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

[Jeffrey Brown]

I spoke too soon; I'm seeing the same problem with MonadError.

    Prelude> :set -XFlexibleContexts
    Prelude> import Control.Monad.Except
    Prelude Control.Monad.Except> let f = (fail "be Left!" :: (MonadError
String m) => m ())
    Loading package transformers-0.4.2.0 ... linking ... done.
    Loading package mtl-2.2.1 ... linking ... done.
    Prelude Control.Monad.Except> f :: Either String ()
    *** Exception: be Left!
    Prelude Control.Monad.Except>



On Mon, Nov 30, 2015 at 6:18 PM, Jeffrey Brown <jeffbrown.the at gmail.com>
wrote:

> Oleg's suggestion works! I just had to add these two lines in order to use
> it:
>
>     {-# LANGUAGE FlexibleContexts #-}
>     import Control.Monad.Except -- mtl library
>
> Thanks, everybody!
>
> On Mon, Nov 30, 2015 at 2:38 PM, Oleg Grenrus <oleg.grenrus at iki.fi> wrote:
>
>>
>> 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
>> [2]
>> 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
>> [4]
>> 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> 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
>>
>>
>> --
>> Jeffrey Benjamin Brown
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>>
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>>
>>
>
>
> --
> Jeffrey Benjamin Brown
>



-- 
Jeffrey Benjamin Brown
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