[Haskell-cafe] About the return type
byorgey at seas.upenn.edu
Sat Jul 11 15:33:00 EDT 2009
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
> Thank you in advance!
> dv_findFigure :: a -> Point -> s (Maybe (Figures))
> fig_contains :: fig -> Point -> m Bool
> anc :: Point
> 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?
More information about the Haskell-Cafe