On Sun, Dec 21, 2008 at 11:20 AM, Jan-Willem Maessen
On Dec 21, 2008, at 8:52 AM, Martijn van Steenbergen wrote:
Hello all,
Data.Ord has a handy function called comparing, and its documentation shows an example of its use.
But what if you want to sort a list of values based on multiple criteria? It turns out there is a neat way to do this:
compareTuple = mconcat [comparing fst, comparing snd]
The default Monoid instances for Ordering and functions work exactly as required here. (Thanks to vixey in #haskell for the hint to look at monoids!)
Indeed, this is great to know. I can't help but notice that there is no documentation of any kind at all for the Monoid instance of Ordering; how were we supposed to know this behavior existed in the first place, except by hunting down the source code for the instance declaration?
This is a great example of why it's a bad idea to introduce new
functionality with a Monoid instance. Even if you know the instance
exists, mappend is so general that it's difficult or impossible to
predict what it will do at a given type.
There should be an explicit function for combining Ordering values
lexicographically, with a note in the documentation saying that it's
the basis of the Monoid instance.
--
Dave Menendez