special in the normal, variable, and
+    // The following characters are special in the normal and
     // switch_expressions modes.
     //
     if (m == lexer_mode::normal             ||
-        m == lexer_mode::variable           ||
         m == lexer_mode::switch_expressions ||
         m == lexer_mode::case_patterns)
     {
@@ -262,9 +263,9 @@ namespace build2
       }
     }
 
-    // The following characters are special in the normal and variable modes.
+    // The following characters are special in the normal mode.
     //
-    if (m == lexer_mode::normal || m == lexer_mode::variable)
+    if (m == lexer_mode::normal)
     {
       switch (c)
       {
@@ -285,6 +286,16 @@ namespace build2
             get ();
             return make_token (type::append);
           }
+          break;
+        }
+      case '?':
+        {
+          if (peek () == '=')
+          {
+            get ();
+            return make_token (type::default_assign);
+          }
+          break;
         }
       }
     }
diff --git a/libbuild2/parser.cxx b/libbuild2/parser.cxx
index 91f2d5c..ec5e161 100644
--- a/libbuild2/parser.cxx
+++ b/libbuild2/parser.cxx
@@ -2749,6 +2749,16 @@ namespace build2
   void parser::
   parse_variable (token& t, type& tt, const variable& var, type kind)
   {
+    // @@ TODO: yet unclear what should the logic be here: we could expect
+    //    the called to handle skipping or skip it here. Need to see how
+    //    everything fits.
+    //
+    // Note that here we treat default assignment (?=) the same as normal
+    // assignment expecting the caller to check whether the assignment is
+    // necessary (and skipping evaluating the value altogether otherwise).
+    //
+    assert (kind != type::default_assign);
+
     value rhs (parse_variable_value (t, tt));
 
     value& lhs (
diff --git a/libbuild2/token.cxx b/libbuild2/token.cxx
index 0f9a189..4975a02 100644
--- a/libbuild2/token.cxx
+++ b/libbuild2/token.cxx
@@ -41,6 +41,7 @@ namespace build2
     case token_type::assign:         os << q << '='  << q; break;
     case token_type::prepend:        os << q << "=+" << q; break;
     case token_type::append:         os << q << "+=" << q; break;
+    case token_type::default_assign: os << q << "?=" << q; break;
 
     case token_type::equal:          os << q << "==" << q; break;
     case token_type::not_equal:      os << q << "!=" << q; break;
diff --git a/libbuild2/token.hxx b/libbuild2/token.hxx
index 4cf1f51..e48c088 100644
--- a/libbuild2/token.hxx
+++ b/libbuild2/token.hxx
@@ -53,6 +53,7 @@ namespace build2
       assign,          // =
       prepend,         // =+
       append,          // +=
+      default_assign,  // ?=
 
       equal,           // ==
       not_equal,       // !=
-- 
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