[Haskell-cafe] Where is "minimumsBy"?

[David Feuer]

Laziness does not make the complexity work out fine. Sorting is still O(n
log n), which isn't needed here.

On Wed, Sep 26, 2018, 10:22 AM Tom Ellis <
tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:

> Hopefully laziness makes the complexity work out fine.  Nonetheless I don't
> like relying on laziness for the correct complexity and it would still be
> nice to have an explicit version.
>
> On Wed, Sep 26, 2018 at 10:13:21AM -0400, Brandon Allbery wrote:
> > Not exactly that, but you can use groupBy fst . sort, then the head of
> the
> > result list is your "minimumsBy" result.
> >
> > On Wed, Sep 26, 2018 at 6:28 AM Tom Ellis <
> > tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
> > > Data.List.minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
> > >
> > >
> > >
> https://www.stackage.org/haddock/lts-12.1/base-4.11.1.0/Data-List.html#v:minimumBy
> > >
> > > but there are many cases where that's quite unhelpful.  Actually what
> we
> > > want is more like
> > >
> > >     minimumsBy :: ... => (a -> a -> Ordering) -> t a -> [a]
> > >
> > > There can be many distinct minimizers.  For example when I want to get
> the
> > > collection of the youngest people from [(Age, Person)] I want
> > >
> > >     minimumsBy (compare `on` fst) [(12, alice), (15, balaji), (12,
> cho)]
> > >
> > > to return
> > >
> > >     [(12, alice), (12, cho)]
> > >
> > > Does "minimumsBy" exist somewhere reasonably standard?  Hoogle doesn't
> > > throw
> > > up anything obvious
> > >
> > >
> > >
> https://www.stackage.org/lts-12.1/hoogle?q=%28a+-%3E+a+-%3E+Ordering%29+-%3E+t+a+-%3E+%5Ba%5D
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