add mx.chain macro

README.md

@@ -6,3 +6,13 @@
 
 
 Julia wrapper of [MXNet](https://github.com/dmlc/mxnet).
+
+```julia
+mlp = @mx.chain mx.Variable(:data)             =>
+  mx.FullyConnected(name=:fc1, num_hidden=128) =>
+  mx.Activation(name=:relu1, act_type=:relu)   =>
+  mx.FullyConnected(name=:fc2, num_hidden=64)  =>
+  mx.Activation(name=:relu2, act_type=:relu)   =>
+  mx.FullyConnected(name=:fc3, num_hidden=10)  =>
+  mx.Softmax(name=:softmax)
+```

examples/mnist/mlp.jl

@@ -1,13 +1,26 @@
 using MXNet
 
+#--------------------------------------------------------------------------------
 # define MLP
-data = mx.Variable(:data)
-fc1  = mx.FullyConnected(data = data, name=:fc1, num_hidden=128)
-act1 = mx.Activation(data = fc1, name=:relu1, act_type=:relu)
-fc2  = mx.FullyConnected(data = act1, name=:fc2, num_hidden=64)
-act2 = mx.Activation(data = fc2, name=:relu2, act_type=:relu)
-fc3  = mx.FullyConnected(data = act2, name=:fc3, num_hidden=10)
-mlp  = mx.Softmax(data = fc3, name=:softmax)
+# the following two ways are equivalent
+
+#-- Option 1: explicit composition
+# data = mx.Variable(:data)
+# fc1  = mx.FullyConnected(data = data, name=:fc1, num_hidden=128)
+# act1 = mx.Activation(data = fc1, name=:relu1, act_type=:relu)
+# fc2  = mx.FullyConnected(data = act1, name=:fc2, num_hidden=64)
+# act2 = mx.Activation(data = fc2, name=:relu2, act_type=:relu)
+# fc3  = mx.FullyConnected(data = act2, name=:fc3, num_hidden=10)
+# mlp  = mx.Softmax(data = fc3, name=:softmax)
+
+#-- Option 2: using the mx.chain macro
+mlp = @mx.chain mx.Variable(:data)             =>
+  mx.FullyConnected(name=:fc1, num_hidden=128) =>
+  mx.Activation(name=:relu1, act_type=:relu)   =>
+  mx.FullyConnected(name=:fc2, num_hidden=64)  =>
+  mx.Activation(name=:relu2, act_type=:relu)   =>
+  mx.FullyConnected(name=:fc3, num_hidden=10)  =>
+  mx.Softmax(name=:softmax)
 
 # data provider
 batch_size = 100

src/symbol.jl

@@ -349,3 +349,32 @@ function _import_atomic_symbol_creators()
     _define_atomic_symbol_creator(creator_hdr)
   end
 end
+
+################################################################################
+# Utility macros to chain up symbols
+################################################################################
+macro chain(layers)
+  exprs = []
+  last_layer = nothing
+  function _chain_layer(layer, last_layer)
+    if isa(last_layer, Void)
+      layer
+    else
+      @assert(isa(layer, Expr) && layer.head == :call, "Do not know how to chain up $layer")
+      return Expr(:call, layer.args[1], last_layer, layer.args[2:end]...)
+    end
+  end
+  while true
+    if layers.head == :(=>)
+      new_layer = gensym()
+      push!(exprs, :($new_layer = $(_chain_layer(layers.args[1], last_layer))))
+      last_layer = new_layer
+      layers = layers.args[2]
+    else
+      push!(exprs, _chain_layer(layers, last_layer))
+      break
+    end
+  end
+  return Expr(:block, exprs...)
+end
+