# If new struct should implement show function to well print? otherwise it may cause error?
**URL:** https://discourse.julialang.org/t/if-new-struct-should-implement-show-function-to-well-print-otherwise-it-may-cause-error/87983
**Category:** General Usage
**Tags:** question, package
**Created:** [September 29, 2022, 12:18pm UTC](https://discourse.julialang.org/t/if-new-struct-should-implement-show-function-to-well-print-otherwise-it-may-cause-error/87983 "2022-09-29T12:18:15Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@xuanquansheng](https://discourse.julialang.org/u/xuanquansheng)
#### Post date: [September 29, 2022, 12:18pm UTC](https://discourse.julialang.org/t/if-new-struct-should-implement-show-function-to-well-print-otherwise-it-may-cause-error/87983/1 "2022-09-29T12:18:15Z")
I’ve recently been learning source codes of pkg `FixedEffectModels`.
```julia
if false
Xall = Combination(y, Xexo, Xendo, Z)
else
Xall = Combination(y, Xexo)
println(typeof(Xall)) # it can been print
println("Xall") # it can been print
println(Xall) # it cause error
end
```
it reports a error as follows:
```julia
CanonicalIndexError: getindex not defined for Combination{Float64}
```
these code in a cell with notebook of VS code can replicate this error：
```julia
struct Combination{T} <: AbstractMatrix{T}
A::Tuple
cumlength::Vector{Int}
end
function Combination(A::Union{AbstractVector{T}, AbstractMatrix{T}}...) where {T}
Combination{T}(A, cumsum([size(x, 2) for x in A]))
end
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)
end
Combination([1,2,3], [4,5,6])
```
and in a new cell with followed code works well.
```julia
# new cell
Combination([1,2,3], [4,5,6]);
```
So
- 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?
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [September 29, 2022, 1:10pm UTC](https://discourse.julialang.org/t/if-new-struct-should-implement-show-function-to-well-print-otherwise-it-may-cause-error/87983/2 "2022-09-29T13:10:13Z")
> [@xuanquansheng](#):
>
> `struct Combination{T} <: AbstractMatrix{T}`
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.
---
### Author:  [@xuanquansheng](https://discourse.julialang.org/u/xuanquansheng)
#### Post date: [September 29, 2022, 1:58pm UTC](https://discourse.julialang.org/t/if-new-struct-should-implement-show-function-to-well-print-otherwise-it-may-cause-error/87983/3 "2022-09-29T13:58:51Z")
> [@stevengj](#):
>
> which means that Julia calls the default `show` method for arrays, which in turn expects `getindex` to be defined. Hence the error.
so where can I learn this information?
> [@stevengj](#):
>
> If you don’t define `getindex`, this shouldn’t be a subtype of `AbstractMatrix`. Remove the `<: AbstractMatrix{T}` and `println` will work fine.
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.
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [September 29, 2022, 2:25pm UTC](https://discourse.julialang.org/t/if-new-struct-should-implement-show-function-to-well-print-otherwise-it-may-cause-error/87983/4 "2022-09-29T14:25:23Z")
> [@xuanquansheng](#):
>
> 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?
---
### Author:  [@xuanquansheng](https://discourse.julialang.org/u/xuanquansheng)
#### Post date: [September 29, 2022, 2:35pm UTC](https://discourse.julialang.org/t/if-new-struct-should-implement-show-function-to-well-print-otherwise-it-may-cause-error/87983/5 "2022-09-29T14:35:04Z")
> [@stevengj](#):
>
> 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.
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [September 29, 2022, 2:37pm UTC](https://discourse.julialang.org/t/if-new-struct-should-implement-show-function-to-well-print-otherwise-it-may-cause-error/87983/6 "2022-09-29T14:37:42Z")
> [@xuanquansheng](#):
>
> view a variable with strange struct?
`dump(x)` works even if you have broken `show`.
---
### Author:  [@xuanquansheng](https://discourse.julialang.org/u/xuanquansheng)
#### Post date: [September 29, 2022, 2:53pm UTC](https://discourse.julialang.org/t/if-new-struct-should-implement-show-function-to-well-print-otherwise-it-may-cause-error/87983/7 "2022-09-29T14:53:56Z")
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