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 data Items lbl = Items (S.Seq (lbl, Items lbl)) deriving (Eq, Show, Typeable, Data) Is there an easy way to understinfing those moves?