Add Data.Eq.equating to match Data.Ord.comparing?
stefanor at cox.net
Thu Sep 20 10:10:34 EDT 2007
On Thu, Sep 20, 2007 at 11:26:48AM +0100, Paul Johnson wrote:
> I recently wanted to group a list of pairs by the second item in the pairs.
> So I tried writing
> > x = groupBy (comparing snd) pairs
> But this threw a type error because groupBy expects its first argument to
> return a Bool and comparing returns an Ordering. So I had to write
> > x = groupBy ((== EQ) . comparing snd) pairs
> Which is clunky. What I needed was a function in Data.Eq such as
> > equating :: Eq a => (b -> a) -> b -> b -> Bool
> This could be generalised to a function such as
> > pairWise :: (b -> b -> c) -> (a -> b) -> a -> a -> c
> > pairWise pair project x y = pair (project x) (project y)
> Then we can write
> > comparing = pairWise compare
> > equating = pairWise (==)
> Should I submit a patch to add this? And is "pairWise" the right name?
No, because it's already in the standard library (Data.Function.on,
intended use (==) `on` snd)
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