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@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@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@iki.fi> wrote:

On 01 Dec 2015, at 00:34, Oleg Grenrus <oleg.grenrus@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


- Oleg

On 01 Dec 2015, at 00:25, Jeffrey Brown <jeffbrown.the@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



--
Jeffrey Benjamin Brown
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe




--
Jeffrey Benjamin Brown



--
Jeffrey Benjamin Brown