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

[Oleg Grenrus]

> On 01 Dec 2015, at 00:34, Oleg Grenrus <oleg.grenrus at iki.fi> wrote:
> 
> Hi, Jeffrey
> 
> in short: `fail` of `Either e` throws an exception (i.e. is not overriden, default implementation is `fail s = error s`) [1, 2]
> 
> For `Maybe`, fail is defined as `fail _ = Nothing`; which is good default. [3]
> 
> You probably want to use for example `throwError from `mtl` package [4]:
> 

I haven’t still tested it, but less wrong context is `MonadError String m`:

    gelemM :: (MonadError String 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 throwError "Node not in Graph"   -- False otherwise
> 
> [1] https://hackage.haskell.org/package/base-4.8.1.0/docs/src/Data.Either.html#line-137 <https://hackage.haskell.org/package/base-4.8.1.0/docs/src/Data.Either.html#line-137>
> [2] https://hackage.haskell.org/package/base-4.8.1.0/docs/src/GHC.Base.html#Monad <https://hackage.haskell.org/package/base-4.8.1.0/docs/src/GHC.Base.html#Monad>
> [3] https://hackage.haskell.org/package/base-4.8.1.0/docs/src/GHC.Base.html#line-642 <https://hackage.haskell.org/package/base-4.8.1.0/docs/src/GHC.Base.html#line-642>
> [4] http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#v:throwError <http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#v:throwError>
> 
> - Oleg
> 
>> On 01 Dec 2015, at 00:25, Jeffrey Brown <jeffbrown.the at gmail.com <mailto:jeffbrown.the at gmail.com>> wrote:
>> 
>> 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 <https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/master/test/monad_fail_problems.hs>
>> 
>> 
>> --
>> Jeffrey Benjamin Brown
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> 
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