From 61af14a9219bff090b238f15c9def5453087aa5d Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Thu, 23 Jul 2015 17:03:02 +0200
Subject: [PATCH] Add foldl' primop

---
 doc/manual/expressions/builtins.xml | 13 +++++++++++++
 src/libexpr/primops.cc              | 24 ++++++++++++++++++++++++
 tests/lang/lib.nix                  |  2 +-
 3 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/doc/manual/expressions/builtins.xml b/doc/manual/expressions/builtins.xml
index 6bdfdd55c..0b11e2f5a 100644
--- a/doc/manual/expressions/builtins.xml
+++ b/doc/manual/expressions/builtins.xml
@@ -313,6 +313,19 @@ stdenv.mkDerivation {
   </varlistentry>
 
 
+  <varlistentry><term><function>builtins.foldl’</function> 
+    <replaceable>op</replaceable> <replaceable>nul</replaceable> <replaceable>list</replaceable></term>
+
+    <listitem><para>Reduce a list by applying a binary operator, from
+    left to right, e.g. <literal>foldl’ op nul [x0 x1 x2 ...] = op (op
+    (op nul x0) x1) x2) ...</literal>. The operator is applied
+    strictly, i.e., its arguments are evaluated first. For example,
+    <literal>foldl’ (x: y: x + y) 0 [1 2 3]</literal> evaluates to
+    6.</para></listitem>
+
+  </varlistentry>
+
+
   <varlistentry><term><function>builtins.fromJSON</function> <replaceable>e</replaceable></term>
 
     <listitem><para>Convert a JSON string to a Nix
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index c36a68ce0..6a4b5b042 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1304,6 +1304,29 @@ static void prim_length(EvalState & state, const Pos & pos, Value * * args, Valu
 }
 
 
+/* Reduce a list by applying a binary operator, from left to
+   right. The operator is applied strictly. */
+static void prim_foldlStrict(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceFunction(*args[0], pos);
+    state.forceList(*args[2], pos);
+
+    Value * vCur = args[1];
+
+    if (args[2]->list.length)
+        for (unsigned int n = 0; n < args[2]->list.length; ++n) {
+            Value vTmp;
+            state.callFunction(*args[0], *vCur, vTmp, pos);
+            vCur = n == args[2]->list.length - 1 ? &v : state.allocValue();
+            state.callFunction(vTmp, *args[2]->list.elems[n], *vCur, pos);
+        }
+    else
+        v = *vCur;
+
+    state.forceValue(v);
+}
+
+
 /*************************************************************
  * Integer arithmetic
  *************************************************************/
@@ -1641,6 +1664,7 @@ void EvalState::createBaseEnv()
     addPrimOp("__elem", 2, prim_elem);
     addPrimOp("__concatLists", 1, prim_concatLists);
     addPrimOp("__length", 1, prim_length);
+    addPrimOp("__foldl'", 3, prim_foldlStrict);
 
     // Integer arithmetic
     addPrimOp("__add", 2, prim_add);
diff --git a/tests/lang/lib.nix b/tests/lang/lib.nix
index 882005dc1..262cdd7e8 100644
--- a/tests/lang/lib.nix
+++ b/tests/lang/lib.nix
@@ -17,7 +17,7 @@ rec {
     then fold (x: y: (flatten x) ++ y) [] x
     else [x];
 
-  sum = fold (x: y: add x y) 0;
+  sum = foldl' (x: y: add x y) 0;
 
   hasSuffix = ext: fileName:
     let lenFileName = stringLength fileName;