ListZipper.findLeft

[Christopher Mazey]

findLeft and findRight fail

-- prop> findLeft (const True) -<< fromList xs == fromList xs

when

xs = 0 :. Nil

Apparently the tests and examples contradict each other; in this case the property seems include the focus of the ListZipper in its search, while the example

-- >>> findLeft (== 1) (zipper [2, 1] 1 [4, 5])
-- [] >1< [2,1,4,5]

does not.

The code:

-- | Seek to the left for a location matching a predicate, starting from the
-- current one.
--
-- -- /Tip:/ Use `break`
--
-- prop> findLeft (const True) -<< fromList xs == fromList xs
--
-- prop> findLeft (const False) (zipper l x r) == IsNotZ
--
-- >>> findLeft (== 1) (zipper [2, 1] 3 [4, 5])
-- [] >1< [2,3,4,5]
--
-- >>> findLeft (== 1) (zipper [2, 1] 1 [4, 5])
-- [] >1< [2,1,4,5]
findLeft :: (a -> Bool) -> ListZipper a -> MaybeListZipper a
findLeft p (ListZipper l x r) =
  case break p l of
    (_,Nil) -> IsNotZ
    (l',h :. t) ->
      IsZ $
      ListZipper t
                 h
                 (reverse l' ++ x :. r)

[Tony Morris]

Thanks Christopher, I have opened an issue:

https://github.com/NICTA/course/issues/126

I will fix it now.

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[Christopher Mazey]

Thanks, Tony.  And thank you very much for the course.  It is very good--I am learning and enjoying it a lot.