[Haskell-cafe] Comparing on multiple criteria

Martijn van Steenbergen martijn at van.steenbergen.nl
Sun Dec 21 08:52:07 EST 2008

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 

To reverse the order of a criterion or a set of criteria, flip can be used:

compareTuple' = mconcat [comparing fst, flip $ comparing snd]

I think it would be really neat if these two use cases were also 
described in comparing's documentation. Who is the right person to ask 
this of?

Thanks in advance,


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