Saving a struct with an anonymous function type in a type parameter is highly non future-proof

[marius311]

I've got a struct which has an anonymous function as a field, with the type in the struct type parameters. I don't need to serialize the function, I can reconstruct it at load-time, but since its anonymous it might have a new "name" upon reconstructing if the order of anonymous functions defined in the running session / containing module ever changes. This seems to cause JLD2 to error since the type parameters may not be exactly the same when reconstructing. Something like this:

using JLD2

# define struct and custom serialization
struct FunWrap{F}
    f :: F
    function FunWrap()
        f = () -> 1
        new{typeof(f)}(f)
    end
end
JLD2.writeas(::Type{<:FunWrap}) = Tuple{}
JLD2.wconvert(::Type{Tuple{}}, f::FunWrap) = ()
JLD2.rconvert(::Type{<:FunWrap}, ::Tuple{}) = FunWrap()

f = FunWrap()
@save "test.jld2" f
# redefine to create a new name for the anonymous function. this
# simulates making some (even completely unrelated) change to the
# contianing module
struct FunWrap{F} 
    f :: F
    function FunWrap()
        f = () -> 1
        new{typeof(f)}(f)
    end
end
@load "test.jld2" f
# ERROR: TypeError: in typeassert, expected FunWrap{var"#7#8"}, got a value of type FunWrap{var"#11#12"}

Is there any way to get this to work? Doesn't seem like the custom serialization helps here, and neither would typemap.

I should mention the real use-case is saving an Interpolations.jl object, which uses an anonymous function. Its not hard to make a custom serialization which just saves the data and tries to recreate the Interpolation object at load-time, but loading can then fail due to the issue above. Thanks for any help.

[JonasIsensee]

Thank you for this detailed report.

This is indeed a rather difficult question. Implementation and (future-proof) representation within files are one thing. However, even at a conceptual level I'm not sure there is a meaningful solution.

I'm happy to discuss, though. See e.g. #316 #338 #377 as proof that I care about this and that I've struggled ...

[JonasIsensee]

I think, the approach tested here might be the trick:

https://github.com/JuliaIO/JLD2.jl/blob/master/test%2Ftest_files.jl#L211