Design and performance issues with function generator macro

General Usage
4

Btw. here is the final macro in case anyone is interested.

function unpack() end

readtype(io, ::Type{T}) where T<:Union{Integer, AbstractFloat} = ntoh(read(io, T))
readtype(io, ::Type{T}) where T<:AbstractVector{UInt8} = read(io, sizeof(T))


macro io(data)
   struct_name = data.args[2]

   types = []
   parametric_types = []
   for f in data.args[3].args
       isa(f, LineNumberNode) && continue
       isa(f, Symbol) && error("Untyped field")
       Meta.isexpr(f, :(::)) || error("")
       push!(types, f.args[2])
   end

   struct_size = sum([sizeof(eval(t)) for t in types])

   quote
       $(esc(data))  # executing the code to create the actual struct
       Base.sizeof(::Type{$(esc(struct_name))}) = $struct_size

       function $(@__MODULE__).unpack(io, ::Type{$(esc(struct_name))})
           $(esc(struct_name))($([:(readtype(io, $t)) for t in types]...))
       end

       nothing  # supress REPL output
   end
end
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