`Functors.children` fails on `Type` in Functors v0.5

[mtfishman]

For example:

julia> using Functors

julia> pkgversion(Functors)
v"0.5.0"

julia> Functors.children(Matrix{Float64})
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] #2
   @ ~/.julia/packages/Functors/1RvWk/src/functor.jl:21 [inlined]
 [2] ntuple(f::Functors.var"#2#5"{DataType, NTuple{8, Symbol}}, n::Int64)
   @ Base ./ntuple.jl:19
 [3] functor(T::Type, x::Type)
   @ Functors ~/.julia/packages/Functors/1RvWk/src/functor.jl:21
 [4] functor(x::Type)
   @ Functors ~/.julia/packages/Functors/1RvWk/src/functor.jl:25
 [5] children(x::Type)
   @ Functors ~/.julia/packages/Functors/1RvWk/src/functor.jl:58
 [6] top-level scope
   @ REPL[11]:1

julia> versioninfo()
Julia Version 1.11.1
Commit 8f5b7ca12ad (2024-10-16 10:53 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin22.4.0)
  CPU: 10 × Apple M1 Max
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, apple-m1)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)

while:

julia> using Functors

julia> pkgversion(Functors)
v"0.4.12"

julia> Functors.children(Matrix{Float64})
()

The cause seems to be that Functors v0.5 has generic code for walking through structs, which fails for types.

Probably it just requires a definition like:

Functors.functor(x::Type) = NoChildren(), _ -> x

[mcabbott]

Oh that's not good. @leaf Type seems to be one fix.

Are there other objects which are also going to go wrong?

julia> using Functors  # master branch

julia> fmap(x -> @show(x), (; a=1, b=Int, c=3))
x = 1
ERROR: UndefRefError: access to undefined reference
Stacktrace:
  [1] #functor##2
    @ ~/.julia/packages/Functors/1RvWk/src/functor.jl:21 [inlined]
  [2] ntuple(f::Functors.var"#functor##2#functor##3"{Type{Int64}, NTuple{8, Symbol}}, n::Int64)
    @ Base ./ntuple.jl:19
  [3] functor(T::Type, x::Type)
    @ Functors ~/.julia/packages/Functors/1RvWk/src/functor.jl:21
  [4] functor(x::Type)
    @ Functors ~/.julia/packages/Functors/1RvWk/src/functor.jl:25
  [5] children(x::Type)
    @ Functors ~/.julia/packages/Functors/1RvWk/src/functor.jl:58

julia> fmap(x -> @show(x), (; a=1, b=Array, c=3))  # gives awful errors on 1.12 with Functors@0.5
x = 1
x = :T
x = Union{}
ERROR: UndefRefError: access to undefined reference
Stacktrace:
  [1] #functor##2
    @ ~/.julia/packages/Functors/be90D/src/functor.jl:21 [inlined]
  [2] ntuple(f::Functors.var"#functor##2#functor##3"{Type{Any}, NTuple{8, Symbol}}, n::Int64)
    @ Base ./ntuple.jl:19
  [3] functor(T::Type, x::Type)
    @ Functors ~/.julia/packages/Functors/be90D/src/functor.jl:21
  [4] functor(x::Type)
    @ Functors ~/.julia/packages/Functors/be90D/src/functor.jl:25
  [5] children(x::Type)

julia> Functors.@leaf DataType  # this is not enough

julia> fmap(x -> @show(x), (; a=1, b=Int, c=3))
x = 1
x = Int64
x = 3
(a = 1, b = Int64, c = 3)

julia> fmap(x -> @show(x), (; a=1, b=Array, c=3))
x = 1
x = :T
x = Union{}
x = Any
x = :N
x = Union{}
x = Array{T, N}
x = 3
(a = 1, b = Array{T, N}, c = 3)

julia> Functors.@leaf Type  # but perhaps this is?

julia> fmap(x -> @show(x), (; a=1, b=Array, c=3))
x = 1
x = Array
x = 3
(a = 1, b = Array, c = 3)

julia> isstructtype(Int)
false

julia> isstructtype(Array)
true