[L03 State] Refining firstRepeat? (SPOILERS)

[David Tchepak]

I'm trying to refine my attempt at firstRepeat.

I started with:

firstRepeat ::

  Ord a =>

  List a

  -> Optional a

firstRepeat =

  flip eval S.empty 

      . findM (\x -> State (\s -> (x `S.member` s, x `S.insert` s)))

(where S is Data.Set)

I noticed I am taking (S.member, S.insert) and applying `x` and `s` to both parts of the tuple, and I was interested in seeing if there was a way to express this. I looked at using (&&&) from Control.Arrow, which gives me:

  (b -> c) -> (b -> d) -> b -> (c,d)    -- specialised to (->)

So I can get this:

    flip eval S.empty

        . findM (\x -> State $ S.member x &&& S.insert x)

Is there a nice way to refine this, getting rid of the explicit `x` reference?

Cheers,

David

[Tony Morris]

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Try this.

State . liftA2 (&&&) S.member S.insert

--
Tony Morris
http://tmorris.net/

[David Tchepak]

That's awesome. Thanks!