- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Places to write and publish projects that "explain ideas"?
by Siddharth Bhat 25 Dec '18

25 Dec '18

Regarding (1), as far as I'm aware, the requirements are flexible, but the general requirement is "published a paper at a good venue, either journal or conference". This implementation was a hobby project of mine, but I was hoping to be able to justify spending more time on it by finding a venue to publish it at. As for (2), it's mostly expository. The techniques that I am interesting in showing off have no known "simple" implementations as far as I'm aware -- scalar evolution, polyhedral compilation, some kinds of inter-procedural analyses, SSA, etc. do not have simple implementations. Indeed, the goal is to show how to write *optmising* compilers. Most compilers books teach one how to write *a compiler, *while there's a second course usually on compiler optimisation. I don't know of readable, clean implementations of, say, the SSA construction algorithm, or the scalar evolution analysis. Regarding nanopass: I think it makes a lot of sense as a philosophy as a way to architect compilers. However, there are a lot of nice haskell-isms to write compilers which are scattered throughout the literature, as far as I can tell: Hoopl, trees that grow, much of Matt might's work on abstracting abstract interpreters, Scrap-your-boilerplate style techniques, equality saturation, etc. which are all fantastic, but I've never seen them under the same umbrella. So that's a sketch of the general story I want to tell :) Thanks. ~Siddharth On Tue, Dec 25, 2018 at 7:30 PM Richard O'Keefe <raoknz(a)gmail.com> wrote: > > Question 1: what does your department require? The first person to > consult > is your supervisor. Helping students figure out where to publish is > definitely > part of a supervisor's job (speaking as a former supervisor of PhD > students). > > Question 2: what is the TOPIC of your publication? Since you are merely > (!) > demonstrating known techniques on a toy language, does it really count as > work on compilers? Is it software engineering? Is it computer science > education? Is the aim to develop something more maintainable? What is the > quality metric by which your work should be judged? > > Observation: if you haven't already read the "Nanopass" paper, > https://www.cs.indiana.edu/~dyb/pubs/nano-jfp.pdf , > you should do so now. http://nanopass.org/ is the associated web site. > There is also a mailing list, > https://groups.google.com/forum/#!forum/nanopass-framework > I think your aims are sufficiently close to theirs for you to have > interesting things to say to each other, despite you using Haskell and > them having used Scheme. > > >

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Places to write and publish projects that "explain ideas"?
by Siddharth Bhat 25 Dec '18

25 Dec '18

Hello all, I have somewhat of an unorthodox problem. I've been writing on and off, a series on implementing compilers "beautifully" in haskell, called tiny-optimising-compiler <http://github.com/bollu/tiny-optimising-compiler>. They're literate haskell files, with the aim of explaining the really elegant ideas that exist in compilers literature - data flow analysis, abstract interpretation, SSA, continuations, scalar evolution, and some more slightly out-there / research-y things, like polyhedral compilation, equality saturation. However, I'm also a research student at my university, and am expected to publish before I graduate. I was looking for possible places to publish a project such as this, whose selling point would be "explains things elegantly, and possibly rewords standard things to nice looking haskell". Are there places where one could conceivably publish about such a project? If not, I forsee myself not being able to finish this project for a while longer, and that would make me sad. Thanks, and a merry christmas to all, ~Siddharth

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Happy Haskell Programming for GHC 8.x
by Kazu Yamamoto 25 Dec '18

25 Dec '18

Hello Cafe, I'm happy to announce that Happy Haskell Programming (HHP) version 0.0.0 has been released: http://hackage.haskell.org/package/hhp In short, HHP is a fork from ghc-mod 4.1.6 and re-written for all GHC 8.x. Emacs front end is included. For more information, please read: https://github.com/kazu-yamamoto/hhp Merry Christmas and Happy Haskell Programming on Emacs with GHC 8.x! --Kazu

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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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