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POCan't seem to implement Either correctly
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5103380
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AcceptedAnswerId
5104328
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2
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0
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CreationDate
2011-02-24T10:29:27.370
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0
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2011-02-24T19:21:18.893
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434445
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1
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296
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text
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Alright so here's my current code: <pre><code>import System.IO import System.Environment import System.Directory main = do unfiltered <- getArgs ; home <- getHomeDirectory ; let db = home ++ "/.grindstone" case unfiltered of (x:xs) -> return () _ -> error "No command given. See --help for more info." command:args <- getArgs createDirectoryIfMissing True db let check = case args of [] -> error "No arguments given. See --help for more info." _ -> do let (params@(param:_),rest) = span (\(c:_) -> c=='-') args if length params > 1 then error ("No arguments given for " ++ param) else do let (pArgs,_) = span (\(c:_) -> c/='-') rest return (param, pArgs) :: Either (IO ()) (String, [String]) let add = print "sup" let cmds = [("add", add)] let action = lookup command cmds case action of Nothing -> error "Unknown command." (Just action) -> action </code></pre> The main problem is with check. I tried implementing the Either type since I want it to either error out, or return something for another function to use, but, it's currently erroring out with: <pre><code>grindstone.hs:21:23: No instance for (Monad (Either (IO ()))) arising from a use of `return' at grindstone.hs:21:23-43 Possible fix: add an instance declaration for (Monad (Either (IO ()))) In the expression: return (param, pArgs) :: Either (IO ()) (String, [String]) In the expression: do { let (pArgs, _) = span (\ (c : _) -> ...) rest; return (param, pArgs) :: Either (IO ()) (String, [String]) } In the expression: if length params > 1 then error ("No arguments given for " ++ param) else do { let (pArgs, _) = ...; return (param, pArgs) :: Either (IO ()) (String, [String]) } </code></pre> I'm only starting out in haskell and haven't dealt too much with monads yet so just thought I'd ask on here. anyone have any ideas?
Tags
<haskell>
Title
Can't seem to implement Either correctly
singulars
PostAcceptedAnswerId
1. PO
 singulars
 PostTypePostTypeId
 PTAnswer
PostParentId
1. This table or related slice is empty.
PostTypePostTypeId
1. PTQuestion
UserLastEditorUserId
1. This table or related slice is empty.
UserOwnerUserId
1. USChris Bolton
plurals
PostLinksPostIdRelatedPostId
1. This table or related slice is empty.
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1. This table or related slice is empty.
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1. This table or related slice is empty.
PostsParentIdCreationDate
1. PO
 singulars
 PostTypePostTypeId
 PTAnswer
2. PO
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 PostTypePostTypeId
 PTAnswer
VotesPostIdCreationDate
1. VO
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 POCan't seem to implement Either correctly
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 This table or related slice is empty.
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 VTUpMod
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1. This table or related slice is empty.

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