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Now comes my question: in the impure values there is always that "s". I was thinking that the whole structure should have s as a parameter:
Yes
data MList s = MList { mlVec :: MVector s Move, mlNextPh :: MList -> ST s (Maybe (MList s)) }
you probably meant: data MList s = MList { ... , mlNextPh :: Mlist s -> ... } Now I'm not sure about your exact problem since the following compiles for me. import Data.Vector import Data.Vector.Mutable import Control.Monad.ST type Move = () data MList s = MList { mvVec :: MVector s Move, mlNextPh :: MList s -> ST s (Maybe (MList s)) } splitMove :: MList s -> ST s (Maybe (Move, MList s)) splitMove ml = do m <- unsafeRead (mvVec ml) 0 undefined Something you always have to watch out for when dealing with ST is not to return something that depends on s in the last statement (the one you use runST on). In other words, if you want to return a vector you have to freeze it, so it's not mutable anymore. If you still can't figure it out paste some complete example that doesn't work. silvio -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iQIcBAEBAgAGBQJP0Uu5AAoJEDLsP+zrbatWKFoP+wYdmAwO3aKPIibOydDwPlcu GmwWLCDoylhBsA1swskPGZTlBevFFeS0kzDMAhZ2dtR18HHf0TVLFCL6mljgQGhu YLsT8a2Y5eepPd7CC0wHD7qLH0t6ln/urRhWNnVEGryVHmsIDCBzuKBzopshaaOm 8awNeEbmZApki193r/YJ21Zsxidx4N2tSGCd712ka9Wr7l19RzBukonTy/wNCTtN 1sj54xCKap3MpnQe4L68nep6WjMovnwn5ucPWlouPP5N99/2umiEPDwX3y9moD/Q VkbYe0HzZtvSX7JJaDM/hJ2dWKHsg5CLdO/aW7Uz3HttTy0/FmvwhxaNAzkmQimw L4uakvyuw1EJuSAwB5XRfeUL6LDpka165jb8V8Iy2gjYg3aGMwf9VVmObjEAA93s nvQd+iH1lDe38cbfz8dfQdTakDVYtFNnYL+kXIF1Z7DiS25IThtS0RJRH//E+CZg MpOtW2LBfa3vwP9NqVryGTAhWFtWHXOtpXfCXOa0+pQNn1zHkTXtIDJ4XoT5qkmd 6GDwFyGfkPZO01qNMoXwj/wBz/eaSa4Vj0qb73jNdNH2MbJ13Ws9Jlp4jwcxbG4a m/fYV0/6LmPEiV8H9+4cG8nhUP2ie2DJqo8tzdjiaZ7C7TEym9jd6gsljMQ8qiAG Q7aAmMed/DBlY/Anh2xY =X9CL -----END PGP SIGNATURE-----