Create Magic macro

- Contributed by @mcabbot in FluxML/Flux.jl#2107

src/Fluxperimental.jl

@@ -8,4 +8,7 @@ export Split, Join
 include("train.jl")
 export shinkansen!
 
+include("magic.jl")
+export @Magic
+
 end # module Fluxperimental

src/magic.jl

@@ -0,0 +1,114 @@
+"""
+    @Magic(forward::Function; construct...)
+
+Creates a layer by specifying some code to construct the layer, run immediately,
+and (usually as a `do` block) a function for the forward pass.
+You may think of `construct` as keywords, or better as a `let` block creating local variables.
+Their names may be used within the body of the `forward` function.
+
+Here is a linear model:
+```
+r = @Magic(w = rand(3)) do x
+  w .* x
+end
+r([1, 1, 1])  # x is set to [1, 1, 1].
+```
+
+Here is a linear model with bias and activation:
+```
+d = @Magic(in=5, out=7, W=randn(out, in), b=zeros(out), act=relu) do x
+  y = W * x
+  act.(y .+ b)
+end
+d(ones(5, 10))  # 7×10 Matrix as output.
+```
+
+Finally, here is a simple MLP:
+```
+using Flux
+
+n_in = 1
+n_out = 1
+nlayers = 3
+
+model = @Magic(
+  w1=Dense(n_in, 128),
+  w2=[Dense(128, 128) for i=1:nlayers],
+  w3=Dense(128, n_out),
+  act=relu
+) do x
+  embed = act(w1(x))
+  for w in w2
+    embed = act(w(embed))
+  end
+  out = w3(embed)
+  return out
+end
+
+model(randn(n_in, 32))  # 1×32 Matrix as output.
+```
+
+We can train this model just like any `Chain`:
+```
+data = [([x], 2x-x^3) for x in -2:0.1f0:2]
+optim = Flux.setup(Adam(), model)
+
+for epoch in 1:1000
+  Flux.train!((m,x,y) -> (m(x) - y)^2, model, data, optim)
+end
+```
+"""
+macro Magic(fex, kwexs...)
+  # check input
+  Meta.isexpr(fex, :(->)) || error("expects a do block")
+  isempty(kwexs) && error("expects keyword arguments")
+  all(ex -> Meta.isexpr(ex, :kw), kwexs) || error("expects only keyword argumens")
+
+  # make strings
+  layer = "@Magic"
+  setup = join(map(ex -> string(ex.args[1], " = ", ex.args[2]), kwexs), ", ")
+  input = join(fex.args[1].args, ", ")
+  block = string(Base.remove_linenums!(fex).args[2])
+
+  # edit expressions
+  vars = map(ex -> ex.args[1], kwexs)
+  assigns = map(ex -> Expr(:(=), ex.args...), kwexs)
+  @gensym self
+  pushfirst!(fex.args[1].args, self)
+  addprefix!(fex, self, vars)
+
+  # assemble
+  return esc(quote
+    let
+      $(assigns...)
+      $MagicLayer($fex, ($layer, $setup, $input, $block); $(vars...))
+    end
+  end)
+end
+
+function addprefix!(ex::Expr, self, vars)
+  for i = 1:length(ex.args)
+    if ex.args[i] in vars
+      ex.args[i] = :($self.$(ex.args[i]))
+    else
+      addprefix!(ex.args[i], self, vars)
+    end
+  end
+end
+addprefix!(not_ex, self, vars) = nothing
+
+struct MagicLayer{F,NT<:NamedTuple}
+  fun::F
+  strings::NTuple{4,String}
+  variables::NT
+end
+MagicLayer(f::Function, str::Tuple; kw...) = MagicLayer(f, str, NamedTuple(kw))
+(m::MagicLayer)(x...) = m.fun(m.variables, x...)
+MagicLayer(args...) = error("MagicLayer is meant to be constructed by the macro")
+Flux.@functor MagicLayer
+
+function Base.show(io::IO, m::MagicLayer)
+  layer, setup, input, block = m.strings
+  print(io, layer, "(", setup, ") do ", input)
+  return print(io, block[6:end])
+end