Add a primop ‘concatLists’

This can serve as a generic efficient list builder.  For instance, the
function ‘catAttrs’ in Nixpkgs can be rewritten from

  attr: l: fold (s: l: if hasAttr attr s then [(getAttr attr s)] ++ l else l) [] l

to

  attr: l: builtins.concatLists (map (s: if hasAttr attr s then [(getAttr attr s)] else []) l)

Statistics before:

  time elapsed: 1.08683
  size of a value: 24
  environments allocated: 1384376 (35809568 bytes)
  list elements: 6946783 (55574264 bytes)
  list concatenations: 37434
  values allocated: 1760440 (42250560 bytes)
  attribute sets allocated: 392040
  right-biased unions: 186334
  values copied in right-biased unions: 591137
  symbols in symbol table: 18273
  number of thunks: 1297673
  number of thunks avoided: 1380759
  number of attr lookups: 430802
  number of primop calls: 628912
  number of function calls: 1333544

Statistics after (including new catAttrs):

  time elapsed: 0.959854
  size of a value: 24
  environments allocated: 1010198 (26829296 bytes)
  list elements: 1984878 (15879024 bytes)
  list concatenations: 30488
  values allocated: 1589760 (38154240 bytes)
  attribute sets allocated: 392040
  right-biased unions: 186334
  values copied in right-biased unions: 591137
  symbols in symbol table: 18274
  number of thunks: 1040925
  number of thunks avoided: 1038428
  number of attr lookups: 438419
  number of primop calls: 474844
  number of function calls: 959366

1 files changed, 10 insertions, 0 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index aa27df6ada11..f5c004044135 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -931,6 +931,7 @@ static void prim_filter(EvalState & state, Value * * args, Value & v)
 }
 
 
+/* Return true if a list contains a given element. */
 static void prim_elem(EvalState & state, Value * * args, Value & v)
 {
     bool res = false;
@@ -944,6 +945,14 @@ static void prim_elem(EvalState & state, Value * * args, Value & v)
 }
 
 
+/* Concatenate a list of lists. */
+static void prim_concatLists(EvalState & state, Value * * args, Value & v)
+{
+    state.forceList(*args[0]);
+    state.concatLists(v, args[0]->list.length, args[0]->list.elems);
+}
+
+
 /* Return the length of a list.  This is an O(1) time operation. */
 static void prim_length(EvalState & state, Value * * args, Value & v)
 {
@@ -1160,6 +1169,7 @@ void EvalState::createBaseEnv()
     addPrimOp("map", 2, prim_map);
     addPrimOp("__filter", 2, prim_filter);
     addPrimOp("__elem", 2, prim_elem);
+    addPrimOp("__concatLists", 1, prim_concatLists);
     addPrimOp("__length", 1, prim_length);
 
     // Integer arithmetic