On 10 juli 2016 at 20:03:57, martin (martin.drautzburg@web.de) wrote: Am 07/10/2016 um 03:17 PM schrieb Martijn Schrage:
Hi Martin,
Ghci defaults the type of 'getHole t1' to 'Maybe ()'. You can see this happening if you enable warnings in ghci with ':set -Wall'. Providing the correct type signature gives you the answer you're expecting:
*Main> getHole t1 :: Maybe [Int] Just [1,2,3]
I understand, thanks. I tried a generic Zipper with a pair and noticed that going down gives me the second component. This does make some sense, but I am still having trouble to anticipate what up, down, left and right do when the the datatype is more complicated and not a tree. E.g. this one gives me trouble: import qualified Data.Sequence as S To understand the navigation, just draw your values as trees, with the constructor on one level and the children below. For example, a list [1,2] becomes: (:) / \ 1 (:) / \ 2 [] And a tuple (1,2) becomes: (,) / \ 1 2 Also note that down not only moves a level downward, but also to the rightmost child (down' moves to the leftmost child instead). I'm not sure how useful it is to use a zipper on a Seq (which, as you can see in the source, is still just a tree, like all data types,) as the implementation uses a balanced FingerTree which is kept abstract. But as it turns out, the Seq Data instance follows the right-associative view pattern, so if you wish you could navigate similar to lists (down for the tail, and down' or down followed by left for the head). Cheers, Martijn data Items lbl = Items (S.Seq (lbl, Items lbl)) deriving (Eq, Show, Typeable, Data) Is there an easy way to understinfing those moves?