Fix issue #111 - nested map calls

source/mir/ndslice/iterator.d

@@ -630,11 +630,46 @@ Creates a mapped iterator. Uses `__map` if possible.
 auto mapIterator(alias fun, Iterator)(Iterator iterator)
 {
  static if (__traits(hasMember, Iterator, "__map"))
- return Iterator.__map!fun(iterator);
+ {
+ static if (is(Iterator : MapIterator!(Iter0, fun0), Iter0, alias fun0)
+ && !__traits(compiles, Iterator.__map!fun(iterator)))
+ {
+ // https://github.com/libmir/mir-algorithm/issues/111
+ pragma(msg, __FUNCTION__~" not coalescing chained map calls into a single lambda, possibly because of multiple embedded context pointers");
+ return MapIterator!(Iterator, fun)(iterator);
+ }
+ else
+ return Iterator.__map!fun(iterator);
+ }
  else
  return MapIterator!(Iterator, fun)(iterator);
 }
 
+@safe pure nothrow @nogc version(mir_test) unittest
+{
+ // https://github.com/libmir/mir-algorithm/issues/111
+ import mir.ndslice.topology : iota, map;
+ import mir.functional : pipe;
+
+ static auto foo(T)(T x)
+ {
+ return x.map!(a => a + 1);
+ }
+
+ static auto bar(T)(T x)
+ {
+ return foo(x).map!(a => a + 2);
+ }
+
+ auto data = iota(5);
+ auto result = iota([5], 3);
+
+ auto x = data.map!(a => a + 1).map!(a => a + 2);
+ assert(x == result);
+
+ auto y = bar(data);
+ assert(y == result);
+}
 
 /++
 `BytegroupIterator` is used by $(SUBREF topology, Bytegroup) and $(SUBREF topology, bytegroup).