1 files changed, 74 insertions, 0 deletions
diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
new file mode 100644
index 000000000..274f49cea
--- /dev/null
+++ b/src/libexpr/attr-path.cc
@@ -0,0 +1,74 @@
+#include "attr-path.hh"
+#include "nixexpr-ast.hh"
+
+
+bool isAttrs(EvalState & state, Expr e, ATermMap & attrs)
+{
+    e = evalExpr(state, e);
+    ATermList dummy;
+    if (!matchAttrs(e, dummy)) return false;
+    queryAllAttrs(e, attrs, false);
+    return true;
+}
+
+
+Expr findAlongAttrPath(EvalState & state, const string & attrPath, Expr e)
+{
+    Strings tokens = tokenizeString(attrPath, ".");
+
+    Error attrError =
+        Error(format("attribute selection path `%1%' does not match expression") % attrPath);
+
+    string curPath;
+    
+    for (Strings::iterator i = tokens.begin(); i != tokens.end(); ++i) {
+
+        if (!curPath.empty()) curPath += ".";
+        curPath += *i;
+
+        /* Is *i an index (integer) or a normal attribute name? */
+        enum { apAttr, apIndex } apType = apAttr;
+        string attr = *i;
+        int attrIndex = -1;
+        if (string2Int(attr, attrIndex)) apType = apIndex;
+
+        /* Evaluate the expression. */
+        e = evalExpr(state, autoCallFunction(evalExpr(state, e)));
+
+        /* It should evaluate to either an attribute set or an
+           expression, according to what is specified in the
+           attrPath. */
+
+        if (apType == apAttr) {
+
+            ATermMap attrs(128);
+
+            if (!isAttrs(state, e, attrs))
+                throw TypeError(
+                    format("the expression selected by the selection path `%1%' should be an attribute set but is %2%")
+                    % curPath % showType(e));
+                
+            e = attrs.get(toATerm(attr));
+            if (!e)
+                throw Error(format("attribute `%1%' in selection path `%2%' not found") % attr % curPath);
+
+        }
+
+        else if (apType == apIndex) {
+
+            ATermList es;
+            if (!matchList(e, es))
+                throw TypeError(
+                    format("the expression selected by the selection path `%1%' should be a list but is %2%")
+                    % curPath % showType(e));
+
+            e = ATelementAt(es, attrIndex);
+            if (!e)
+                throw Error(format("list index %1% in selection path `%2%' not found") % attrIndex % curPath);
+            
+        }
+        
+    }
+    
+    return e;
+}