August 2017 - Beginners

Fwd: Path {Abs|Rel} {Dir|File} type question
by Yitzchak Gale 30 Aug '17

30 Aug '17

You wrote: >>> [In the path library] the `Path` has different representation >>> on Windows and POSIX which seems that no way >>> to use both on the same platform > IMHO it will be good to have like in Python: all kind of paths (NT, > POSIX, etc) available as well as local path (local platform > dependent)... Why like in Python? You mean like in Haskell. The standard and popular "filepath" library in Haskell does all that very nicely. The "path" library you are looking at is a newer library that wraps "filepath" and provides more type safety. It looks very nice. I hope the issue with multiplatform paths will soon be fixed, and then perhaps the path library will become more widely used. In the meantime, I suggest you use the "filepath" library. Regards, Yitz

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Path {Abs|Rel} {Dir|File} type question
by Baa 30 Aug '17

30 Aug '17

Hello, List! There is a library https://hackage.haskell.org/package/path-0.6.1 of Chris Done from FPCOMPLETE. `Path` - to be safe - can be constructed only with parsing (with TH or directly). OK. But the `Path` has different representation on Windows and POSIX which seems that no way to use both on the same platform (source of path can be network peer which has another platform). Does anyone use this library and if does - how do you workaround this? === Best regards, Paul

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Is it possible to "run" JSON Parser?
by Baa 30 Aug '17

30 Aug '17

Hello List again! :) If I have a `FromJSON` instance, with `parseJSON` which translates `Value` to MyType, how can I run this parser like I "run" state/writer/reader monads? I mean not decoding of a bytestring representation: decode::ByteString -> Maybe MyType but run::Value -> Maybe MyType The reason is that I have already collected Value's , and I have parsers, so what is the problem, it would seem, to run parser on the Value - to convert `Value` into `Maybe MyType`... Is it possible? === Best regards, Paul

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Multiline code in GHCi
by Shishir Srivastava 29 Aug '17

29 Aug '17

Hi, I am trying to run this basic multiline code in GHCi. I am not sure if this is even possible without writing it in a file and compiling it in a traditional way. ------- Prelude> :set +m Prelude> let main1 = do Prelude| a <- readLn Prelude| return a Prelude| Prelude> Prelude> main1 <interactive>:75:1: No instance for (Show (IO b0)) arising from a use of `print' In the first argument of `print', namely `it' In a stmt of an interactive GHCi command: print it ------- Could someone please confirm if the function can be defined and called like this in GHCi or point to where am going wrong. Thanks, Shishir Srivastava

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Installing Idris for Linux Dummies: "The following packages are likely to be broken..."
by Manny Romero 29 Aug '17

29 Aug '17

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Re: [Haskell-beginners] Installing Idris for Linux Dummies: "The following packages are likely to be broken..."
by Manny Romero 29 Aug '17

29 Aug '17

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How to parse JSON into existential type
by Baa 29 Aug '17

29 Aug '17

Hello List! In some of my JSONs I have part "list of any values", for example: [1, "hi", "false", 2.71] To parse it I create type like: data RespItem = forall a. FromJSON a => RespItem { riItem :: [a] } ,------------------------------------------------------------------- | btw, I try `(FromJSON a, Show a, Generic a) => ...` also but don't | see any difference, so I'm not sure what is right constraints list. `------------------------------------------------------------------- And GHC does not allow to derive automatically with "deriving" Show and Generic instances for existential types, as I see. So I write: instance Generic RespItem instance Show RespItem Also this type `RespItem` in part of another one: data Resp = Resp { rHeadings :: [T.Text] , rRow :: [RespItem] } deriving (Generic, Show) Interesting is that now auto-`deriving` works! OK, but I need to to instantiate `FromJSON` for RespItem because next step must be `FromJSON` for Resp. So I'm trying: instance FromJSON RespItem where parseJSON ??? = ??? and here I get different errors, and absolutely don't understand how to parse it, I get "ambiguous type error..." and so on (which is right, sure). === Best regards, Paul

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Re: [Haskell-beginners] Beginners Digest, Vol 110, Issue 20
by Dmitry Mamontov 25 Aug '17

25 Aug '17

Hello, Maybe, using fmap would be enough? data Proxy = Proxy String f :: Maybe Proxy -> Maybe String f = fmap (\(Proxy a) -> a) 2017-08-25 15:00 GMT+03:00 <beginners-request(a)haskell.org>: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Maybe a -> Maybe b (PICCA Frederic-Emmanuel) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 25 Aug 2017 08:05:34 +0000 > From: PICCA Frederic-Emmanuel > <frederic-emmanuel.picca(a)synchrotron-soleil.fr> > To: "beginners(a)haskell.org" <beginners(a)haskell.org> > Subject: [Haskell-beginners] Maybe a -> Maybe b > Message-ID: > <A2A20EC3B8560D408356CAC2FC148E53BB41B8D4@SUN-DAG3. > synchrotron-soleil.fr> > > Content-Type: text/plain; charset="us-ascii" > > Hello, I have this > > data Proxy = Proxy String > > And I want to write a function > > f :: Maybe Proxy -> Maybe String > f ma = case ma of > (Just (Proxy a)) -> Just a > Nothing -> Nothing > > I was wondering if there is no simpler way to do this ? > > thanks > > Frederic > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > > ------------------------------ > > End of Beginners Digest, Vol 110, Issue 20 > ****************************************** >

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Maybe a -> Maybe b
by PICCA Frederic-Emmanuel 25 Aug '17

25 Aug '17

Hello, I have this data Proxy = Proxy String And I want to write a function f :: Maybe Proxy -> Maybe String f ma = case ma of (Just (Proxy a)) -> Just a Nothing -> Nothing I was wondering if there is no simpler way to do this ? thanks Frederic

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Re: [Haskell-beginners] Beginners Digest, Vol 110, Issue 20
by Andrey Klaus 25 Aug '17

25 Aug '17

Hello f ma = fmap ((Proxy a) -> Just a) ma or even shorter f = fmap ((Proxy a) -> Just a) 2017-08-25 15:00 GMT+03:00 <beginners-request(a)haskell.org>: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Maybe a -> Maybe b (PICCA Frederic-Emmanuel) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 25 Aug 2017 08:05:34 +0000 > From: PICCA Frederic-Emmanuel > <frederic-emmanuel.picca(a)synchrotron-soleil.fr> > To: "beginners(a)haskell.org" <beginners(a)haskell.org> > Subject: [Haskell-beginners] Maybe a -> Maybe b > Message-ID: > <A2A20EC3B8560D408356CAC2FC148E53BB41B8D4@SUN-DAG3. > synchrotron-soleil.fr> > > Content-Type: text/plain; charset="us-ascii" > > Hello, I have this > > data Proxy = Proxy String > > And I want to write a function > > f :: Maybe Proxy -> Maybe String > f ma = case ma of > (Just (Proxy a)) -> Just a > Nothing -> Nothing > > I was wondering if there is no simpler way to do this ? > > thanks > > Frederic > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > > ------------------------------ > > End of Beginners Digest, Vol 110, Issue 20 > ****************************************** >

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