Herbert In tests/monadfail/MonadFailErrors.stderr I see MonadFailErrors.hs:59:5: error: No instance for (MonadFail Identity) arising from a do statement with the failable pattern ‘Newtype x’ In a stmt of a 'do' block: Newtype x <- undefined In the expression: do { Newtype x <- undefined; undefined } In an equation for ‘newtypeMatch’: newtypeMatch = do { Newtype x <- undefined; undefined } MonadFailErrors.hs:67:5: error: No instance for (MonadFail Identity) arising from a do statement with the failable pattern ‘Data x’ In a stmt of a 'do' block: Data x <- undefined In the expression: do { Data x <- undefined; undefined } In an equation for ‘singleConMatch’: singleConMatch = do { Data x <- undefined; undefined } But ‘Newtype’ and ‘Data’ are single constructor types and hence cannot fail. So why are these errors happening? I’m tiding up your implementation (deleting code!) and accidentally fixed this. So I wanted to check that it is indeed wrong as-is. Simon