julia interpolation of own type with string()

2
votes

I'm quite new to Julia and I'm looking into porting some Python code to Julia. This code uses __repr__() overloading to display cutsom types. I understand that Julia provides the string() method for this functionality. But I can't figure it out.

julia> type Thomas
         t::Integer
       end 
julia> function Base.string(t::Thomas)
         "---> $(t.t) <---"
       end
julia> r = Thomas(8);

With these definitions I expected my string(::Thomas) function to be called whenever a value of type Thomas needed to be converted to a string. In one case, it works as expected:

julia> println("$r")
---> 8 <---

But, for the most cases it does not:

julia> println(r)
Thomas(8)

julia> println(" $r")
 Thomas(8)

julia> println("r = $r")
r = Thomas(8)

julia> repr(r)
"Thomas(8)"

What did I get wrong ? Is there some other function I should define for my new custom type ?

I am running Julia 0.4.0-dev. (the code above was pasted from the REPL of Version 0.4.0-dev+3607 (2015-02-26 07:41 UTC), Commit bef6bf3*, x86_64-linux-gnu)

3
julia

3 Answers

4
votes

At the moment just overriding Base.show should be enough, as follows.

type Thomas
    t::Int   # note Int not Integer
end

Base.show(io::IO, x::Thomas) = print(io, "Thomas with $(x.t)")

Note that in the definition of the type, you should use the concrete type Int (equivalent to Int64 or Int32, depending on your machine's word size), not the abstract type Integer, which will lead to bad performance.

The situation with Base.show, Base.print etc. is indeed confusing at the moment, but with some recent work (look up IOContext) should get simplified and clarified soon.

1
votes

You have to override two versions of Base.print actually to get a consistent behavior of string interpolation:

Base.print(io::IOBuffer, t::Thomas) = Base.print(io, "---> $(t.t) <---")
Base.print(t::Thomas) = Base.print("---> $(t.t) <---")

Then you'll have:

print(t)
string(t)
string(t, t, ...)
"$t"
"t = $t"
"$t $t $t"

and co.

0
votes

You may want to override the show method as well.

Base.show(io::IO, x::Thomas) = show(io, string(x))