[GitHub] junrushao1994 edited a comment on issue #12154: the cond operator doesn't support a function that doesn't require inputs.

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junrushao1994 edited a comment on issue #12154: the cond operator doesn't support a function that doesn't require inputs.
URL: https://github.com/apache/incubator-mxnet/issues/12154#issuecomment-412925662
 
 
   @zheng-da Thank you so much for providing this example and very detailed explanation!
   
   To be clear, please allow me to summarize the problem as follows: currently, the implementation of three control flow operators, `foreach`, `cond` and `while_loop`, requires every subgraph to have at least one input.
   
   To my understanding, this is actually a restriction of `CachedOp`: our implementation simply creates a `CachedOp` for each subgraph, but `CachedOp` refuses to create because it requires at least one input.  This is reflected in the stack trace information you provide:
   ```
   [bt] (2) 2   libmxnet.so                         0x000000010673209c mxnet::CachedOp::CachedOp(nnvm::Symbol const&, std::__1::vector<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::allocator<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > > const&) + 3164
   [bt] (3) 3   libmxnet.so                         0x00000001068d6865 mxnet::op::LoopState::MakeSharedOp(nnvm::Symbol const&) + 453
   [bt] (4) 4   libmxnet.so                         0x0000000106cbdb74 mxnet::op::LoopState::LoopState(nnvm::Symbol const&) + 164
   ```
   
   I guess it will be great if it could be fixed in ongoing work #11641.

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