If new struct should implement show function to well print? otherwise it may cause error?

I’ve recently been learning source codes of pkg FixedEffectModels.

if false
    Xall = Combination(y, Xexo, Xendo, Z)
    Xall = Combination(y, Xexo)
    println(typeof(Xall))   # it can been print
    println("Xall")            #  it can been print 
    println(Xall)               #  it cause error

it reports a error as follows:

CanonicalIndexError: getindex not defined for Combination{Float64}

these code in a cell with notebook of VS code can replicate this error:

struct Combination{T} <: AbstractMatrix{T}

function Combination(A::Union{AbstractVector{T}, AbstractMatrix{T}}...) where {T}
    Combination{T}(A, cumsum([size(x, 2) for x in A]))

Base.size(c::Combination) = (size(c.A[1], 1), c.cumlength[end])
Base.size(c::Combination, i::Integer) = size(c)[i]

function Base.view(c::Combination, ::Colon, j)
    index = searchsortedfirst(c.cumlength, j)
    newj = index == 1 ? j : j   - c.cumlength[index-1]
    view(c.A[index], :, newj)
Combination([1,2,3], [4,5,6])

and in a new cell with followed code works well.

# new cell
Combination([1,2,3], [4,5,6]);


  • why this error occurs?
  • Is it cause by the lack of proper show function for struct Combination
    • if it’s true, is it a flaw of julia?
  • a more complicated question is that how can I know how many methods current object/instance can use?

You are defining a subtype of AbstractMatrix, which means that Julia calls the default show method for arrays, which in turn expects getindex to be defined. Hence the error.

If you don’t define getindex, this shouldn’t be a subtype of AbstractMatrix. Remove the <: AbstractMatrix{T} and println will work fine.


so where can I learn this information?

so is there a general way to view a variable with strange struct? It seems not so convenient to define getindex just for view a variable.

You should define getindex if it is a subtype of AbstractMatrix, otherwise your type is broken. Why do you want to call Combination a matrix subtype if it doesn’t support indexing?

I am sorry I get it. I just what to see what the content of a instance(sometimes we can see the definition directly. here I complement it by draw it from other place) and in other language a easy is to print it.

dump(x) works even if you have broken show.

thanks a lot! It works well. If it wasn’t for you, I wouldn’t have thought that dump could be used in this way