jax.lax.while_loop#

jax.lax.while_loop(cond_fun, body_fun, init_val)[source]#

Call body_fun repeatedly in a loop while cond_fun is True.

The Haskell-like type signature in brief is

while_loop :: (a -> Bool) -> (a -> a) -> a -> a

The semantics of while_loop are given by this Python implementation:

def while_loop(cond_fun, body_fun, init_val):
  val = init_val
  while cond_fun(val):
    val = body_fun(val)
  return val

Unlike that Python version, while_loop is a JAX primitive and is lowered to a single XLA While HLO. That makes it useful for reducing compilation times for jit-compiled functions, since native Python loop constructs in an @jit function are unrolled, leading to large XLA computations.

Also unlike the Python analogue, the loop-carried value val must hold a fixed shape and dtype across all iterations (and not just be consistent up to NumPy rank/shape broadcasting and dtype promotion rules, for example). In other words, the type a in the type signature above represents an array with a fixed shape and dtype (or a nested tuple/list/dict container data structure with a fixed structure and arrays with fixed shape and dtype at the leaves).

Another difference from using Python-native loop constructs is that while_loop is not reverse-mode differentiable because XLA computations require static bounds on memory requirements.

Note

while_loop() compiles cond_fun and body_fun, so while it can be combined with jit(), it’s usually unnecessary.

Parameters

  • cond_fun (Callable[[TypeVar(T)], Any]) – function of type a -> Bool.

  • body_fun (Callable[[TypeVar(T)], TypeVar(T)]) – function of type a -> a.

  • init_val (TypeVar(T)) – value of type a, a type that can be a scalar, array, or any pytree (nested Python tuple/list/dict) thereof, representing the initial loop carry value.

Return type

TypeVar(T)

Returns

The output from the final iteration of body_fun, of type a.