I once used `traverse optional` combined with `catMaybes`. For example:

Prelude Control.Applicative Data.Maybe> catMaybes <$> traverse optional [[1,2],[3]]

[[1,3],[1],[2,3],[2],[3],[]]

This is totally out of order. I think a more natural output is:

[[],[3],[1],[1,3],[2],[2,3]]

2019년 9월 1일 (일) 오후 11:23, Edward Kmett <ekmett@gmail.com>님이 작성:

It would also render the combinator useless for its normal purpose.

optional is used mostly to try to run a parser and to either succeed with its result (wrapped in a Just) or _failing that_ to just return Nothing and carry on.

For monads like parsec, the first parse is the one that gets returned, so the definition isn't symmetric in behavior.

-Edward

On Sun, Sep 1, 2019 at 2:28 AM Dannyu NDos <ndospark320@gmail.com> wrote:
The current 'one or none' definition breaks the order of elements.

It is more Ord-friendly to define it as 'none or one'.
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