3 files changed, 20 insertions, 0 deletions
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 02df4a4a325a..3334e4bbded8 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -176,6 +176,16 @@ Path evalPath(EvalState & state, Expr e)
 }
 
 
+int evalInt(EvalState & state, Expr e)
+{
+    e = evalExpr(state, e);
+    int i;
+    if (!matchInt(e, i))
+        throw TypeError(format("value is %1% while an integer was expected") % showType(e));
+    return i;
+}
+
+
 bool evalBool(EvalState & state, Expr e)
 {
     e = evalExpr(state, e);
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index b34e91055a37..a7f4e69437be 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -62,6 +62,7 @@ Expr strictEvalExpr(EvalState & state, Expr e,
 /* Specific results. */
 string evalString(EvalState & state, Expr e);
 Path evalPath(EvalState & state, Expr e);
+int evalInt(EvalState & state, Expr e);
 bool evalBool(EvalState & state, Expr e);
 ATermList evalList(EvalState & state, Expr e);
 ATerm coerceToString(Expr e);
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 190d587330d1..1739b6656d45 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -771,6 +771,14 @@ static Expr primRelativise(EvalState & state, const ATermVector & args)
 }
 
 
+static Expr primAdd(EvalState & state, const ATermVector & args)
+{
+    int i1 = evalInt(state, args[0]);
+    int i2 = evalInt(state, args[1]);
+    return makeInt(i1 + i2);
+}
+
+
 void EvalState::addPrimOps()
 {
     addPrimOp("builtins", 0, primBuiltins);
@@ -801,6 +809,7 @@ void EvalState::addPrimOps()
     addPrimOp("__hasAttr", 2, primHasAttr);
     addPrimOp("removeAttrs", 2, primRemoveAttrs);
     addPrimOp("relativise", 2, primRelativise);
+    addPrimOp("__add", 2, primAdd);
 }