Creating sub types

I am trying to create an abstract subtype:

abstract type StructuredMatrix{T} <: AbstractMatrix{T} 

However when i run this line code i get: “invalid redefinition of constant”

edit: i am using juno and entering command + enter to run the lines of code

You likely defined StructuredMatrix previously in the same session. Redefining structs requires that Julia be restarted.
(There also needs to be an end at the end of the definition, but I assume this is just missing from the quoted code).

Thank that seems to fix things. Still getting comforable with things.

Now that we on the topic. May I also ask why the folllowing gives me an error:

abstract type StructuredMatrix{T} <: AbstractMatrix{T}     end

struct  HSS{T}  <: StructuredMatrix{T}

    A :: Array{T,2}


A = HSS{Int64}([2 2;
                4 4])

When you created a subtype of AbstractMatrix you implicitly promised that your type would support the interface of an AbstractArray.

When you try to show your HSS object, that promise fails and show tries to call a method (size) which is not defined for HSS.

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