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

[Jeffrey Brown]

Right! That's what I was doing when I got it to work and then I forgot. I
will correct the habit. Sorry for the annoyance!

On Tue, Dec 1, 2015 at 10:46 AM, Oleg Grenrus <oleg.grenrus at iki.fi> wrote:

> Use `throwError`, not `fail`. :)
>
> Forget `fail`. Luckily we are taking it out of `Monad`.
>
> - Oleg
>
>
> On 01 Dec 2015, at 20:44, Jeffrey Brown <jeffbrown.the at gmail.com> wrote:
>
> 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
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe at haskell.org
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>>>
>>>
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>>>
>>>
>>
>>
>> --
>> Jeffrey Benjamin Brown
>>
>
>
>
> --
> Jeffrey Benjamin Brown
>
>
>


-- 
Jeffrey Benjamin Brown
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