:assoc-fn and :default do not really play nice together
Description
Consider an option that can have zero or more values so we store it in a vector (or set). The obvious approach is to specify the empty vector (or set) in :default and use :assoc-fn to conj additional values into it. However, if you want the option to have a non-empty default, if you specify that for :default then :assoc-fn adds the extra values in and you end up with both the default AND the user-supplied values.
What you really want is for the default to only be applied if nothing was assoc'd in.
Currently, there's history that an incrementing argument should have a :default of 0 and then :assoc-fn can be (fn [m k _] (update m k inc)) – and that example is given specifically.
The assumption is that every option starts with the :default value (if given) and the other functions are called as each argument comes in but there's no way to provide a starting value for the :assoc-fn separate from the :default value for the option.
I'm not sure what the best approach here would be.
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import September 27, 2018 at 11:30 PM
Comment made by: dorab
Thanks much.
Sean Corfield September 25, 2018 at 6:06 AM
I just pushed some updates to both the README and the docstring for parse-opts.
import September 24, 2018 at 7:06 PM
Comment made by: dorab
OK. Fair enough. Thanks.
Sean Corfield September 24, 2018 at 7:03 PM
The docstring for parse-opts is already huge – I can't put all the docs in it!
import September 24, 2018 at 6:55 PM
Comment made by: dorab
Thanks. Sorry, I though I read all the docs first, but obviously not.
That Example Usage section on :default vs {:default-fn}} describes the process well. It might be good to add that to the parse-opts docstring as well.
Major
Consider an option that can have zero or more values so we store it in a vector (or set). The obvious approach is to specify the empty vector (or set) in :default and use :assoc-fn to conj additional values into it. However, if you want the option to have a non-empty default, if you specify that for :default then :assoc-fn adds the extra values in and you end up with both the default AND the user-supplied values.
What you really want is for the default to only be applied if nothing was assoc'd in.
Currently, there's history that an incrementing argument should have a :default of 0 and then :assoc-fn can be (fn [m k _] (update m k inc)) – and that example is given specifically.
The assumption is that every option starts with the :default value (if given) and the other functions are called as each argument comes in but there's no way to provide a starting value for the :assoc-fn separate from the :default value for the option.
I'm not sure what the best approach here would be.