[Haskell-cafe] Does somebody know about these functions?

[Johan Holmquist]

>> inter :: (a -> a -> b) -> [a] -> [b]
>> inter f [] = []
>> inter f l  = map (uncurry f) $ zip l (tail l)
>
> This is the same as
>
>  inter :: (a -> a -> b) -> [a] -> [b]
>  inter f l = zipWith f l (tail l)

Except when l == [], but the second equation can be replaced by this nicer one.

> and you can use it to define the good old Fibonacci sequence:
>
>  fibs = 0 : 1 : inter (+) fibs

Another use :-)

(sorry Holger for duplicate -- hit wrong answer button at first)


Together, these functions can be used to define a variant of groupBy
that does the "expected thing" in the case of groupBy (<) for example.

groupBy f l = gby $ zip (undefined : inter f l) l where
    gby []         = []
    gby ((_,x):ps) = withPair (:) ((x:) . map snd) gby (span fst ps)


>>> groupBy (<) [1,2,3, 2,3, 1,2]
[[1,2,3],[2,3],[1,2]]

/Johan