This foldl operator repeatedly applies the callable fn to a sequence
of elements from first to last. The elements are made of the tensors
unpacked from elems on dimension 0. The callable fn takes two tensors as
arguments. The first argument is the accumulated value computed from the
preceding invocation of fn, and the second is the value at the current
position of elems. If initializer is None, elems must contain at least
one element, and its first element is used as the initializer.

Suppose that elems is unpacked into values, a list of tensors. The shape
of the result tensor is fn(initializer, values[0]).shape`.

This method also allows multi-arity elems and output of fn. If elems
is a (possibly nested) list or tuple of tensors, then each of these tensors
must have a matching first (unpack) dimension. The signature of fn may
match the structure of elems. That is, if elems is
(t1, [t2, t3, [t4, t5]]), then an appropriate signature for fn is:
fn = lambda (t1, [t2, t3, [t4, t5]]):.

Args

fn

The callable to be performed.

elems

A tensor or (possibly nested) sequence of tensors, each of which will
be unpacked along their first dimension. The nested sequence of the
resulting slices will be the first argument to fn.

initializer

(optional) A tensor or (possibly nested) sequence of tensors,
as the initial value for the accumulator.

parallel_iterations

(optional) The number of iterations allowed to run in
parallel.

back_prop

(optional) Deprecated. False disables support for back
propagation. Prefer using tf.stop_gradient instead.

swap_memory

(optional) True enables GPU-CPU memory swapping.

name

(optional) Name prefix for the returned tensors.

Returns

A tensor or (possibly nested) sequence of tensors, resulting from applying
fn consecutively to the list of tensors unpacked from elems, from first
to last.

Raises

TypeError

if fn is not callable.

Example

elems = tf.constant([1, 2, 3, 4, 5, 6])
sum = tf.foldl(lambda a, x: a + x, elems)
# sum == 21