fix: add a workaround for redeclaration

5 files changed, 35 insertions, 0 deletions

diff --git a/ast.h b/ast.h
index d17eaef..aaa5f7a 100644
--- a/ast.h
+++ b/ast.h
@@ -32,6 +32,7 @@ enum NodeKind
     COND_EQUAL,
     COND_LESS,
     STMT_DECLARE,
+    STMT_UNDECLARE,
     STMT_EMIT,
     STMT_STATIC_ASSERT
 };
diff --git a/eval.c b/eval.c
index fc6e6d7..cb967c2 100644
--- a/eval.c
+++ b/eval.c
@@ -222,6 +222,19 @@ static enum RunStatus declare(struct Machine* machine, const struct Node* node)
     return RUN_DONE;
 }
 
+static enum RunStatus undeclare(struct Machine* machine, const struct Node* node)
+{
+    struct Variable* variable = find_variable(machine, node->name);
+    if (variable == NULL)
+    {
+        (void)complain(node, "cannot undeclare what was never declared");
+        return RUN_FAILED;
+    }
+
+    variable->declared = false;
+    return RUN_DONE;
+}
+
 static enum RunStatus emit(struct Machine* machine, const struct Node* node)
 {
     struct Type type;
@@ -261,6 +274,7 @@ static enum RunStatus execute_one(struct Machine* machine, const struct Node* no
     switch (node->kind)
     {
         case STMT_DECLARE: return declare(machine, node);
+        case STMT_UNDECLARE: return undeclare(machine, node);
         case STMT_EMIT: return emit(machine, node);
         case STMT_STATIC_ASSERT: return static_assert_statement(machine, node);
         default: break;
diff --git a/examples/reassign.sfinae b/examples/reassign.sfinae
new file mode 100644
index 0000000..9863cae
--- /dev/null
+++ b/examples/reassign.sfinae
@@ -0,0 +1,16 @@
+// there is no assignment. to change x, undeclare it and declare it again
+// if the new value depends on the old one, park it in a second variable first,
+// because the old one is gone by the time you declare
+
+::declare<::name<'x'>, ::decltype(::difference<'F', 'A'>)>::commit;
+::emit<::decltype(::value_of<::name<'x'>>)>::commit;
+
+::declare<
+    ::name<'n', 'e', 'x', 't'>,
+    ::decltype(::sum<::value_of<::name<'x'>>, ::difference<'B', 'A'>>)
+>::commit;
+::undeclare<::name<'x'>>::commit;
+::declare<::name<'x'>, ::decltype(::value_of<::name<'n', 'e', 'x', 't'>>)>::commit;
+::undeclare<::name<'n', 'e', 'x', 't'>>::commit;
+
+::emit<::decltype(::value_of<::name<'x'>>)>::commit;
diff --git a/sfinae.l b/sfinae.l
index 7c745ef..17bf28e 100644
--- a/sfinae.l
+++ b/sfinae.l
@@ -36,6 +36,7 @@ static int fail(const char* message)
 "static_assert"         return TOK_STATIC_ASSERT;
 "commit"                return TOK_COMMIT;
 "declare"               return TOK_DECLARE;
+"undeclare"             return TOK_UNDECLARE;
 "name"                  return TOK_NAME;
 "value_of"              return TOK_VALUE_OF;
 "void"                  return TOK_VOID;
diff --git a/sfinae.y b/sfinae.y
index e142aaa..1acb4d0 100644
--- a/sfinae.y
+++ b/sfinae.y
@@ -22,6 +22,7 @@ void yyerror(const char* message);
 %token TOK_STATIC_ASSERT
 %token TOK_COMMIT
 %token TOK_DECLARE
+%token TOK_UNDECLARE
 %token TOK_NAME
 %token TOK_VALUE_OF
 %token TOK_VOID
@@ -72,6 +73,8 @@ statement
         { $$ = node_unary(STMT_STATIC_ASSERT, $4); }
     | TOK_COLONCOLON TOK_DECLARE '<' name ',' type '>' TOK_COLONCOLON TOK_COMMIT
         { $$ = node_named(STMT_DECLARE, &$4, $6); }
+    | TOK_COLONCOLON TOK_UNDECLARE '<' name '>' TOK_COLONCOLON TOK_COMMIT
+        { $$ = node_named(STMT_UNDECLARE, &$4, NULL); }
     ;
 
 name