[Haskell-cafe] About the return type
douyaxu at gmail.com
Thu Jul 9 10:57:19 EDT 2009
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
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