[Haskell-cafe] Monadic function fails as Nothing in Maybe context but as Exception in Either context.
Oleg Grenrus
oleg.grenrus at iki.fi
Tue Dec 1 18:46:03 UTC 2015
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 <mailto: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 <mailto:oleg.grenrus at iki.fi>> wrote:
>
>> On 01 Dec 2015, at 00:34, Oleg Grenrus <oleg.grenrus at iki.fi <mailto: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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>
>
>
>
> --
> Jeffrey Benjamin Brown
>
>
>
> --
> Jeffrey Benjamin Brown
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