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Callable taking subset of TFF AST constructs to CompiledComputations.
tff.framework.TFParser()
When this function is applied via transformation_utils.transform_postorder
to a TFF AST node satisfying its assumptions, the tree under this node will
be reduced to a single instance of building_blocks.CompiledComputation
representing the same logic.
Notice that this function is designed to be applied to what is essentially
a subtree of a larger TFF AST; once the processing on a single device has
been aligned at the AST level, and placement separated from the logic of
this processing, we should be left with a function wrapped via
federated_map
or federated_apply
to a federated argument. It is this
function which we need to reduce to TensorFlow, and it is to the root
node of this function which we are looking to apply TFParser
. Because of
this, we assume that there is a lambda expression at the top of the AST
we are looking to parse, as well as the rest of the assumptions below.
 All called lambdas have been converted to blocks.
 All blocks have been inlined; that is, there are no block/LET constructs remaining.
 All compiled computations are called.
 No compiled computations have been partially called; we believe this should be handled correctly today but we haven't reasoned explicitly about this possibility.
 The only leaf nodes present under
comp
are compiled computations and references to the argument of the toplevel lambda which we are hoping to replace with a compiled computation. Further, every leaf node which is a reference has as its parent abuilding_blocks.Call
, whose associated function is a TF graph. This prevents us from needing to deal with arbitrary nesting of references and TF graphs, and significantly clarifies the reasoning. This can be accomplished by "decorating" the appropriate leaves with called identity TF graphs, the construction of which is provided by a utility module.  There is only a single lambda binding any references present in the AST,
and it is placed at the root of the AST to which we apply
TFParser
.  There are no intrinsics present in the AST.
Methods
__call__
__call__(
comp
)
Transforms comp
by checking all elements of the parser library.
This function is roughly performing intermediatecode generation, taking TFF and generating TF. Calling this function is essentially checking the stack and selecting a semantic action based on its contents, and only one of these actions should be selected for a given computation.
Notice that since the parser library contains mutually exclusive options, it is safe to return early.
Args  

comp

The building_blocks.ComputationBuildingBlock to check for
possibility of reduction according to the parsing library.

Returns  

A tuple whose first element is a possibly transformed version of comp ,
and whose second is a Boolean indicating whether or not comp was
transformed. This is in conforming to the conventions of
transformation_utils.transform_postorder .
