- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Parsing error in Haskell2010
by Stuart A. Kurtz 19 Nov '14

19 Nov '14

Dear Andy, > If that is the actual indentation in the file (and not just one or both of our mail clients breaking it), the problem is that the `let` line needs to start in the same column as the lines with `gen` and `putStr`. The gen, let, and putStr were properly aligned. After all, it compiled as Haskell98 ;-). > (Also, the inner do-block needs to be indented strictly further than the word `log`, in case it isn't in the original file.) This seems to be the issue. Haskell98 didn't require this, Haskell2010 does, and this seems less desirable to me. Isn't it reasonable to assume that the it's the do that dominates syntactically here, and not the let? Peace, Stu

3 4

Dependently typed fold in GHC/Haskell
by Christiaan Baaij 19 Nov '14

19 Nov '14

Dear Cafe, I'm trying to convert the following Idris code, which compiles: > gfold : {p : Nat -> Type} > -> ((l : Nat) -> a -> p l -> p (S l)) > -> p 0 -> Vect k a -> p k > gfold _ z Nil = z > gfold f z ((::) {n} x xs) = f n x (gfold f z xs) > > gconcat : {m : Nat} -> Vect n a -> Vect m a -> Vect (n + m) a > gconcat {m} xs ys = gfold {p} f ys xs > where > p : Nat -> Type > p l = Vect (l + m) a > > f : (l : Nat) -> a -> Vect (l + m) a -> Vect (S (l + m)) a > f _ x r = x :: r To Haskell + various GHC extensions, which gives me type errors: > {-# LANGUAGE DataKinds, GADTs, KindSignatures, TypeOperators, RankNTypes, > ScopedTypeVariables, TypeFamilies #-} > > import Data.Proxy > > data Nat = Z | S Nat > > type family Plus (x :: Nat) (y :: Nat) :: Nat where > Plus Z y = y > Plus (S n) y = S (Plus n y) > > data Vec :: * -> Nat -> * where > Nil :: Vec a Z > (:>) :: a -> Vec a n -> Vec a (S n) > > gfold :: Proxy (p :: Nat -> *) > -> (forall l . Proxy l -> a -> p l -> p (S l)) > -> p Z -> Vec a k -> p k > gfold Proxy f z Nil = z > gfold Proxy f z (x :> xs) = f Proxy x (gfold Proxy f z xs) > > type family P (a :: *) (m :: Nat) (l :: Nat) :: * where > P a m l = Vec a (Plus l m) > > gconcat :: forall n m a . Vec a n -> Vec a m -> Vec a (Plus n m) > gconcat xs ys = gfold p f ys xs > where > p :: Proxy (P a m) > p = Proxy > > f :: forall (l :: Nat) . Proxy l -> a -> Vec a (Plus l m) > -> Vec a (S (Plus l m)) > f _ x r = x :> r I get the following errors: > VecTest.hs:25:25: > Couldn't match type ‘l’ with ‘Plus l m’ > ‘l’ is a rigid type variable bound by > a type expected by the context: > Proxy l -> a -> Vec a l -> Vec a ('S l) > at VecTest.hs:25:17 > Expected type: Proxy l -> a -> Vec a l -> Vec a ('S l) > Actual type: Proxy l > -> a -> Vec a (Plus l m) -> Vec a ('S (Plus l m)) > Relevant bindings include > p :: Proxy (P a m) (bound at VecTest.hs:28:5) > f :: forall (l :: Nat). > Proxy l -> a -> Vec a (Plus l m) -> Vec a ('S (Plus l m)) > (bound at VecTest.hs:32:5) > ys :: Vec a m (bound at VecTest.hs:25:12) > gconcat :: Vec a n -> Vec a m -> Vec a (Plus n m) > (bound at VecTest.hs:25:1) > In the second argument of ‘gfold’, namely ‘f’ > In the expression: gfold p f ys xs > > VecTest.hs:25:27: > Couldn't match type ‘m’ with ‘'Z’ > ‘m’ is a rigid type variable bound by > the type signature for > gconcat :: Vec a n -> Vec a m -> Vec a (Plus n m) > at VecTest.hs:24:21 > Expected type: Vec a 'Z > Actual type: Vec a m > Relevant bindings include > p :: Proxy (P a m) (bound at VecTest.hs:28:5) > f :: forall (l :: Nat). > Proxy l -> a -> Vec a (Plus l m) -> Vec a ('S (Plus l m)) > (bound at VecTest.hs:32:5) > ys :: Vec a m (bound at VecTest.hs:25:12) > gconcat :: Vec a n -> Vec a m -> Vec a (Plus n m) > (bound at VecTest.hs:25:1) > In the third argument of ‘gfold’, namely ‘ys’ > In the expression: gfold p f ys xs Is this kind of type-level programming just not yet possible in GHC? Or am I simply doing it wrong? Cheers, Christiaan

3 3

How to reverse ghc encoding of command line arguments
by Ben Franksen 19 Nov '14

19 Nov '14

I have a question about how to reverse the text encoding as done by ghc and the base library for stuff that comes from the command line or the environment. Assume the user's environment specifies a non-Unicode locale, e.g. some latin encoding. In this case, the String we get from e.g. System.Environment.getArgs does *not* contain the Unicode code points of the characters the user has entered. Instead the input bytes are mapped one-to- one to Char. This has probably been done for compatibility reasons, and I do not want to discuss this choice here. Rather, I want to find out how I can convert such a string back to some proper Unicode representation, for instance in order to store the value in a file with a defined encoding such as utf-8. This should be done in a generic way, i.e. without making ad-hoc assumptions about what the user's encoding might be. There is the iconv package. However, it takes ByteString as input and output and it also requires that I give it the encoding as input. How do I find out which is this encoding? On the command line I could simply do ben@sarun[1]: ~ > locale charmap ISO-8859-1 Is there a Haskell function that does the equivalent or do I have to use getEnv "LC_CTYPE", then parse the result? Let's assume I get this to work, so now I have a String that represents the user's encoding, such as "ISO-8859-1". Now, in order to use iconv, I have to convert the string I got via getArgs into a ByteString. But to do that properly, I would have to decode it according to the user's current locale, which is exactly what I want to achieve in the first place. How do I break this cycle? Perhaps it is simpler to write our own getArgs/getEnv functions and directly convert the data we get from the system to a proper (Unicode) String? Any suggestions would be highly appreciated. Cheers Ben -- "Make it so they have to reboot after every typo." -- Scott Adams

4 12

Call for Participation: BOB Conference in Berlin (Jan 23, 2015)
by Torsten Grust 19 Nov '14

19 Nov '14

================================================================ BOB 2015 Conference "What happens if we simply use what's best?" January 23.2015 Berlin http://bobkonf.de/2015/ Program: http://bobkonf.de/2015/programm.html Registration: http://bobkonf.de/2015/registration.html ================================================================ BOB is the conference for developers, architects and decision-makers to explore technologies beyond the mainstream in software development, and to find the best tools available to software developers today. Our goal is for all participants of BOB to return home with new insights that enable them to improve their own software development experiences. The program features 14 talks and 8 tutorials on current topics: http://bobkonf.de/2015/programm.html The subject range of talks includes functional programming, microservices, package management, and data management. The tutorials feature introductions to Erlang, Haskell, Swift, and ClojureScript, and their applications. Anil Madhavapeddy will hold the keynote talk - about unikernels and functional programming. Registration is open online: http://bobkonf.de/2015/registration.html NOTE: The early-bird rates expire on Dec. 19, 2014! BOB cooperates with the :clojured conference on the following day. There is a registration discount available for participants of both events. http://www.clojured.de/

1 0

Code review: list-like datatype with constant time appending based on function composition.
by Nicola Gigante 19 Nov '14

19 Nov '14

Hello everybody I would kindly ask you to take a look at this code and tell me your opinions: https://gist.github.com/nicola-gigante/43533ce907f88a6aba16 <https://gist.github.com/nicola-gigante/43533ce907f88a6aba16> The file try to implement a list-like data type, based on function composition to achieve a constant-time appending operation (in contrast to linear (++)). I’m sure this is something old. If you have material that covers an approach like this, please tell me. I’ve tried to implement all the type classes that I currently use, and it seems to work. Also, all the operations of those typeclasses seems to preserve the original laziness properties, despite I was expecting the contrary. A list in this code is basically a function that’s constructed by composing a lot of partially applied cons operators. To obtain a plain list from it, it’s sufficient to apply the empty list at the end. So I was expecting that to evaluate even the first element of the list, the code would have to evaluate all the function composition chain to obtain the entire function to being able to apply the final argument. It seems not true, since this code can still manipulate infinite lists. For example: > fmap (*2) . fromList $ repeat 42 This code prints an infinite stream of “84” numbers in GHCI. It doesn’t wait to have the entire list to then print it out. In my understanding that means I’m building the list sufficiently lazily, or am I missing something? In the case this actually means my datatype is lazy enough, why isn’t it more strict instead, following the above reasoning? Is it the case that haskell laziness extends to partially constructed functions? Also, the implementation is very simple. Basically I use the list functions and go back and forth with toList/fromList. It all seems too easy… am I wasting a lot of time or space somewhere? Thank you a lot for your help, Greetings, Nicola

5 8

NOTE: the primary webserver is going down for immediate maintenance.
by Austin Seipp 19 Nov '14

19 Nov '14

Hello *, The primary haskell.org domain, www.haskell.org, is hosted on a system which seems to have lost one of its RAID disks completely. We were planning on moving this machine in the next few weeks to new infrastructure, but we are now expediting this plan and will be doing it ASAP. As we move this server, both the webserver and the mailing system will be going down. Please don't be alarmed if your emails aren't delivered or things go quiet. Many services will continue to work, but we do realize this will be upsetting for many. You can follow the progress on #haskell-infrastructure on Freenode, and see updates on https://status.haskell.org If you need to download something like a GHC binary or Haskell Platform package, you can use https://downloads.haskell.org in the mean time, which is a new service we were hoping to announce more officially soon, but is already working today. Unfortunately we cannot give an expected time of completion for the move, but we'll try to keep people well informed through IRC or something like Reddit. Thanks -- Regards, Austin Seipp, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

2 1

[ANNOUNCE] New release of SBV (v3.2)
by Levent Erkok 19 Nov '14

19 Nov '14

I'm pleased to announce v3.2 release of SBV, providing facilities for SMT based theorem proving in Haskell. This is mainly a bug-fix/maintenance release, together with one new feature: SBV now implements 'sAssert', which works similar to 'assert' calls in Haskell or other languages to check that certain program invariants always hold. Except, SBV allows the condition to be symbolic, and all violations will be caught and reported as the underlying program is symbolically executed. (For instance, this facility can be useful in other DSL's built on top of SBV to make sure the code they generate never divides by 0, or indexes out of bounds.) Full release notes: https://github.com/LeventErkok/sbv /blob/master/CHANGES.md SBV web page: http://leventerkok.github.io/sbv/ As usual, bug reports and feedback are most welcome! -Levent.

1 0

Parsing error in Haskell2010
by Stuart A. Kurtz 18 Nov '14

18 Nov '14

Dear Cafe, I recently ran into a Haskell2010 parsing error with code that parses just fine under Haskell98, and it seems to me that this might be an unintended consequence. Briefly, I have runModel :: StdGen -> Model () -> Result where Model is a monadic type, and then my main was main :: IO () main = do gen <- getStdGen let log = runModel gen $ do initialize 72 report replicateM_ 50 $ do replicateM_251 migrate report putStr . format $ log Haskell 2010 doesn't like the additional level of indentation after the "let log" line. If I pull the content of the "do" into a separate definition, so this becomes let log = runModel gen simulate the parsing error goes away. Questions: 1) Is this intended? 2) Can this code be formatted in a way to make Haskell 2010 happy? Many thanks. Peace, Stu

3 2

ISO 8601 Date time format without old-locale?
by Bram Neijt 18 Nov '14

18 Nov '14

I want to do ISO8601 formatting of a date with a numeric zone offset and I can't seem to find a way of doing it without requiring the old-locale package[1]. What is the current method of obtaining and formatting time representation if old-locale is old? Greetings, Bram [1] A sketch of the code using old-locale: -- 2014-11-18T06:37:04+00:00 iso8601DateTimeFormat :: String iso8601DateTimeFormat = "%FT%T%z" main = do now <- getCurrentTime putStrLn $ formatTime defaultTimeLocale iso8601DateTimeFormat now Which requires "defaultTimeLocale" from the old-locale package.

5 4

The next 7000... abstractions to learn!
by Gautier DI FOLCO 18 Nov '14

18 Nov '14

Hi all, I'm using Haskell since nearly three years now and I think I have well understood that types are the most important part of our programs. I have looked all around to learn as many datatypes/typeclasses as possible. But I suffer for a lack of direction to pursue my learning. I have well understood Prelude's one and some random ones (include someones for Category Theory), but I can't handle Kmett's code. Are there some intermediates abstractions I can learn? (For example in some librairies). Thanks by advance for your help. Regards.

2 2