Editing Talk:Typeclassopedia (section)

== §6.4 Other monoidal classes ==

This section states that instances of Alternative/MonadPlus/ArrowPlus are sometimes the same as the Monoid instance, and sometimes different. It would be helpful (imo) if it was explained why this is: whether it would be better if instances of Alternative/MonadPlus/ArrowPlus were the same as the Monoid instance, and that sometimes they are not due to accidents of history; or whether sometimes these instances should be different, and giving guidance as to when those times are.

--[[User:Dave4420|Dave4420]] 17:16, 26 November 2011 (UTC)

Hmm, where does it say that? [[User:Byorgey|Byorgey]] 17:40, 26 November 2011 (UTC)

Search for <blockquote>A
simple example of a <code>MonadPlus</code> instance is <code>[]</code>, which is exactly the
same as the <code>Monoid</code> instance for <code>[]</code>: the empty list represents
failure, and list concatenation represents choice.  In general,
however, a <code>MonadPlus</code> instance for a type need not be the same as its
<code>Monoid</code> instance; <code>Maybe</code> is an example of such a type.</blockquote>--[[User:Dave4420|Dave4420]] 09:54, 27 November 2011 (UTC)

:Ah, I see now.  I think it's mostly due to history.  Many of these types could have many different <code>Monoid</code> instances.  I'll put in some additional discussion. --[[User:Byorgey|Byorgey]] 03:44, 28 November 2011 (UTC)