# [Haskell-cafe] Monadic function fails as Nothing in Maybe context but as Exception in Either context.

Jeffrey Brown jeffbrown.the at gmail.com
Mon Nov 30 22:25:49 UTC 2015

```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

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