allow keyword arguments in bind.

docs/user-guide/overview.md

@@ -111,4 +111,25 @@ end
 # fc1_output => (10,64)
 ```
 
+## Binding and Executing
+
+In order to execute the computation graph specified a composed symbol, we will *bind* the free variables to concrete values, specified as `mx.NDArray`s. This will create an `mx.Executor` on a given `mx.Context`. A context describes the computation devices (CPUs, GPUs, etc.) and an executor will carry out the computation (forward/backward) specified in the corresponding symbolic composition.
+```julia
+A = mx.variable(:A)
+B = mx.variable(:B)
+C = A .* B
+a = mx.ones(3) * 4
+b = mx.ones(3) * 2
+c_exec = mx.bind(C, context=mx.cpu(), args=Dict(:A => a, :B => b))
+
+mx.forward(c_exec)
+copy(c_exec.outputs[1])  # copy turns NDArray into Julia Array
+# =>
+# 3-element Array{Float32,1}:
+#  8.0
+#  8.0
+#  8.0
+```
+**TODO** Provide pointers to further details.
+
 # Low Level Interface

src/executor.jl

@@ -48,27 +48,26 @@ function _get_ndarray_inputs(arg_key::AbstractString, args::Dict{Base.Symbol,NDA
     end
     arr
   end
+  # help the type inference
+  if allow_missing
+    args_vec = Union{NDArray,Void}[args_vec...]
+  else
+    args_vec = NDArray[args_vec...]
+  end
   args_hdr = MX_handle[(isa(x,Void) ? MX_handle(0) : x) for x in args_vec]
   return (args_hdr, args_vec)
 end
 
 @enum GRAD_REQ GRAD_NOP=0 GRAD_WRITE=1 GRAD_ADD=3
 function bind(self :: Symbol, ctx :: Context, args :: Union{Vector{NDArray},Dict{Base.Symbol,NDArray}};
-              args_grad  :: Union{Void,Vector{NDArray},Dict{Base.Symbol,NDArray}} = nothing,
-              aux_states :: Union{Void,Vector{NDArray},Dict{Base.Symbol,NDArray}} = nothing,
+              args_grad  :: Union{Vector{NDArray},Dict{Base.Symbol,NDArray}} = Dict{Base.Symbol,NDArray}(),
+              aux_states :: Union{Vector{NDArray},Dict{Base.Symbol,NDArray}} = Dict{Base.Symbol,NDArray}(),
               grad_req   :: Union{GRAD_REQ,Vector{GRAD_REQ},Dict{Base.Symbol,GRAD_REQ}} = GRAD_WRITE)
 
   arg_names = list_arguments(self)
 
-  args_hdr, args = _get_ndarray_inputs("args", args, arg_names, false)
-  if isa(args_grad, Void)
-    args_grad = [nothing for i=1:length(args)]
-    args_grad_hdr = MX_handle[Ptr{Void}(0) for i=1:length(args)]
-  else
-    args_grad_hdr, args_grad = _get_ndarray_inputs("args_grad", args_grad, arg_names, true)
-  end
-
-  if isa(aux_states, Void); aux_states = NDArray[]; end
+  args_hdr, args           = _get_ndarray_inputs("args", args, arg_names, false)
+  args_grad_hdr, args_grad = _get_ndarray_inputs("args_grad", args_grad, arg_names, true)
   aux_args_hdr, aux_states = _get_ndarray_inputs("aux_states", aux_states, list_auxiliary_states(self), false)
 
   if isa(grad_req, GRAD_REQ)
@@ -90,6 +89,17 @@ function bind(self :: Symbol, ctx :: Context, args :: Union{Vector{NDArray},Dict
   executor = Executor(MX_ExecutorHandle(ref_hdr[]), self,
                       args, args_grad, aux_states)
 end
+function bind(self :: Symbol; kwargs...)
+  kwargs = Dict(kwargs)
+  @assert(haskey(kwargs, :args), "Must specify args")
+  args = pop!(kwargs, :args)
+  if haskey(kwargs, :context)
+    context = pop!(kwargs, :context)
+  else
+    context = cpu()
+  end
+  bind(self, context, args; kwargs...)
+end
 
 function simple_bind(self :: Symbol, ctx :: Context; grad_req :: GRAD_REQ=GRAD_WRITE, kwargs...)
   arg_shapes, out_shapes, aux_shapes = infer_shape(self; kwargs...)