I've written a monadic function which in a Maybe context produces a Nothing when it fails (as intended), but in an Either context produces an Exception rather than a Left.
Here's a tiny demonstration. "tinyGraph" below has one Node, 0, with the label "dog". If I try to change the label at Node 0 to "cat", it works. If I try to change the label at Node 1 to "cat", it fails, because Node 1 is not in the graph.
type MyGraph = Gr String String
tinyGraph = mkGraph [(0, "dog")] [] :: MyGraph
maybeSucceed = replaceStringAtNodeM tinyGraph 0 "cat" :: Maybe MyGraph
-- == Just (mkGraph [(0,"cat")] [])
maybeFail = replaceStringAtNodeM tinyGraph 1 "cat" :: Maybe MyGraph
-- == Nothing
eitherSucceed = replaceStringAtNodeM tinyGraph 0 "cat" :: Either String MyGraph
-- == Right (mkGraph [(0,"cat")] [])
eitherFail = replaceStringAtNodeM tinyGraph 1 "cat" :: Either String MyGraph
-- *** Exception: Node not in Graph
Here's the code:
import Data.Graph.Inductive -- FGL, the Functional Graph Library
gelemM :: (Monad m) => MyGraph -> Node -> m ()
gelemM g n = if gelem n g -- FGL's gelem function returns
then return () -- True if the node is in the graph
else fail "Node not in Graph" -- False otherwise
replaceStringAtNode :: MyGraph -> Node -> String -> MyGraph
replaceStringAtNode g n e = let (Just (a,b,c,d),g') = match n g
in (a,b,e,d) & g'
replaceStringAtNodeM :: (Monad m) => MyGraph -> Node -> String -> m MyGraph
replaceStringAtNodeM g n s = do
gelemM g n
return $ replaceStringAtNode g n s
-- if evaluated, the pattern match in replaceStringAtNode must succeed,
-- because gelemM catches the case where n is not in the graph
--