Why not define this function in a normal way?

[hurricane007]

Hello guys,
I just started using SimpleChains.jl.
For me, Julia is a tool and I didn't learn the language very carefully so I don't know all the syntaxes. It's a little bit hard for me to understand the function below, which is found in the first example of SimpleChains.jl. I think there must be a lot of users like me who don't have sophisticated knowledge of Julia.
So could the authors write documentation and examples in a simpler/common way?
Thanks!

report = let mtrain = mlpdloss, X=X, Xtest=Xtest, mtest = mlpdtest
  p -> begin
    let train = mlpdloss(X, p), test = mlpdtest(Xtest, p)
      @info "Loss:" train test
    end
  end
end

[chriselrod]

Julia has bad semantics around capturing variables in closures, causing frequent bugs like this

julia> x = rand(2,3);

julia> f = y -> y + x
#3 (generic function with 1 method)

julia> a = rand(2,3);

julia> f(a)
2×3 Matrix{Float64}:
 1.39317   1.18612  1.0031
 0.303721  1.17959  0.689349

julia> x = 4 # I'm now doing something else, so redefining x
4

julia> f(a) # let me call f again... oops!
ERROR: MethodError: no method matching +(::Matrix{Float64}, ::Int64)
For element-wise addition, use broadcasting with dot syntax: array .+ scalar

Closest candidates are:
  +(::Any, ::Any, ::Any, ::Any...)
   @ Base operators.jl:585
  +(::T, ::T) where T<:Union{Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8}
   @ Base int.jl:87
  +(::Rational, ::Integer)
   @ Base rational.jl:332
  ...

Stacktrace:
 [1] (::var"#3#4")(y::Matrix{Float64})
   @ Main ./REPL[2]:1
 [2] top-level scope
   @ REPL[6]:1

Using let in this way lets us avoid the problem:

julia> x = rand(2,3);

julia> f = let x = x; y -> y + x; end
#5 (generic function with 1 method)

julia> a = rand(2,3);

julia> f(a)
2×3 Matrix{Float64}:
 0.600399  1.78474  1.07068
 0.416237  1.025    0.708488

julia> x = 4
4

julia> f(a) # f still works!
2×3 Matrix{Float64}:
 0.600399  1.78474  1.07068
 0.416237  1.025    0.708488

[hurricane007]

If I understand correctly, report = let mtrain = mlpdloss, X=X, Xtest=Xtest, mtest = mlpdtest uses mtrain = mlpdloss to avoid bugs when mlpdloss is redefined. I am confused as mtrain is not used in the function.
Or do I understand it wrong?

[chriselrod]

I am confused as mtrain is not used in the function.
Or do I understand it wrong?

Ha ha, no, you understand it correctly. That is a bug.