Error printing custom types

Tooling Juno
,
1

If we define a new show method for a custom class and then create a variable with said class, Juno will keep printing the variable to the REPL. Here is a MWE:

import Base.show
mutable struct foo
    bar
end
function show(io::IO, a::foo)
    print(a.bar)
    return
end
t = foo(1)

If we run this, it will print 11 to the REPL (which is already wrong, since it’s showing t twice). Furthermore, all following REPL prompts will show julia>1.

This does not happen if we execute the code directly from the REPL.

2

That’s happening because your definition of show is wrong. You’re printing to stdout instead of io, so whenever show is called you’ll see a 1 pop up in the REPL. When Juno shows your variable in the Workspace show is called, just like when displaying a result in the REPL – you’ll end up seeing 11.

Basically, your code works in the REPL only by accident, because usually io == stdout in that case.

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