Just a bit of info

Whilst I don't have the code here to show this, an interesting thing that we found this week regarding a Moose attribute.

has q{attr} => (isa => q{Str}, is => q{rw});

This makes $class->attr() both reader and writer.

Now, it is mentioned in the manual that if you specify an attribute option, it will override the default by Moose, but what I was interested in was the error message.

has q{attr} => (isa => q{Str}, is => q{rw}, writer => q{_set_attr});

The error message you get here, if you try to use

$class->attr($some_string);

Is that you are trying to modify a read_only attribute. I was expecting it to have something different to this, since 'is => q{rw}'.

Using 'is => q{ro}' still allows you to use your private writer to set the value.

Personally, I always use 'is => q{ro}' unless I want the attr name to be the writer, but this is mildly interesting that setting a writer effectively overwrites the declaration of rw to be ro. Does this therefore make it more or less confusing to the user?

Looking at it from two different perspectives:

Public attribute setting:

Someone inspecting the code for this attribute would see rw and this would tell them, hey you can set this. Hopefully they would look for a writer option before diving in to use the attr name itself. But I can see the use of declaring rw in order to give some hint.

Private attribute setting:

Someone inspecting the code would see rw, and may assume that this means they should be able to set this attribute. However, the writer should only be used internally to the class, therefore giving a hint that it can, and perhaps should, be overwritten if needed. Of course, as with any situation, let the buyer beware, this functionality may change.
However, declaring ro, you are dropping the hint that outside of the class, if you didn't set it on construction, you really shouldn't be thinking of doing so.

I'll leave it up to someone else to determine a best practice. It was an interesting thing to discover. I shall stick with declaring 'is => q{ro}, writer => q{_set_attr}' since most of the time, it is private to the class to set anyway.