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

[Jeffrey Brown]

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

[1]
https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/master/test/monad_fail_problems.hs


-- 
Jeffrey Benjamin Brown
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