Hi John,

this is a nice way of expressing it! I was hoping there would be some more fundamental function that I would have missed though. By fundamental I mean that I was hoping there was some function of a "core" typeclass like Alternative that I had missed that was a general case of my function. Thanks anyway, I learned about lens' partsOf :)

Paul

On Sat, Apr 8, 2017 at 11:20 PM John Wiegley <johnw@newartisans.com> wrote:

>>>>> "PB" == Paul Brauner <polux2001@gmail.com> writes:

PB> Oops, there's a free variable in there, let me fix it:
PB> first f [] = Nothing
PB> first f (x:xs) = fmap (:xs) (f x) <|> fmap (x:) (first f xs)

If you break up the test from the modification (instead of fusing the
operation into 'f'), this sort of thing becomes quite easy to express
using lens:

[1..10] & partsOf (traverse.filtered (>2))._head +~ 3

If the list has no elements >2, the operation is a no-op.

--
John Wiegley GPG fingerprint = 4710 CF98 AF9B 327B B80F
http://newartisans.com 60E1 46C4 BD1A 7AC1 4BA2