# How to use \`TOML.print\` to print a \`Dict\` with keys as different \`MyStruct\` objects?
**URL:** https://discourse.julialang.org/t/how-to-use-toml-print-to-print-a-dict-with-keys-as-different-mystruct-objects/100804
**Category:** General Usage
**Tags:** toml, dict
**Created:** [June 25, 2023, 6:22am UTC](https://discourse.julialang.org/t/how-to-use-toml-print-to-print-a-dict-with-keys-as-different-mystruct-objects/100804 "2023-06-25T06:22:22Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@singularitti](https://discourse.julialang.org/u/singularitti)
#### Post date: [June 25, 2023, 6:22am UTC](https://discourse.julialang.org/t/how-to-use-toml-print-to-print-a-dict-with-keys-as-different-mystruct-objects/100804/1 "2023-06-25T06:22:22Z")
I’m currently working with a dictionary whose keys are instances of a custom struct named `MyStruct`. Here is the definition:
```julia
struct MyStruct
a::Int
b::String
end
```
I need to use the `TOML.print` function to print this dictionary in a TOML-compatible format. When I print a `Dict` where `MyStruct` is the value, like the example below, everything works fine:
```julia
TOML.print(Dict("foo" => MyStruct(5, "bar"))) do x
x isa MyStruct && return [x.a, x.b]
error("unhandled type $(typeof(x))")
end
```
However, I’m struggling to adapt this approach for a dictionary where `MyStruct` instances are used as keys. I understand that TOML doesn’t natively support composite types like my custom struct as keys, so I’m looking for a workaround or a best practice to follow.
```julia
dict = Dict(MyStruct(1, "one") => "value1", MyStruct(2, "two") => "value2")
```
```julia-repl
julia> dict = Dict(MyStruct(1, "one") => "value1", MyStruct(2, "two") => "value2")
Dict{MyStruct, String} with 2 entries:
MyStruct(2, "two") => "value2"
MyStruct(1, "one") => "value1"
julia> TOML.print(dict) do x
x isa MyStruct && return Dict("x" => x.x, "y" => x.y)
end
ERROR: MethodError: no method matching String(::MyStruct)
Closest candidates are:
String(::String)
@ Core boot.jl:360
String(::Core.Compiler.LazyString)
@ Core strings/lazy.jl:80
String(::LazyString)
@ Base strings/lazy.jl:80
...
Stacktrace:
[1] print_table(f::var"#7#8", io::Base.TTY, a::Dict{MyStruct, String}, ks::Vector{String}; indent::Int64, first_block::Bool, sorted::Bool, by::Function)
@ TOML.Internals.Printer ~/.asdf/installs/julia/1.9.1/share/julia/stdlib/v1.9/TOML/src/print.jl:152
[2] print_table (repeats 2 times)
@ ~/.asdf/installs/julia/1.9.1/share/julia/stdlib/v1.9/TOML/src/print.jl:131 [inlined]
[3] #print#6
@ ~/.asdf/installs/julia/1.9.1/share/julia/stdlib/v1.9/TOML/src/print.jl:204 [inlined]
[4] print(f::var"#7#8", a::Dict{MyStruct, String}; sorted::Bool, by::Function)
@ TOML.Internals.Printer ~/.asdf/installs/julia/1.9.1/share/julia/stdlib/v1.9/TOML/src/print.jl:205
[5] print(f::var"#7#8", a::Dict{MyStruct, String})
@ TOML.Internals.Printer ~/.asdf/installs/julia/1.9.1/share/julia/stdlib/v1.9/TOML/src/print.jl:205
[6] top-level scope
@ REPL[6]:1
```
Does anyone have suggestions on how I might achieve this? Any help would be appreciated!
---
### Author:  [@haberdashPI](https://discourse.julialang.org/u/haberdashPI)
#### Post date: [June 25, 2023, 4:31pm UTC](https://discourse.julialang.org/t/how-to-use-toml-print-to-print-a-dict-with-keys-as-different-mystruct-objects/100804/2 "2023-06-25T16:31:29Z")
I’m not at a computer right now so can’t type up a complete answer. But you can do this pretty easily by passing a function as the first argument to TOML print (see docstring using ? In the repl). You could then use e.g. propertynames and getproperty inside that function to generate a Dict.
---
### Author:  [@singularitti](https://discourse.julialang.org/u/singularitti)
#### Post date: [June 25, 2023, 8:47pm UTC](https://discourse.julialang.org/t/how-to-use-toml-print-to-print-a-dict-with-keys-as-different-mystruct-objects/100804/3 "2023-06-25T20:47:59Z")
Do you mean this example? It’s from the documentation:
```julia
TOML.print(Dict("foo" => MyStruct(5, "bar"))) do x
x isa MyStruct && return [x.a, x.b]
error("unhandled type $(typeof(x))")
end
```
However, it’s for printing `MyStruct` as values, not keys.
---
### Author:  [@singularitti](https://discourse.julialang.org/u/singularitti)
#### Post date: [June 25, 2023, 10:41pm UTC](https://discourse.julialang.org/t/how-to-use-toml-print-to-print-a-dict-with-keys-as-different-mystruct-objects/100804/4 "2023-06-25T22:41:49Z")
OK, using a `Dict`s as keys in a `TOML` file seems to be illegal syntax, which marks this question meaningless.