Hello I try to have a generic way to open and close ressources for my software class DataResource a where data OpenedDataResource a :: * acquireDataResource :: a -> IO (OpenedDataResource a) releaseDataResource :: OpenedDataResource a -> IO () the idea is to have a generic method like this withDataResource :: DataResource a => a -> (OpenedDataResource a -> IO r) -> IO r withDataResource r = bracket (acquireDataResource r) releaseDataResource and later another one for the bracket of Pipes. So I try with something simple -- File newtype H5FilePath = H5FilePath FilePath instance DataResource H5FilePath where data OpenedDataResource H5FilePath = File acquireDataResource (H5FilePath f) = openFile (pack f) [ReadOnly] Nothing releaseDataResource = closeFile but I end up with this sort of error src/Hkl/H5.hs:231:40: error: • Couldn't match type ‘File’ with ‘OpenedDataResource H5FilePath’ Expected type: IO (OpenedDataResource H5FilePath) Actual type: IO File • In the expression: openFile (pack f) [ReadOnly] Nothing In an equation for ‘acquireDataResource’: acquireDataResource (H5FilePath f) = openFile (pack f) [ReadOnly] Nothing In the instance declaration for ‘DataResource H5FilePath’ | 231 | acquireDataResource (H5FilePath f) = openFile (pack f) [ReadOnly] Nothing | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ src/Hkl/H5.hs:232:25: error: • Couldn't match type ‘OpenedDataResource H5FilePath’ with ‘File’ Expected type: OpenedDataResource H5FilePath -> IO () Actual type: File -> IO () • In the expression: closeFile In an equation for ‘releaseDataResource’: releaseDataResource = closeFile In the instance declaration for ‘DataResource H5FilePath’ | 232 | releaseDataResource = closeFile | ^^^^^^^^^ cabal: Failed to build hkl-0.1.0.0 (which is required by exe:binoculars-ng from hkl-0.1.0.0). I do not understand why ? and more seriously, I do not know how to fix this. thanks for your help Frederic