Add ‘deepSeq’ primop

Note that unlike ‘lib.deepSeq’ in Nixpkgs, this handles cycles.

4 files changed, 14 insertions, 0 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 2da15b3c53eb..a25bd8854e0b 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -392,6 +392,16 @@ void prim_seq(EvalState & state, const Pos & pos, Value * * args, Value & v)
 }
 
 
+/* Evaluate the first argument deeply (i.e. recursing into lists and
+   attrsets), then return the second argument. */
+void prim_deepSeq(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceValueDeep(*args[0]);
+    state.forceValue(*args[1]);
+    v = *args[1];
+}
+
+
 /* Evaluate the first expression and print it on standard error.  Then
    return the second expression.  Useful for debugging. */
 static void prim_trace(EvalState & state, const Pos & pos, Value * * args, Value & v)
@@ -1435,6 +1445,7 @@ void EvalState::createBaseEnv()
 
     // Strictness
     addPrimOp("__seq", 2, prim_seq);
+    addPrimOp("__deepSeq", 2, prim_deepSeq);
 
     // Debugging
     addPrimOp("__trace", 2, prim_trace);
diff --git a/tests/lang/eval-fail-deepseq.nix b/tests/lang/eval-fail-deepseq.nix
new file mode 100644
index 000000000000..9baa49b063ec
--- /dev/null
+++ b/tests/lang/eval-fail-deepseq.nix
@@ -0,0 +1 @@
+builtins.deepSeq { x = abort "foo"; } 456
diff --git a/tests/lang/eval-okay-deepseq.exp b/tests/lang/eval-okay-deepseq.exp
new file mode 100644
index 000000000000..8d38505c1686
--- /dev/null
+++ b/tests/lang/eval-okay-deepseq.exp
@@ -0,0 +1 @@
+456
diff --git a/tests/lang/eval-okay-deepseq.nix b/tests/lang/eval-okay-deepseq.nix
new file mode 100644
index 000000000000..53aa4b1dc251
--- /dev/null
+++ b/tests/lang/eval-okay-deepseq.nix
@@ -0,0 +1 @@
+builtins.deepSeq (let as = { x = 123; y = as; }; in as) 456