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@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@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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