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get rid of IO in [IO XXX]
by info＠maximka.de 04 Sep '17

04 Sep '17

What is the way to transform a list of [IO XXX] type to [XXX]? Thanks, Alexei

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I have a -> f a. How to get m a -> f m a ?
by Baa 04 Sep '17

04 Sep '17

Hello List! I have function a -> IO a. How to get function: Maybe a -> IO (Maybe a) ? I found in Haskell mails archive such thing: class IFunctor f where imap :: Idiom i => (s -> i t) -> f s -> i (f t) which looks similar, but I didn't find any helpfull instances of `IFunctor` class in its package (and unfortunately I don't know what are the indexed types: IMonad, IFunctor, etc). Sure, there is the primitive solution like: myfunc :: a -> IO a ... f x = case x of Nothing -> return Nothing Just x' -> Just <$> myfunc x' but more interesting is to know more standard and Haskelish solution (like lift's, etc). May be I miss something very obvious.. === Best regards, Paul

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Re: [Haskell-beginners] Restrict type in phantom data-type
by Baa 01 Sep '17

01 Sep '17

I pereputal emails. --- David, hello! 1. Is it the same/different as: data family Day a data Sunny data Rainy data instance Day Sunny = SunnyDay deriving Show data instance Day Rainy = RainyDay deriving Show ..and here you can not create `Day Int` object because no `Day Int` constructor (but you can create such constructor) ? Or in case with type families there is possibility to extend it to `Day Int` and in case with DayaKinds it's totally impossible? 2. I read somewhere (on forums) that restrictions on data types... I don't remember exactly, but something like they are not real restrictions or are related to old extension which is/will be deprecated. I'm not sure. Also, I'm not sure is it - in your example - restriction (constraint) or something else. Am I wrong? > This is maybe edging toward haskell-cafe territory, but you can > definitely do this in haskell. > > {-# LANGUAGE DataKinds, KindSignatures #-} > > data DayType = Sunny | Rainy > > data Day (a :: DayType) = Day > > > sunnyDay :: Day Sunny > sunnyDay = Day > > rainyDay :: Day Rainy > rainyDay = Day > > -- impossibleDay :: Day () > -- impossibleDay = Day > > On Fri, Sep 1, 2017 at 10:18 AM, Baa <aquagnu(a)gmail.com> wrote: > > Hello, List! > > > > For example, I have specialized (right nameis phantom?) type: > > > > data Day a = Day { ... no `a` here } > > data Sunny > > data Rainy > > > > joyToday :: Day Sunny -> IO () > > joyToday day = ... > > > > melancholyToday :: Day Rainy -> IO () > > melancholyToday day = ... > > > > And I can create (in spite of that it's phantom) some day: > > > > let day1 = Day {...} :: Day Sunny > > joyToday day1 > > > > but no problem to create `Day Int`, `Day Char`, etc which is > > pointless actually (sure "creator"-function can be exported from the > > module only, but I'm talking about type-level solution). > > > > I know that constraints (`... =>`) on data types are > > redundant/removed from the language. And I'm not sure how it's > > possible to restrict that parameter `a` (I know that it's possible > > to Java/C++/Perl6 (not sure), some other languages but how to add > > such restriction in Haskell? IMHO type families can help but I'm > > not sure how it will look (Sunny, Rainy are "nullary" type, so...). > > > > Is it possible for Haskell too? > > > > === > > Best regards, Paul > > _______________________________________________ > > Beginners mailing list > > Beginners(a)haskell.org > > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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Restrict type in phantom data-type
by Baa 01 Sep '17

01 Sep '17

Hello, List! For example, I have specialized (right nameis phantom?) type: data Day a = Day { ... no `a` here } data Sunny data Rainy joyToday :: Day Sunny -> IO () joyToday day = ... melancholyToday :: Day Rainy -> IO () melancholyToday day = ... And I can create (in spite of that it's phantom) some day: let day1 = Day {...} :: Day Sunny joyToday day1 but no problem to create `Day Int`, `Day Char`, etc which is pointless actually (sure "creator"-function can be exported from the module only, but I'm talking about type-level solution). I know that constraints (`... =>`) on data types are redundant/removed from the language. And I'm not sure how it's possible to restrict that parameter `a` (I know that it's possible to Java/C++/Perl6 (not sure), some other languages but how to add such restriction in Haskell? IMHO type families can help but I'm not sure how it will look (Sunny, Rainy are "nullary" type, so...). Is it possible for Haskell too? === Best regards, Paul

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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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