[Haskell-cafe] What's wrong in my Data.Generic.Zipper code?

Martijn Schrage martijn at oblomov.com
Sun Jul 10 13:17:13 UTC 2016

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]


On 10 juli 2016 at 11:55:48, martin (martin.drautzburg at web.de) wrote:

Hello all,

I do not understand why this simple piece of code returns Nothing instead
of [1,2,3]. Can anybody help?

{-# LANGUAGE BangPatterns,NoMonomorphismRestriction, DeriveDataTypeable #-}
import Data.Typeable
import Data.Data
import Data.Generics.Zipper

ex_list1 :: [Int]
ex_list1 = [1,2,3]

t1 = toZipper ex_list1

-- *Main> getHole t1
-- Nothing
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