libexpr: add findDerivationFilename

extract the derivation to filename:lineno heuristic

src/libexpr/attr-path.cc

@@ -93,4 +93,32 @@ Value * findAlongAttrPath(EvalState & state, const string & attrPath,
 }
 
 
+std::tuple<std::string, int> findDerivationFilename(EvalState & state, Value & v, std::string what)
+{
+    Value * v2;
+    try {
+        auto dummyArgs = state.allocBindings(0);
+        v2 = findAlongAttrPath(state, "meta.position", *dummyArgs, v);
+    } catch (Error &) {
+        throw Error("package '%s' has no source location information", what);
+    }
+
+    auto pos = state.forceString(*v2);
+
+    auto colon = pos.rfind(':');
+    if (colon == std::string::npos)
+        throw Error("cannot parse meta.position attribute '%s'", pos);
+
+    std::string filename(pos, 0, colon);
+    int lineno;
+    try {
+        lineno = std::stoi(std::string(pos, colon + 1));
+    } catch (std::invalid_argument & e) {
+        throw Error("cannot parse line number '%s'", pos);
+    }
+
+    return std::make_tuple(filename, lineno);
+}
+
+
 }

src/libexpr/attr-path.hh

@@ -4,10 +4,15 @@
 
 #include <string>
 #include <map>
+#include <tuple>
 
 namespace nix {
 
 Value * findAlongAttrPath(EvalState & state, const string & attrPath,
     Bindings & autoArgs, Value & vIn);
 
+/* Heuristic to find the filename and lineno or a derivation. */
+std::tuple<std::string, int> findDerivationFilename(EvalState & state,
+    Value & v, std::string what);
+
 }

src/nix/edit.cc

@@ -36,28 +36,9 @@ struct CmdEdit : InstallableCommand
 
         auto v = installable->toValue(*state);
 
-        Value * v2;
+        std::string filename;
-        try {
-            auto dummyArgs = state->allocBindings(0);
-            v2 = findAlongAttrPath(*state, "meta.position", *dummyArgs, *v);
-        } catch (Error &) {
-            throw Error("package '%s' has no source location information", installable->what());
-        }
-
-        auto pos = state->forceString(*v2);
-        debug("position is %s", pos);
-
-        auto colon = pos.rfind(':');
-        if (colon == std::string::npos)
-            throw Error("cannot parse meta.position attribute '%s'", pos);
-
-        std::string filename(pos, 0, colon);
         int lineno;
-        try {
+        std::tie(filename, lineno) = findDerivationFilename(*state, *v, installable->what());
-            lineno = std::stoi(std::string(pos, colon + 1));
-        } catch (std::invalid_argument & e) {
-            throw Error("cannot parse line number '%s'", pos);
-        }
 
         stopProgressBar();
 

src/nix/repl.cc

@@ -481,28 +481,7 @@ bool NixRepl::processLine(string line)
             lineno = 0;
         } else {
             // assume it's a derivation
-            Value * v2;
+            std::tie(filename, lineno) = findDerivationFilename(state, v, arg);
-            try {
-                auto dummyArgs = state.allocBindings(0);
-                v2 = findAlongAttrPath(state, "meta.position", *dummyArgs, v);
-            } catch (Error &) {
-                throw Error("package '%s' has no source location information", arg);
-            }
-
-            auto pos = state.forceString(*v2);
-            debug("position is %s", pos);
-
-            auto colon = pos.rfind(':');
-            if (colon == std::string::npos)
-                throw Error("cannot parse meta.position attribute '%s'", pos);
-
-            filename = std::string(pos, 0, colon);
-            try {
-                lineno = std::stoi(std::string(pos, colon + 1));
-            } catch (std::invalid_argument & e) {
-                throw Error("cannot parse line number '%s'", pos);
-            }
-
         }
 
         // Open in EDITOR