T16628.hs:10:5:
   No instance for (MonadFail Identity)
        arising from a do statement
        with the failable pattern ‘[x]’
     In a stmt of a 'do' block: [x] <- i1
      In the expression:
        do _ <- i0
           [x] <- i1
           pure (x + 42)
      In an equation for ‘f’:
          f i0 i1
            = do _ <- i0
                 [x] <- i1
                 pure (x + 42)