Reconstruct Types

[stakaz]

Hello, one of the features for using BSON you mention is the backwards compatibility. But I have a very general question about this.

Suppose I save a type (struct) which is designed in a certain way and comes from a package, like this:

struct Foo 
  foo::String 
  bar::Int 
end

After some time the design of this structure changes due to some improvements of the package or whatever...

struct Foo 
  foo::String 
  bar::Int 
  added_feature::Float64
end

Now I will not be able to load my old saved BSON file anymore because I cannot reproduce the right struct in my environment and I cannot recreate it from the saved data because BSON does not load anything.

I do not expect that a reconstructed struct will work properly but at least it would be great if it still would be possible to load it and, e.g., define a conversion method or just extract some data.

[Roboneet]

You could define a conversion method by creating BSON.newstruct!(x::Foo, fs...)

In this example, it would be

function BSON.newstruct!(x::Foo, fs...)
         for (i, f) = enumerate(fs)
                 f = convert(fieldtype(typeof(x),i), f)
                 ccall(:jl_set_nth_field, Nothing, (Any, Csize_t, Any), x, i-1, f)
         end
         # initialise added_feature
         val = 100.0
         index = 3
         ccall(:jl_set_nth_field, Nothing, (Any, Csize_t, Any), x, index-1, val)
         return x
end