On Thu, Jul 09, 2009 at 10:57:19AM -0400, xu zhang wrote: > I have trouble in returning a list of Figures. I want return a type of m > (Maybe [Figure IO]), but the type of dv_findFigure is :: a -> Point -> s > (Maybe (Figure s)). How can change the code below to get a s (Maybe [Figure > s])? > Thank you in advance! > > dv_findFigure :: a -> Point -> s (Maybe (Figures)) > fig_contains :: fig -> Point -> m Bool > anc :: Point > do > fs <- dv_getSelFigs dv > fs' <- filterM (`fig_contains` anc) fs > f <- case fs' of > [] -> dv_findFigure dv anc > fig : _ -> return $ Just fig > case f of > Just f' -> tool_dragtrack self f' > Nothing -> dv_clearSel dv >> tool_areatrack self > > Couldn't match expected type `Figure m' > against inferred type `[Figure IO]' > Expected type: m (Maybe (Figure m)) > Inferred type: m (Maybe [Figure IO]) > In the expression: return $ Just fs > In a case alternative: fig : _ -> return $ Just fs
I'm not sure what you're trying to do here, and the code you gave does not match the error message (the error message cites the code 'fig : _ -> return $ Just fs', but the code you gave has a line 'fig : _ -> return $ Just fig'.) If you still need help, can you provide a more detailed explanation of what this code is supposed to do, and an up-to-date version of the code and error message? -Brent _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
