Invalid subtype error message could be better

[Keno]

Currently as follows:

julia> abstract type Foo{S}; end

julia> struct Bar <: Foo; end
ERROR: invalid subtyping in definition of Bar
Stacktrace:
 [1] top-level scope at REPL[2]:1

Ideally it would at least tell you you forgot a type parameter, and maybe if it's extra nice show you the signature of the thing you're trying to subtype to show you what type parameters it has.

[ShriprajwalK]

Hey, I'd like to work on this. Is it still open? If so, how do I go about this?
Thanks!

[kamalojasv181]

I was working on this issue. Can someone give me a relevant pointer regarding origin of following functions since there is nothing about them in the documentation.
jl_is_abstracttype
jl_is_datatype
jl_subtype
jl_is_tuple_type
jl_is_namedtuple_type

and the datatypes:
jl_datatype_t
jl_value_t

[Keno]

Probably the way to do this is to add a new error type and expose it to C (e.g. search for jl_undefvarerror_type and UndefVarError to see how this is done). That error struct would then have both the LHS and the RHS of the error. Then in Julia, define a show method for the new error that shows something more intelligent.

Can someone give me a relevant pointer regarding origin of following

You can search the code for them. They're defined in julia.h.

[kamalojasv181]

Can someone give me a pointer to the explanation of the use of jl_subtype. I have tried following the function but there is a never ending chain of functions.

[timholy]

It's what gets called by S<:T, is that what you mean?

But you probably don't have to dive in there at all. If you search for the invalid subtyping error in the src/ directory you'll see it's in interpreter.c in a function called jl_set_datatype_super. Rather than calling jl_errorf, you could have it craft a new kind of error (SubtypeError?) that stores both tt and super. Then in base/errorshow.jl you'd need to come up with a way to display such errors. Keno gave you the other tip you'll probably need, how to share an error between C and Julia.

[kamalojasv181]

@timholy jl_set_datatype_super is exactly the place I am at. The reason why I asked about this function is to handle jl_subtype(super, (jl_value_t*)jl_vararg_type) and other conditions that use jl_subtype as I am not sure what this function is doing that is triggering the error.

[timholy]

You can check those same conditions in pure Julia, once the error gets returned. Using Int as a standin for tt and Foo for super, here are some examples:

julia> Int isa DataType  # jl_is_datatype
true

julia> isabstracttype(Foo)    # jl_is_abstracttype
true

julia> typeof(Int.name)    # checking the typename
Core.TypeName

julia> Int <: Foo       # subtyping
false

The names of those C functions hopefully make it fairly clear what's actually going on.

[kamalojasv181]

@timholy in errorshow.jl , how do we know what methods does ex in an error has? I think the only thing I am unclear about this issue is how to use tt and super in errorshow.jl and I think knowledge of ex will solve the problem. Please correct me if i am wrong.

[kamalojasv181]

I just created a pull request to the issue #34510. Couldn't understand how to create error in Julia therefore I edited the intrepreter.c file.Kindly ignore previous commits as I was not very clear with the concept of branches.

[ravibitsgoa]

Hi, Is this issue still open?

[KristofferC]

It is open but there has been some work on it over at #34510.