Fieldnames for an instance of a structure

New to Julia
1

Hi, been a while since I have asked a dumb question in New to Julia so here we go.

Is there a way to define the function fieldnamesor similar to give me the field names for an instance of a structure? I would like to see the fields of mine (or other people’s) custom structures.

AFAIK, fieldnames works on the type, and it throws an error if you use it on an instance of the structure. But for that you need to know the custom type name, that you might have forgotten (not that that would ever happen to me).

However, a hack, that I have been using is misspelling a field on purpose, then the error shows all the available field names. Is there a better way? I have seen some topics on the question, such as this one, but that solution seems even worse than my hack.

Here is what I mean
Code snippet

struct Foo{T}
    a::T
    b::T
end                 # define a struct and forget its fields 3 secs later because I have a memory span of a goldfish
Fooinst = Foo(4,2)  # create an instance
fieldnames(Foo)     # this works, prints the fields
fieldnames(Fooinst) # this does not work because it is not on the type, but I need to know the type name "Foo"
Fooinst.c           # misspell on purpose, this works, because the error throws the field names

Output:

julia> struct Foo{T}
           a::T
           b::T
       end                 # define a struct and forget its fields 3 secs later because I have a memory span of a goldfish

julia> Fooinst = Foo(4,2)  # create an instance
Foo{Int64}(4, 2)

julia> fieldnames(Foo)     # this works, prints the fields
(:a, :b)

julia> fieldnames(Fooinst) # this does not work because it is not on the type, but I need to know the type name "Foo"
ERROR: MethodError: no method matching fieldnames(::Foo{Int64})
The function `fieldnames` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  fieldnames(::Core.TypeofBottom)
   @ Base runtime_internals.jl:339
  fieldnames(::Type{<:Tuple})
   @ Base runtime_internals.jl:341
  fieldnames(::UnionAll)
   @ Base runtime_internals.jl:338
  ...

Stacktrace:
 [1] top-level scope
   @ c:\Users\bblagov\Dropbox\JuliaNew\BEAVARs\dev\devCPZ.jl:27

julia> Fooinst.c       # misspell on purpose, this works, because the error throws the field names
ERROR: FieldError: type Foo has no field `c`, available fields: `a`, `b`
Stacktrace:
 [1] getproperty(x::Foo{Int64}, f::Symbol)
   @ Base .\Base_compiler.jl:54
 [2] top-level scope
   @ c:\Users\bblagov\Dropbox\JuliaNew\BEAVARs\dev\devCPZ.jl:28

The last line gives me the info that I need:

ERROR: FieldError: type Foo has no field c, available fields: a, b

2

What about fieldnames(typeof(Fooinst))?