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optparse-applicative in stack LTS <= 9.3
by Viktor Dukhovni 06 Sep '17

06 Sep '17

FYI, the version of optparse-applicative (0.13.2.0) in Stack LTS-9.3 and earlier does not appear to handle the "noIntersperse" info modifier correctly. Instead of stopping parsing flags after the first non-option argument, it instead simply never parses any options. This is fixed in 0.14.0.0, which is in "Stackage Nightly". Perhaps someone else will run into the same issue. And it would of course be nice to see 0.14.0.0 appear in an upcoming LTS version (say 9.4). -- Viktor. Demo program, this fails to even process "--help" with the outdated optparse-applicative, which just reports all arguments as positional: module Main (main) where import Options.Applicative import Data.Monoid ((<>)) data Opts = Opts { bl :: Bool , fl :: Bool , st :: Maybe String , pos :: String , args :: [String] } deriving (Show) main :: IO () main = do let parse = Opts <$> switch ( long "bool" <> short 'b' ) <*> switch ( long "flag" <> short 'f' ) <*> optional ( strOption ( long "string" <> short 's' ) ) <*> strArgument ( metavar "FILE" ) <*> many ( strArgument ( metavar "ARGS..." ) ) i = info (helper <*> parse) $ fullDesc <> noIntersperse <> header "Option parser" <> progDesc "- test POSIX-style options" execParser i >>= print -- Viktor.

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[ANN] Haskell Workshop in Vancouver, BC
by Román González 06 Sep '17

06 Sep '17

`Hello, Is my pleasure to share the news about a new Study Group to learn Haskell using "The Haskell Book". If you live in Vancouver, British Columbia, or know someone that lives here and might be interested in learning Haskell, please share this link with them: https://workshops.vanfp.org/haskell Best Regards. Roman.-

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 169, Issue 4
by Michal J Gajda 06 Sep '17

06 Sep '17

Hi I can tell a few words about ETL and ML problems in in practice. -- Best regards Michal

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Haskell Numerics / Data Science at ICFP
by dominic＠steinitz.org 05 Sep '17

05 Sep '17

Hello Fellow Haskellers, The organisers of ICFP have very kindly given a room for anyone interested in Haskell Numerics / Data Science on Saturday 9th September 1230-1400. All are welcome and there will be a sign at the reception desk on Saturday morning. I will present a few slides on what we currently have available in our armoury and what might be possible. I plan to spend less than 10 minutes on this. Trevor McDonell has kindly offered to say a few words on accelerate. If anyone wants to give a lightning talk please feel free. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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How to import hidden modules
by Ricardo Peña 03 Sep '17

03 Sep '17

Hi, I need to define a class instance for the algebraic datatype Data.Map.Map in the Hierarchical GCH libraries, but I do not find the way to import Data.Map.Internal which it is assumed to be hidden. Is there any way to access to this definition? Thanks -Ricardo

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Fwd: issues with ad-hoc polymorphic typeclass instances
by Weylin Lam 02 Sep '17

02 Sep '17

---------- Forwarded message ---------- From: Weylin Lam <lam.dev.hs(a)gmail.com> Date: Sat, Sep 2, 2017 at 8:37 AM Subject: Re: [Haskell-cafe] issues with ad-hoc polymorphic typeclass instances To: Jonas Scholl <anselm.scholl(a)tu-harburg.de> i actually hadn't thought of that! It's an good idea, indeed. The issue is, in terms of DSLs, there'd be no way to prevent a potential mix between the more abstract language and its translation, because expressions are connected to their DSLs via constraints, and here without a class for the language OpEnv, there'd be no way to prevent say the use of ask mixed with addWithEnv, since to use the latter i'd need to put a (Op e, OpEnv e m) constraint... Also, without class i simply couldn't define and use the language OpEnv as is, regardless of the existence of its implementation in terms of Op and Env. Sure for my trivial example it's not very important, but it *is* just a trivial example, it doesn't mean it's always irrelevant... at least i think so. On Sat, Sep 2, 2017 at 7:48 AM, Jonas Scholl <anselm.scholl(a)tu-harburg.de> wrote: > Do addWithEnv and mulWithEnv need to be functions in a type class? > Couldn't you just define them as normal functions? Then all the problems > with undecidable instances are gone. I also don't see how having them in > a type class helps you in any way if you want to define such a far > reaching instance for it. Are there cases where you would need another > instance? If not, then putting it in a plain function is the way to go. > If yes, then you can not define Op and Env for the type of your OpEnv > instance, which seems counterintuitive. Otherwise you would run into > overlapping instances and I don't know if you want that, probably not. > > On 09/02/2017 07:10 AM, Weylin Lam wrote: > > Hello ! > > > > I'm trying to write a polymorphic class instance in function of another > > class. So far it's pretty hard, I have trouble with headsizes and so on. > > I actually had to switch to type families pretty fast, and i'm still not > > out of the woods. > > http://lpaste.net/2650337298029215744 > > <http://lpaste.net/2650337298029215744> > > > > The context: > > It concerns "tagless final interpreters" for EDSLs as described > > in http://okmij.org/ftp/tagless-final/course/lecture.pdf > > <http://okmij.org/ftp/tagless-final/course/lecture.pdf> > > A comparison with free > > monads: http://yowconference.com.au/slides/yowlambdajam2016/ > Hopkins-StopPayingForFreeMonads.pdf > > <http://yowconference.com.au/slides/yowlambdajam2016/Hopkins > -StopPayingForFreeMonads.pdf> > > > > The technique is fairly simple: instead of using ADTs to encode the > > operations of the language, we use haskell's class system. Languages > > become type classes, and interpreters are the instances which give > > various different meanings to the language expressions, which become > > simple haskell, ad-hoc polymorphic values. > > > > It has strong advantages. For example, to write some expression in the > > composition of two languages (two typeclasses), you only need to combine > > the constraints: > > class WriteFile h m where > > writeFile :: h -> String -> m () > > class ReadFile h m where > > readFile :: h -> m String > > > > -- we use Monad m to have access to monadic operations, but the language > > itself > > -- does not care that m be a monad or not. > > myExpression :: (WriteFile h m, ReadFile h m, Monad m) => h -> h -> > > String -> m String > > myExpression h h2 str = writeFile h str *> readFile h2 > > > > The fusion of both languages is utterly seamless, which is the beauty of > > typeclasses. > > > > When writing DSLs, there are at least two basic operations that need to > > be done: combining DSLs to create richer languages (and as just shown, > > it's really simple with this technique), and translating one DSL into > > another. > > > > That latter operation between DSLs is a bit more complicated with > > classes. I haven't found examples of how to do so in the wild (the > > technique doesn't seem very much used, esp compared with free monads), > > so if anybody knows how to do it or has references on that, it'd be much > > appreciated. > > > > Theoretical example of what i'm trying to perform: > > class A a where > > subtract :: a -> a -> a > > class B a where > > add :: a -> a -> a > > neg :: a -> a > > > > -- interpreter from A to B: (doesn't work of course) > > class (B a) => A a where > > subtract x y = add x (neg y) > > > > -- interpreter for B in terms of Int: > > instance B Int where > > add = (+) > > neg = negate > > > > expression :: (A a) => a > > expression = subtract (subtract 4 5) 6 > > -- to get it to be interpreted, we need to select the type. usually we > > use a monomorphic version of id: > > asInt = identity :: Int -> Int > > intExp = asInt expression :: Int > > > > For the instance B Int to be re-usable as instance/interpreter of (A > > Int) expressions, we need a way to write an interpretation of A a in > > terms of any pre-existing interpretation of B b. > > Usually there are some issues, the need to wrap the types into newtype > > wrappers among others. > > > > But in the case i'm trying to solve, it still doesn't work. To see where > > i'm stuck, see above my lpaste. > > > > Any help or ideas would be very welcome! :) Thanks a lot in advance. > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > To (un)subscribe, modify options or view archives go to: > > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > > Only members subscribed via the mailman list are allowed to post. > > > > > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. >

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issues with ad-hoc polymorphic typeclass instances
by Weylin Lam 02 Sep '17

02 Sep '17

Hello ! I'm trying to write a polymorphic class instance in function of another class. So far it's pretty hard, I have trouble with headsizes and so on. I actually had to switch to type families pretty fast, and i'm still not out of the woods. http://lpaste.net/2650337298029215744 The context: It concerns "tagless final interpreters" for EDSLs as described in http://okmij.org/ftp/tagless-final/course/lecture.pdf A comparison with free monads: http://yowconference.com.au/slides/ yowlambdajam2016/Hopkins-StopPayingForFreeMonads.pdf The technique is fairly simple: instead of using ADTs to encode the operations of the language, we use haskell's class system. Languages become type classes, and interpreters are the instances which give various different meanings to the language expressions, which become simple haskell, ad-hoc polymorphic values. It has strong advantages. For example, to write some expression in the composition of two languages (two typeclasses), you only need to combine the constraints: class WriteFile h m where writeFile :: h -> String -> m () class ReadFile h m where readFile :: h -> m String -- we use Monad m to have access to monadic operations, but the language itself -- does not care that m be a monad or not. myExpression :: (WriteFile h m, ReadFile h m, Monad m) => h -> h -> String -> m String myExpression h h2 str = writeFile h str *> readFile h2 The fusion of both languages is utterly seamless, which is the beauty of typeclasses. When writing DSLs, there are at least two basic operations that need to be done: combining DSLs to create richer languages (and as just shown, it's really simple with this technique), and translating one DSL into another. That latter operation between DSLs is a bit more complicated with classes. I haven't found examples of how to do so in the wild (the technique doesn't seem very much used, esp compared with free monads), so if anybody knows how to do it or has references on that, it'd be much appreciated. Theoretical example of what i'm trying to perform: class A a where subtract :: a -> a -> a class B a where add :: a -> a -> a neg :: a -> a -- interpreter from A to B: (doesn't work of course) class (B a) => A a where subtract x y = add x (neg y) -- interpreter for B in terms of Int: instance B Int where add = (+) neg = negate expression :: (A a) => a expression = subtract (subtract 4 5) 6 -- to get it to be interpreted, we need to select the type. usually we use a monomorphic version of id: asInt = identity :: Int -> Int intExp = asInt expression :: Int For the instance B Int to be re-usable as instance/interpreter of (A Int) expressions, we need a way to write an interpretation of A a in terms of any pre-existing interpretation of B b. Usually there are some issues, the need to wrap the types into newtype wrappers among others. But in the case i'm trying to solve, it still doesn't work. To see where i'm stuck, see above my lpaste. Any help or ideas would be very welcome! :) Thanks a lot in advance.

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To merge package documentation with the platform doc tree
by John Beattie 01 Sep '17

01 Sep '17

Hi, Suppose I download a package and install it. I would like to be able to have the html documentation for that package and I would like it to be integrated with the local doc tree in the haskell platform. Is this a recognised usecase and is there a recommended way to do that? My first thought was to use haddock but I haven't managed to work out if there is a combination of haddock options which would get me there. As an alternative, I was wondering about cloning the GHC platform source and pasting the new library into the source tree alongside the other libraries and building the documentation. I might have to tweak a makefile or somesuch but would that work? Thanks, John Beattie

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megaparsec 6 migration
by ☂Josh Chia (謝任中) 01 Sep '17

01 Sep '17

I'm trying to migrate my codebase from megaparsec 5 to megaparsec 6. There appears to be major changes. For example, Text.Megaparsec.String.Parser & Text.Megaparsec.Error.Dec have both disappeared. I'm sure I could slowly plod through the new documentation, figure out the types and figure out how to migrate the code on my own, but examples & tutorials would be more time-efficient. Are there examples or tutorials compatible with megaparsec 6? For a start, what should I do with the above two things that have 'disappeared'? What do I replace them with? Josh

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Crossreferenced codebase for Stackage LTS 9.2 available
by Robin Palotai 01 Sep '17

01 Sep '17

To find out who else uses your favorite functions and how: http://stuff.codereview.me/lts/9.2 . For example, click 'async' at http://stuff.codereview.me/lts/9.2/#async-2.1.1.1/Control/Concurrent/Async.… to get all the callsites of that method. There are additional files available at http://stuff.codereview.me/ if you want to serve the xrefs locally, or merge your own packages into the index (assuming you use the same stackage lts version). A request: the crossreferencing is pretty usable, but far from complete. If you find any issues (missing crossreferences or bad ones), please have a look at the buglist at https://github.com/google/haskell-indexer/issues, and thumbs-up any that you find hurting, or open new ones! Thank you & happy code browsing, Robin

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