- Haskell-Cafe - Haskell.org

Reminder Dutch FP-Day 2019
by Bas Lijnse 24 Dec '18

24 Dec '18

Hi Everybody, Just a quick reminder. You can still register for the "FP Dag" 2019 on January 11 in Breda. The program and additional details are available at https://wiki.clean.cs.ru.nl/NL-FP_dag_2019 Registration closes on January 4. Participation is free, but registration is mandatory. To register simply reply to this e-mail with your name, affiliation (optional) and whether you intend to join us for dinner afterwards. Happy Holidays! Bas and Jan Martin

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Re: [Haskell-cafe] Upload new version of indexed-extras or let me take over
by Yuji Yamamoto 24 Dec '18

24 Dec '18

Confirmed! Thanks! 2018年12月24日(月) 16:04 Alexis Williams <alexis(a)typedr.at>: > It's done. > > Alexis Williams > Sent from Polymail > <https://polymail.io/?utm_source=polymail&utm_medium=referral&utm_campaign=s…> > > On Sun, Dec 23rd, 2018 at 11:1 PM, Yuji Yamamoto < > whosekiteneverfly(a)gmail.com> wrote: > >> Thank you Reiner, and Alexis. >> >> But I'm afraid the new version doesn't look uploaded yet... >> http://hackage.haskell.org/package/indexed-extras >> If you're okay, I'd like you to add me to the GitHub Project and the >> maintainer on Hackage. >> Here're my accounts: >> >> http://hackage.haskell.org/user/igrep >> https://github.com/igrep/ >> >> 2018年12月20日(木) 6:49 Reiner Pope <reiner.pope(a)gmail.com>: >> >> (haskell-cafe to bcc) >> >> Hi Yuji, thanks for the ping and sorry for the delay. >> >> Alexis (cc'd) took over ownership of indexed and indexed-extras at the >> beginning of this year, but apparently her permissions to upload packages >> to Hackage got reset by a Hackage reset. I've now fixed that, so she can >> reupload now (she was generally much more prompt than me). >> >> I'll upload a new version tonight. >> >> Not required, but if you like I'm also happy to add you on the GitHub >> project and the Hackage packages if you'd like to give me your details. >> >> Reiner >> >> On Sat, Dec 15, 2018 at 1:42 AM Yuji Yamamoto < >> whosekiteneverfly(a)gmail.com> wrote: >> >> Hello, this is a reminder of the message I sent (only to Reiner) about >> three weeks ago. >> >> I'm developing some libraries in Haskell with your package indexed-extras >> <http://hackage.haskell.org/package/indexed-extras>. >> I found the package can't be built with the latest bifunctor package due >> to its version constraint. >> I sent a pull request <https://github.com/reinerp/indexed-extras/pull/8> >> to fix the problem and it has already been merged. >> Can you release the new version on Hackage? >> It's now the biggest obstacle developing my libraries depending on it. >> >> If you can't maintain the package anymore, I'm willing to take over. >> Thanks in advance. >> >> >> -- >> 山本悠滋 >> twitter: @igrep >> GitHub: https://github.com/igrep >> GitLab: https://gitlab.com/igrep >> Facebook: http://www.facebook.com/igrep >> >> >> >> -- >> 山本悠滋 >> twitter: @igrep >> GitHub: https://github.com/igrep >> GitLab: https://gitlab.com/igrep >> Facebook: http://www.facebook.com/igrep >> > > -- 山本悠滋 twitter: @igrep GitHub: https://github.com/igrep GitLab: https://gitlab.com/igrep Facebook: http://www.facebook.com/igrep

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Upload new version of indexed-extras or let me take over
by Yuji Yamamoto 24 Dec '18

24 Dec '18

Hello, this is a reminder of the message I sent (only to Reiner) about three weeks ago. I'm developing some libraries in Haskell with your package indexed-extras <http://hackage.haskell.org/package/indexed-extras>. I found the package can't be built with the latest bifunctor package due to its version constraint. I sent a pull request <https://github.com/reinerp/indexed-extras/pull/8> to fix the problem and it has already been merged. Can you release the new version on Hackage? It's now the biggest obstacle developing my libraries depending on it. If you can't maintain the package anymore, I'm willing to take over. Thanks in advance. -- 山本悠滋 twitter: @igrep GitHub: https://github.com/igrep GitLab: https://gitlab.com/igrep Facebook: http://www.facebook.com/igrep

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handling NULL value in database query with Maybe (or other ...)
by Damien Mattei 20 Dec '18

20 Dec '18

Hi, i have this query in SQL used by my Haskell code: let name = "A 20" -- return the list of N°BD from WDS for a given name let qry_head_WDS = "select DNUM from WDS where DISC = ?" :: Query putStrLn "before query WDS" bd_rows_WDS <- query conn qry_head_WDS (Only (name::String)) putStrLn "after query WDS" forM_ bd_rows_WDS $ \(Only a) -> putStrLn $ Text.unpack a works well if there is no NULL values in a database, but if NULL value for field N°BD i got this exception at runtime: before query WDS *** Exception: UnexpectedNull {errSQLType = "String", errHaskellType = "Text", errFieldName = "DNUM", errMessage = "unexpected null in table WDS of database sidonie"} *Main> i need some code ,perheaps uing Maybe to handle the NULL value in the field N°BD i think Haskell have special feature to program this , i read the article: https://lukeplant.me.uk/blog/posts/null-pointers-vs-none-vs-maybe/ but for now i have no concrete solution.... Regards, Damien -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS

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Re: [Haskell-cafe] handling NULL value in database query with Maybe (or other ...)
by Damien Mattei 19 Dec '18

19 Dec '18

thank tou for your C/HASKELL "rosetta stone" about the Monad it helps a lot understanding the thing... i always thougt Monads are powerful but they are not easy sot manipulates syntactically for a beginner in Haskell. yes in fact what i secretly expected was a way to discard NULL fields automagically and it seems really possible with Haskell. Le 19/12/2018 11:27, Mario Lang a écrit : > Damien Mattei <mattei(a)oca.eu> writes: > >> using Nothing and Just compared to other language, it's the same than >> if i had written in C something like : >> if (x == NULL) printf("NULL\n"); >> else printf("%s",*x); >> >> i expected more from Haskell... > > Haskell actually offers more. > > 1. The type of "Maybe a" prevents you from ignoring the fact that "a" might > be null. In C, you can accidentally write > > printf("%s",*x); > > and risking a segfault. The compiler will not care. IN Haskell, the > types dont even allow you to write something like that. You would be > forced to check for Just or Nothing in one of several ways. > > Now, "Maybe a" doesn't only limit your abilities, it also offers you the > functions from the Functor, Applicative and Monad typeclasses. > > 2. Functor allows you to apply a function to the "a", as long as there is > some "a" inside the "Maybe a". So, "fmap f x" is roughly equivalent to: > > typeOfA f(typeOfA); > typeOfA *x func(typeOfA *x) { > if (x) { > typeOfA *val = malloc(sizeof(typeOfA)); > *val = f(*x); > return val; > } > return NULL; > } > func(x); > > (note that pure Haskell doesn't have assignment, so the translation to C > is a little bit more verbose then it might be if you were using values > on the stack.) i tried this: let resFMAP = fmap (\(Only a) -> (putStrLn ("a =" ++ Text.unpack a))) bd_rows_WDS but compile complains now that i use a Maybe variable instead of a plain one,strange because this exactly what i expected from Maybe use it to automatically discard the Nothing values (avoiding to putStrln them): Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:293:75: error: • Couldn't match type ‘Maybe Text’ with ‘Text’ Expected type: [Only Text] Actual type: [Only (Maybe Text)] • In the second argument of ‘fmap’, namely ‘bd_rows_WDS’ In the expression: fmap (\ (Only a) -> (putStrLn ("a =" ++ unpack a))) bd_rows_WDS In an equation for ‘resFMAP’: resFMAP = fmap (\ (Only a) -> (putStrLn ("a =" ++ unpack a))) bd_rows_WDS | 293 | let resFMAP = fmap (\(Only a) -> (putStrLn ("a =" ++ Text.unpack a))) bd_rows_WDS | ^^^^^^^^^^^ Failed, no modules loaded. bd_rows_WDS is defined like this with a Maybe: (bd_rows_WDS :: [Only (Maybe Text)]) <- query conn qry_head_WDS (Only (name::String)) anyway i know enought in haskell now to remove myself the Nothing (NULL) values of the list doing that (it worked): -- remove the records having N°BD NULL let fltWDS = Prelude.filter (\(Only a) -> case a of Nothing -> False Just a -> True) bd_rows_WDS > > So you do not need to write an explicit if in Haskell. The Functor > instance of Maybe "knows" how to deal will null cases. > In particular, if it encounters a Nothing, it returns a Nothing, > ignoring the provided function. > > 3. Applicative allows you to apply a function to the "a"s of several "Maybe a" > values, returning a Maybe wrapping the result of that function. > So "f <$> x <*> y" is equivalent to > > typeOfA f(TypeOfA); > typeOfA *func(typeOfA *x, typeOfA *y) { > if (x && y) { > typeOfA *val = malloc(sizeof(typeOfA)); > *val = f(*x, *y); > return val; > } > return NULL; > } > func(x, y); > > The amount of boilerplate eliminated grows. > > 3. Monad allows you to chain several null-checks without having to write > out the nesting. This time, f, g and h return a "Maybe a", not just an "a": > > do > a <- f x > b <- g a > h b > > or > > f x >>= \a -> g a >>= \b -> h b > > is equvalent to something like > > typeOfA *f(typeOfA); > typeOfA *g(typeOfA); > typeOfA *h(typeOfA); > typeOfA *func(typeOfA x) { > if (x) { > type *a = f(*x); > if (a) { > type *b = g(*a); > if (b) { > return f(*b); > } > } > } > return NULL; > } > > or, in Haskell: > > case f x of > Just a -> case g a of > Just b -> h b > Nothing -> Nothing > Nothing -> Nothing > > The Monad typeclass really pays off, as it saves you from doing the null > checks and dereferencing. The nested "if null then null" stuff goes all > away, and you can chain several functions together that return a > possible null value. > >> ok reading twice the article in link, i admit there should be some >> adavantage using Nothing instead of null, such as type, and genericity >> and not being spcecific to alloated memory as in C ... > > Exactly. As I see it, the Functor, Applicative and Monad typeclasses > of the Maybe type give you the tools > necessary to reduce the boilerplate that results from forcing you to > deal with each and every null case in your code. > -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS

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Timing out a pure evaluation of an expression I did not write myself
by Ryan Reich 19 Dec '18

19 Dec '18

I want to time out a pure computation. My experience, and that described in various previous questions here and elsewhere (the best of which is https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html) is that this doesn't always work: for instance, >>> timeout 1 $ evaluate $ let x = 0 : x in last x does not time out because, apparently, the fact that the expression evaluates in constant space (i.e. never allocates) means that it never yields to the timeout monitor thread that would kill it. The solution that is described in the other iterations is to embed checkpoints in the expression that do allocate, giving the RTS a chance to switch contexts. However, in my application, the expression is /arbitrary/ and I do not have the freedom to inject alterations into it. (Don't argue this point, please. The expression is arbitrary.) How can I time out a tight loop like the above? Clearly, it can be done, because I can, say, alt-tab over to another terminal and kill the process, which exploits the operating system's more aggressively pre-emptive scheduling. Is there a solution using bound threads, say 'forkOS' instead of 'forkIO' in the implementation of 'timeout'? Unix signals? Some FFI-based workaround? Etc. Keep in mind that notwithstanding that comment, I don't actually want to kill the whole process, but just the one evaluation. Thanks in advance, Ryan Reich

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Diagnosing build problems when cross-compiling hslua
by Vanessa McHale 18 Dec '18

18 Dec '18

Hi all, I recently ran into some troubles cross-compiling hslua for Raspberry Pi, viz. |cabal new-build --with-ghc arm-linux-gnueabihf-ghc --with-ghc-pkg arm-linux-gnueabihf-ghc-pkg Build profile: -w ghc-8.6.3 -O1 In order, the following will be built (use -v for more details): - hslua-1.0.1 (lib) (configuration changed) Configuring library for hslua-1.0.1.. Preprocessing library for hslua-1.0.1.. Types.hsc: In function ‘_hsc2hs_test1’: Types.hsc:59:12: error: ‘LUA_INTEGER’ undeclared (first use in this function) Types.hsc:59:12: note: each undeclared identifier is reported only once for each function it appears in Types.hsc:59:24: error: expected ‘;’ before numeric constant compilation failed| Any hints as to what's failing? Is this something wrong with hslua, hsc2hs, or cabal? And how would I go about fixing it? Cheers, Vanessa McHale

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How to unify these three types with identical structures into one definition?
by ducis 16 Dec '18

16 Dec '18

Hello, Say I have a certain type for parse trees: data Expr = Var String | Enclosed String Expr String | Prefix Expr Expr | Ternary Expr String Expr String Expr deriving (Show,Eq,Ord,Generic)--,Typeable) Then I want a new type where every occurence of "Expr" definition changed to "[Expr]": data ExprL = VarL String | EnclosedL String [Expr] String | PrefixL [Expr] [Expr] | TernaryL [Expr] String [Expr] String [Expr] deriving (Show,Eq,Ord,Generic) Sometimes, I also want to use a DList instead of list. data ExprD = VarD String | EnclosedD String (DList Expr) String | PrefixD (DList Expr) (DList Expr) | TernaryD (DList Expr) String (DList Expr) String (DList Expr) deriving (Show,Eq,Ord,Generic) They have exactly the same structure, is there a way to unify the three definitions into one? Furthurmore, is it possible to generalise the latter two for all Functors ? Thanks, ducis

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fmap use
by Damien Mattei 14 Dec '18

14 Dec '18

hello again, another question with Haskell, again with my database but that is again related to Monads: i have this code in main: rows <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278" let lg = Prelude.length rows let tst = if lg > 1 then "some" else "one or zero" it compiles and i assume it works, and i have a new (for those that have follow my previous posts) function like this one: -- this function will return the list of N°BD from WDS for a given name -- note: there could be multiple result for a given name in WDS due to components getBD_WDS :: Connection -> String -> IO Float getBD_WDS conn name = trace "Entering getBD_WDS" noBDfp where qry_head = "select DNUM from WDS where DISC = ?" :: Query bd_rows :: IO [Only Text] bd_rows = query conn qry_head (Only (name::String)) lg = fmap Prelude.length bd_rows -- tst = if (fmap lg) > 1 then "some" else "one or zero" noBDtxt :: IO Text -- noBDtxt = trace "lg " (fmap (fromOnly . Prelude.head) bd_rows) noBDtxt = (fmap (fromOnly . Prelude.head) bd_rows) -- noBDstr :: String -- noBDstr = Text.unpack noBDtxt noBDstr :: IO String noBDstr = fmap Text.unpack noBDtxt -- noBDfp = read $ noBDstr :: Float noBDfp :: IO Float noBDfp = trace "Exiting getBD" (fmap read noBDstr) the problem is that first i noticed that in a function to make working the same code than in Main i had to had almost always fmap before the line, i admit ,even if i'm teacher in functional programming in Scheme at University, i begin in Haskell and i don't always understand what i'm using, i try to learn it by instinct even if sometimes i read pages about Functor and Monads i have no time to get in the Mathematics theoretical knowledge behind Haskell, i try to learn by getting some solutions in Haskell-cafe and it works, i know now a lot more about Haskell than 2 weeks ago,so.... if someon could explain me a few more about fmap or just give what i must place in my function instead of let lg = Prelude.length rows let tst = if lg > 1 then "some" else "one or zero" i replace this in the function by: lg = fmap Prelude.length bd_rows OK for compilation, but then tst = if lg > 1 then "some" else "one or zero" does not compile: *Main> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:80:18: error: • No instance for (Ord (IO Int)) arising from a use of ‘>’ • In the expression: lg > 1 In the expression: if lg > 1 then "some" else "one or zero" In an equation for ‘tst’: tst = if lg > 1 then "some" else "one or zero" | 80 | tst = if lg > 1 then "some" else "one or zero" | ^^^^^^ UpdateSidonie.hs:80:23: error: • No instance for (Num (IO Int)) arising from the literal ‘1’ • In the second argument of ‘(>)’, namely ‘1’ In the expression: lg > 1 In the expression: if lg > 1 then "some" else "one or zero" | 80 | tst = if lg > 1 then "some" else "one or zero" | ^ Failed, no modules loaded. and even the "fmap miraculous function" as i call it no more works: tst = if (fmap lg) > 1 then "some" else "one or zero" Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:80:24: error: • Couldn't match expected type ‘a0 -> b0’ with actual type ‘IO Int’ • In the first argument of ‘fmap’, namely ‘lg’ In the first argument of ‘(>)’, namely ‘(fmap lg)’ In the expression: (fmap lg) > 1 | 80 | tst = if (fmap lg) > 1 then "some" else "one or zero" | ^^ Failed, no modules loaded. a solution to this proble i think , will help me a lot, i sticked with problems in haskell i would have already solved in Scheme or Python, that's really discouraging me of using Haskell, i'm obstinating myself because i really think it's a great language that push the programming to high levels of abstractions but for now it's a bit a nightmare, i must admit it... -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS

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MonadThrow, MonadReader and shake
by PICCA Frederic-Emmanuel 14 Dec '18

14 Dec '18

Hello , I would like your opinion, about this thread. http://mail.haskell.org/pipermail/beginners/2018-December/018403.html I do not know how to solve the issue. thanks Frederic

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