back_propagation.jl

mutable struct Variable
    data 
    grad
    creator 
end

mutable struct Func    
    input :: Variable
    output :: Variable
    forward :: Function
    backward :: Function
    name
end


function Base.println(x::Variable,debug=false)
    print("Variable(")
    print("data:",x.data)
    if debug
        println()
        (x.grad != nothing) && println("grad:",x.grad)
        (x.creator != nothing) && println("creator:",x.creator.name)
    end
    println(")")
end

function Base.println(x::Func)
    println("Func[")
    print("input: ")
    println(x.input)
    print("output: ")
    println(x.output)
    if x.name != nothing
        println("function:",x.name)
    end
    println("]")
end



function set_func(functions::Array{Function},name=nothing)
    none_variable = Variable(nothing,nothing,nothing) 
    return Func(none_variable,none_variable,functions[1],functions[2],name)
end

function set_var(data,grad=nothing,cretor=nothing)
    return Variable(data,grad,cretor)
end

square_forward(x) =  x ^ 2

square_backward(x) = 2x

exp_forward(x) = exp(x)

exp_backward(x) = exp(x)



Square = [square_forward, square_backward] 
Exp = [exp_forward, exp_backward]



function Base.run(f::Func,input::Variable)
    x = input.data 
    f.input = input 
    y = f.forward(x)
    output = Variable(y,nothing,f)
    f.output = output
    return  output
end


function backward!(var::Variable)
    f = var.creator
    if f != nothing
        x = f.input
        x.grad = f.backward(f.input.data) * var.grad
        backward!(x)
    end
end




a = set_var(0.5)
A = set_func(Square,"A")
B = set_func(Exp,"B")
C = set_func(Square,"C")

b = run(A,a)
c = run(B,b)
y = run(C,c)


y.grad = 1
backward!(y)
println(a.grad)