* When something goes wrong in the evaluation of a Nix expression,

  print a nice backtrace of the stack, rather than vomiting a gigantic
  (and useless) aterm on the screen.  Example:

    error: while evaluating file `.../pkgs/system/test.nix':
    while evaluating attribute `subversion' at `.../pkgs/system/all-packages-generic.nix', line 533:
    while evaluating function at `.../pkgs/applications/version-management/subversion/default.nix', line 1:
    assertion failed at `.../pkgs/applications/version-management/subversion/default.nix', line 13

  Since the Nix expression language is lazy, the trace may be
  misleading.  The purpose is to provide a hint as to the location of
  the problem.    
  

1 files changed, 5 insertions, 1 deletions

diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 924f8b912f77..ecd2534088f6 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -52,6 +52,9 @@ private:
 ATerm string2ATerm(const string & s);
 string aterm2String(ATerm t);
 
+/* Show a position. */
+string showPos(ATerm pos);
+
 /* Generic bottomup traversal over ATerms.  The traversal first
    recursively descends into subterms, and then applies the given term
    function to the resulting term. */
@@ -63,11 +66,12 @@ ATerm bottomupRewrite(TermFun & f, ATerm e);
 
 /* Query all attributes in an attribute set expression.  The
    expression must be in normal form. */
-void queryAllAttrs(Expr e, ATermMap & attrs);
+void queryAllAttrs(Expr e, ATermMap & attrs, bool withPos = false);
 
 /* Query a specific attribute from an attribute set expression.  The
    expression must be in normal form. */
 Expr queryAttr(Expr e, const string & name);
+Expr queryAttr(Expr e, const string & name, ATerm & pos);
 
 /* Create an attribute set expression from an Attrs value. */
 Expr makeAttrs(const ATermMap & attrs);