<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On 01 Dec 2015, at 00:34, Oleg Grenrus <<a href="mailto:oleg.grenrus@iki.fi" class="">oleg.grenrus@iki.fi</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=us-ascii" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi, Jeffrey<div class=""><br class=""></div><div class="">in short: `fail` of `Either e` throws an exception (i.e. is not overriden, default implementation is `fail s = error s`) [1, 2]</div><div class=""><br class=""></div><div class="">For `Maybe`, fail is defined as `fail _ = Nothing`; which is good default. [3] </div><div class=""><br class=""></div><div class="">You probably want to use for example `throwError from `mtl` package [4]:</div><div class=""><br class=""></div><div class=""><div dir="ltr" class=""><div class=""></div></div></div></div></div></blockquote><div><br class=""></div><div>I haven’t still tested it, but less wrong context is `MonadError String m`:</div><br class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><div dir="ltr" class=""><div class=""><font face="monospace, monospace" class=""> gelemM :: (MonadError String m) => MyGraph -> Node -> m ()</font></div><div class=""><font face="monospace, monospace" class=""> gelemM g n = if gelem n g -- FGL's gelem function returns</font></div><div class=""><font face="monospace, monospace" class=""> then return () -- True if the node is in the graph</font></div><div class=""><font face="monospace, monospace" class=""> else throwError "Node not in Graph" -- False otherwise</font></div></div></div></div></div><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><br class=""></div><div class="">[1] <a href="https://hackage.haskell.org/package/base-4.8.1.0/docs/src/Data.Either.html#line-137" class="">https://hackage.haskell.org/package/base-4.8.1.0/docs/src/Data.Either.html#line-137</a></div><div class="">[2] <a href="https://hackage.haskell.org/package/base-4.8.1.0/docs/src/GHC.Base.html#Monad" class="">https://hackage.haskell.org/package/base-4.8.1.0/docs/src/GHC.Base.html#Monad</a></div><div class="">[3] <a href="https://hackage.haskell.org/package/base-4.8.1.0/docs/src/GHC.Base.html#line-642" class="">https://hackage.haskell.org/package/base-4.8.1.0/docs/src/GHC.Base.html#line-642</a></div><div class="">[4] <a href="http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#v:throwError" class="">http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#v:throwError</a></div><div class=""><br class=""></div><div class="">- Oleg</div><div class=""><br class=""><div class=""><div class=""><blockquote type="cite" class=""><div class="">On 01 Dec 2015, at 00:25, Jeffrey Brown <<a href="mailto:jeffbrown.the@gmail.com" class="">jeffbrown.the@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><div class="">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.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class=""><font face="monospace, monospace" class=""> type MyGraph = Gr String String</font></div><div class=""><font face="monospace, monospace" class=""><br class=""></font></div><div class=""><font face="monospace, monospace" class=""> tinyGraph = mkGraph [(0, "dog")] [] :: MyGraph</font></div><div class=""><font face="monospace, monospace" class=""><br class=""></font></div><div class=""><font face="monospace, monospace" class=""> maybeSucceed = replaceStringAtNodeM tinyGraph 0 "cat" :: Maybe MyGraph</font></div><div class=""><font face="monospace, monospace" class=""> -- == Just (mkGraph [(0,"cat")] [])</font></div><div class=""><font face="monospace, monospace" class=""> maybeFail = replaceStringAtNodeM tinyGraph 1 "cat" :: Maybe MyGraph</font></div><div class=""><font face="monospace, monospace" class=""> -- == Nothing</font></div><div class=""><font face="monospace, monospace" class=""><br class=""></font></div><div class=""><font face="monospace, monospace" class=""> eitherSucceed = replaceStringAtNodeM tinyGraph 0 "cat" :: Either String MyGraph</font></div><div class=""><font face="monospace, monospace" class=""> -- == Right (mkGraph [(0,"cat")] [])</font></div><div class=""><font face="monospace, monospace" class=""> eitherFail = replaceStringAtNodeM tinyGraph 1 "cat" :: Either String MyGraph</font></div><div class=""><font face="monospace, monospace" class=""> -- *** Exception: Node not in Graph</font></div><div class=""><br class=""></div><div class="">Here's the code:</div><div class=""><br class=""></div><div class=""><font face="monospace, monospace" class=""> import Data.Graph.Inductive -- FGL, the Functional Graph Library</font></div><div class=""><font face="monospace, monospace" class=""><br class=""></font></div><div class=""><font face="monospace, monospace" class=""> gelemM :: (Monad m) => MyGraph -> Node -> m ()</font></div><div class=""><font face="monospace, monospace" class=""> gelemM g n = if gelem n g -- FGL's gelem function returns</font></div><div class=""><font face="monospace, monospace" class=""> then return () -- True if the node is in the graph</font></div><div class=""><font face="monospace, monospace" class=""> else fail "Node not in Graph" -- False otherwise</font></div><div class=""><font face="monospace, monospace" class=""><br class=""></font></div><div class=""><font face="monospace, monospace" class=""> replaceStringAtNode :: MyGraph -> Node -> String -> MyGraph</font></div><div class=""><font face="monospace, monospace" class=""> replaceStringAtNode g n e = let (Just (a,b,c,d),g') = match n g</font></div><div class=""><font face="monospace, monospace" class=""> in (a,b,e,d) & g'</font></div><div class=""><font face="monospace, monospace" class=""><br class=""></font></div><div class=""><font face="monospace, monospace" class=""> replaceStringAtNodeM :: (Monad m) => MyGraph -> Node -> String -> m MyGraph</font></div><div class=""><font face="monospace, monospace" class=""> replaceStringAtNodeM g n s = do</font></div><div class=""><font face="monospace, monospace" class=""> gelemM g n</font></div><div class=""><font face="monospace, monospace" class=""> return $ replaceStringAtNode g n s</font></div><div class=""><font face="monospace, monospace" class=""> -- if evaluated, the pattern match in </font><span style="font-family:monospace,monospace" class="">replaceStringAtNode must succeed,</span></div><div class=""><span style="font-family:monospace,monospace" class=""> -- because gelemM catches the case where n is not in the graph</span></div><div class=""><br class=""></div><div class="">[1] <a href="https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/master/test/monad_fail_problems.hs" class="">https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/master/test/monad_fail_problems.hs</a></div><div class=""><br class=""></div><div class=""><br class=""></div>-- <br class=""><div class="gmail_signature"><div dir="ltr" class="">Jeffrey Benjamin Brown</div></div>
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