April 2016 - Haskell-Cafe

ghc-mod errors in Emacs
by Đorđe Grbić 21 Apr '16

21 Apr '16

Greetings dear Haskell community, I'm trying to use Spacemacs for Haskell development but as soon as I open a .hs file ghc-mod dumps errors: ghc-mod: <command line>: cannot satisfy -package-id base-4.6.0.1-8aa5d403c45ea59dcd2c39f123e27d57 (use -v for more information) <command line>: cannot satisfy -package-id base-4.6.0.1-8aa5d403c45ea59dcd2c39f123e27d57 (use -v for more information) <command line>: cannot satisfy -package-id base-4.6.0.1-8aa5d403c45ea59dcd2c39f123e27d57 (use -v for more information) I have ghc-mod version: 5.5.0.0 compiled by GHC 7.8.4. GHC version : 7.8.4 Emacs: 24.5.1 Spacemacs: 0.105.18 OS: Ubuntu 14.04.4 LTS Any help is appreciated, Djordje Grbic

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Re: [Haskell-cafe] Documenting extensions
by Doug McIlroy 21 Apr '16

21 Apr '16

>>> Later I discovered >>> that by default GHC rejected almost all my report-conforming (H98 >>> and H2010) code. >> >> Do you mean that GHC can't compile Haskell2010 conformant code? >> Can you please post (or point to) some examples? > > This page [1] lists the ways in which GHC diverges from the standard. > > Erik > > [1] https://downloads.haskell.org/~ghc/master/users-guide/bugs.html The main problem was that my code relied on constraint (Num a) implying (Eq a) per the standard. I also had context clauses in data declarations. Thus I had to make pervasive, albeit trivial, changes to get past GHC's nonconformance. (Presumably option -XHaskell2010 would have papered over the trouble, but I prefer to maximize code portability.) Doug McIlroy

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Hugs
by David Feuer 21 Apr '16

21 Apr '16

Apparently, Eric Meijer is now teaching a large class using Hugs, which leads to many of his students asking questions about it on StackOverflow. I therefore suggest that we change the status of Hugs on the website from "is no longer in development" to "has been maintained by Eric Meijer since 2015". I think we should also include on the website a statement to the effect that people seeking help using Hugs should contact Eric Meijer directly, and provide his email address to facilitate that. Obviously, he'll need full write access to the Hugs source repo to fulfill his duties as maintainer. Yes, this is all a bit tongue-in-cheek, but I really do think that pointing students to an unmaintained language implementation (regardless of the pedagogical reasons) has negative consequences for the functional programming community as a whole.

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[ANN] Stratosphere: AWS CloudFormation EDSL in Haskell
by David Reaver 20 Apr '16

20 Apr '16

Github: https://github.com/frontrowed/stratosphere Hackage: https://hackage.haskell.org/package/stratosphere CloudFormation is a system that provisions and updates Amazon Web Services (AWS) resources based on declarative templates. Common criticisms of CloudFormation include the use of JSON as the template language and limited error-checking, often only available in the form of run-time errors and stack rollbacks. By wrapping templates in Haskell, we are able to easily construct them and help ensure correctness. The goals of stratosphere are to: - Build a Haskell EDSL to specify CloudFormation templates. Since it is embedded in Haskell, it is type-checked and generally much easier to work with than raw JSON. - Have a simple checking/linting system outside of the types that can find common errors in templates. - Be able to also read valid CloudFormation JSON templates so they can be type-checked. This also gives us free integration tests by using the huge amount of example templates available in the AWS docs. Most of the commonly used CloudFormation resources are implemented, and adding new resources is very straightforward. (We created a web scraper that generates a JSON file from a given CloudFormation documentation page URL, and from that we generate a Haskell module.) So far, we have implemented resources we use at Front Row Education, and we will add more resources over time. The library is very much in a usable state as-is. However, we want to make any sweeping changes while it is still young. If you have any suggestions at all, you want us to add your favorite resource, or if you find a bug, please file an issue on Github! Also, we want to give a huge thanks to Brendan Hay, the author of amazonka, for his ideas and feedback on the project.

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Typeclass Constraint Causing Typechecker to Fail
by James M 19 Apr '16

19 Apr '16

Below I have a contrived example. Please do not take this to be real world code. f :: (Show a) => Int -> (Int -> a) -> Int -> IO a f i g x = do print i return $ g (x + i) foo :: Bool -> IO (Either [Int] [String]) foo b = do let helper = f 2 if b then Left <$> sequence (fmap (helper negate) [0,1]) else Right <$> sequence (fmap (helper show) [0,1]) The above will fail stating Example.hs:14:35: Couldn't match type ‘Int’ with ‘[Char]’ Expected type: Int -> IO String Actual type: Int -> IO Int In the first argument of ‘fmap’, namely ‘(helper show)’ In the first argument of ‘sequence’, namely ‘(fmap (helper show) [0, 1])’ Example.hs:14:42: Couldn't match type ‘[Char]’ with ‘Int’ Expected type: Int -> Int Actual type: Int -> String In the first argument of ‘helper’, namely ‘show’ In the first argument of ‘fmap’, namely ‘(helper show)’ However, if I simply change f's type signature to not have the typeclass constraint, the type checker is happy: f :: Int -> (Int -> a) -> Int -> IO a Another possibility to remove the problem is to remove the let statement and instead put the entire expression in. foo :: Bool -> IO (Either [Int] [String]) foo b = do if b then Left <$> sequence (fmap (f 2 negate) [0,1]) else Right <$> sequence (fmap (f 2 show) [0,1]) This seems to be a bug in the typechecker, and maybe it is a well known issue. Can someone please confirm that this is a bug and whether or not it is known? James

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Re: [Haskell-cafe] Is it possible to make lazy combinators for IO?
by David Feuer 19 Apr '16

19 Apr '16

lazyMap :: (a -> b) -> IO a -> IO b, hypothetically. Let me take a step back. There are two ways I know of to use the result of an IO action. The most common way, by far, is to use >>= to send it on to the next computation. The other notable way is to use fixIO. No part of the result of an IO action becomes available until that action "completes" (in the shady case of lazy IO, this may be immediate, but I'm ignoring lazy IO for this discussion). This causes the well-known inefficiency of mapM in IO: individual results cannot be consed until the entire computation is complete. It seems to cause much more painful effects for fixIO--lazy programming techniques may simply be unavailable. For example, it's possible for a pure computation using a queue in a single-threaded fashion to represent that queue as a list, tying the knot to pass the list of all enqueued elements to the beginning. If an IO computation wants to use a queue, this technique does not seem to be available. The knot can be tied with fixIO, but conses are delayed until it's too late--attempting to dequeue an element will block forever. I'm wondering if there's some way to make this sort of thing work, either with plain Haskell IO or some thin layer built on top. On Apr 18, 2016 5:12 PM, "David Turner" <dct25-561bs(a)mythic-beasts.com> wrote: > Sorry, you've lost me. I suspect you're trying to give a minimal example > of the problem you're having, but you've stripped away too much context. > What are lazyMap and f? At least, what are their types? > On 18 Apr 2016 22:03, "David Feuer" <david.feuer(a)gmail.com> wrote: > >> Consider the implementation of `second` for pairs: >> >> second f ~(a,b) = (a, f b) >> >> Now >> >> fix $ second (3 :) >> >> Will be (undefined, [3,3,....]) >> >> Translating this to IO, I'd want >> >> lazyMap f undefined >> >> to produce as much as possible of the result, although it cannot produce >> the final State# RealWorld token. >> On Apr 18, 2016 4:47 PM, "David Turner" <dct25-561bs(a)mythic-beasts.com> >> wrote: >> >>> You can't know that the final result of the computation (x `seq` >>> (3:...)) will begin with 3, because sometimes it doesn't! More >>> specifically, it diverges (without yielding the 3) if x diverges. >>> >>> I don't think this is anything special about mfix: (let x = x `seq` >>> 3:... in x) also diverges for the same reason. >>> >>> Hope that helps, >>> >>> David >>> On 18 Apr 2016 21:19, "David Feuer" <david.feuer(a)gmail.com> wrote: >>> >>>> If >>>> >>>> f :: a -> IO a >>>> >>>> for some a, and I want to use >>>> >>>> mfix f >>>> >>>> then f must not inspect its argument in any way, or the computation >>>> will get stuck. In some cases, this seems a bit harsh. For example, >>>> >>>> mfix (\x -> fmap (3 :) (x `seq` readLn)) >>>> >>>> looks perfectly reasonable. There is no need to inspect the return [] >>>> action to know that the final result of the computation will begin >>>> with 3:. Is there a lazy IO mapping function somewhere that can work >>>> such magic? >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe(a)haskell.org >>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>>> >>>

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Existential quantification of config data types
by Charlie Durham 19 Apr '16

19 Apr '16

Hi, I'm running into a weird problem i've never had before. I want to define a datatype like: data Network = forall a. Layer a => Network [a] for which I have things that implement Layer, like data TypicalLayer = ... instance Layer TypicalLayer where ... with this, I want to be able to load a Network from a config file where Network can consist of different data types all implementing Layer. Is there a nice way to do this without making a data type that combines all of my layer types, something like: data LayerType = TypicalLayerType TypicalLayer | AnotherLayerType AnotherLayer ... deriving (Show,Read) Thanks for the help! Charlie Durham

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Re: [Haskell-cafe] Sending email
by David Escobar 19 Apr '16

19 Apr '16

No explicit mention is made anywhere in the documentation about TLS or even SSL, so perhaps not? Some libraries I've come across specifically mention that they don't support TLS or SSL. Being relatively new to this part of Haskell, what is the most standard library the community uses for email that supports modern protocols such as those used by GMail? Thanks. On Mon, Apr 18, 2016 at 2:49 PM, Alex Feldman-Crough <alex(a)fldcr.com> wrote: > Does the library support TLS? Does it have to be configured differently? > It sounds like a negotiation error to me. > On Mon, Apr 18, 2016 at 2:43 PM David Escobar <davidescobar1976(a)gmail.com> > wrote: > >> Hi everyone, >> I'm trying to use the *Network.Mail.SMTP* library to send email: >> >> *{-# LANGUAGE OverloadedStrings #-}* >> >> *module Main where* >> >> *import Control.Exception* >> >> *import qualified Data.Text as T* >> *import qualified Data.Text.Lazy as LT* >> *import Network.Mail.SMTP* >> >> *main :: IO ()* >> *main = do* >> * sendEmail (“Person sender”, “sender(a)somewhere.com >> <sender(a)somewhere.com>”)* >> * [(“Person recipient“, “recipient(a)somewhere.com >> <recipient(a)somewhere.com>”)]* >> * "Test email"* >> * "Some message goes here."* >> >> >> *sendEmail :: (T.Text, T.Text) -> [(T.Text, T.Text)] -> T.Text -> T.Text >> -> IO ()* >> *sendEmail (fromName, fromEmail) toAddresses subject' body' = do* >> * let toNameAddrs = map (\(toName, toEmail) -> Address (Just toName) >> toEmail) toAddresses* >> * msg = simpleMail (Address (Just fromName) fromEmail)* >> * toNameAddrs* >> * []* >> * []* >> * subject'* >> * [ plainTextPart $ LT.fromStrict body' ]* >> * result <- try $ sendMailWithLogin' "smtp.gmail.com >> <http://smtp.gmail.com>"* >> * 465 -- SSL port* >> * “sender_login”* >> * “sender_password”* >> * msg :: IO (Either SomeException ())* >> * case result of* >> * Left e -> putStrLn $ "Exception caught: " ++ (displayException e)* >> * Right _ -> putStrLn "Sent email successfully."* >> >> >> The program compiles, but when I run it, I get: >> >> *Exception caught: <socket: 49>: Data.ByteString.hGetLine: end of file* >> >> I tried using the TLS port of 587, but then I just get an authentication >> failure. Am I using the wrong library or is it just the wrong >> configuration. Thanks. >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > -- > Alex Feldman Crough > 602 573-9588

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diffs on HaskellWiki
by Patrick Pelletier 19 Apr '16

19 Apr '16

I've noticed that when viewing changes on HaskellWiki, the diffs are missing. We had a similar problem on the OpenSSL wiki (which, like HaskellWiki, is based on MediaWiki), and the sysadmin said he fixed the problem by switching from the external diff engine to the internal diff engine. Is there someone who could look into this and possibly make that change to the wiki's configuration? I looked around the HaskellWiki and couldn't find any mention of how to contact a maintainer. Thanks, --Patrick

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Is it possible to make lazy combinators for IO?
by David Feuer 18 Apr '16

18 Apr '16

If f :: a -> IO a for some a, and I want to use mfix f then f must not inspect its argument in any way, or the computation will get stuck. In some cases, this seems a bit harsh. For example, mfix (\x -> fmap (3 :) (x `seq` readLn)) looks perfectly reasonable. There is no need to inspect the return [] action to know that the final result of the computation will begin with 3:. Is there a lazy IO mapping function somewhere that can work such magic?

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