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

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 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 wrote:

...

On 01 Dec 2015, at 00:34, Oleg Grenrus 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#l... [2] https://hackage.haskell.org/package/base-4.8.1.0/docs/src/GHC.Base.html#Mona... [3] https://hackage.haskell.org/package/base-4.8.1.0/docs/src/GHC.Base.html#line... [4] http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#...

- Oleg

On 01 Dec 2015, at 00:25, Jeffrey Brown 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/...

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

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-- Jeffrey Benjamin Brown

-- Jeffrey Benjamin Brown