TOML: using to_toml option may produce incorrect TOML

[stephenll]

Here is is a simplified example to illustrate the issue I'm seeing with using the TOML writer with the to_toml option for writing a custom type. It seems that when using the to_toml option, TOML print doesn't print the correct TOML.

I expect the hello = "world" to not be combined with the [b] table.

I'm using Julia 1.8 on both a M1 Mac (arm64) and an Intel Mac.

Just looking to see if anyone else is seeing this and if my understanding is correct.

using TOML

struct Foo
    a::Int64
    b::Float64
end

struct Bar
    c::Float64
    d::String
end


f = Foo(2,9.9)
b = Bar(1.345, "hello")

dd = Dict("hello"=>"world", "f"=>f,  "b"=>b)

toDict(foo::Foo) = Dict("a"=>foo.a, "b"=>foo.b)
toDict(bar::Bar) = Dict("c"=>bar.c, "d"=>bar.d)

TOML.print(toDict,dd)

which results in the "hello" being added to the b table in the TOML.

[f]
b = 9.9
a = 2

[b]
c = 1.345
d = "hello"
hello = "world"

[KristofferC]

At a glance

julia/stdlib/TOML/src/print.jl

Lines 109 to 114 in 18fa383

	is_tabular(value) && continue
	if !isa(value, TOMLValue)
	if f === nothing
	error("type `$(typeof(value))` is not a valid TOML type, pass a conversion function to `TOML.print`")
	end
	toml_value = f(value)

the f(value) conversion comes after the is_tabular(value) which seems like could cause something like this

[vtjnash]

Added in #41009 (@aviatesk). Looks like the added logic didn't follow the comment on the loop here that is_tabular data shouldn't be printed in this pass

[stephenll]

If anyone is following and needs a temporary fix... I think I found a work around via custom sorting. First, create a vector of strings of fields that are non-tabular. Next, create a function to see if x is not in the list. We want false assigned to those in the list, so when it comes to sorting, those are forced first since they are effectively 0. Here is a snippet:

list = ["hello"]
TOML.print(toDict, dd, sorted=true, by = x->x ∉ list)

hello = "world"

[f]
b = 9.9
a = 2

[b]
c = 1.345
d = "hello"

So far it works on the few test cases I tried.

[KristofferC]

Can be closed when TOML is updated in the Julia repo.