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Suggestions for "Advanced" Topics for Haskell-based Class
by Conrad Cunningham 27 Jan '17

27 Jan '17

I am teaching a class in Haskell-based functional programming for advanced undergraduates and beginning graduate students at my institution. None of the students have previously used Haskell and for most of my students functional programming is new. Because I am teaching this in a "multiparadigm programming" course, I want to expand beyond what I have usually covered in the Haskell-based "functional programming" course and cover a few topics in areas such as parallel, concurrent, distributed, reactive, or metaprogramming (domain-specific languages, Template Haskell, etc.). Assuming my course has more or less covered the topics in *Learn You a Haskell for Great Good *(with likely shallow coverage of monads) at that point, what would be good additional topics to cover, libraries to use, and tutorial or teaching resources to use? Although I have taught fundamental Haskell FP topics for many years, I have not delved into any of these "advanced" topics. Thanks, Conrad -- H. Conrad Cunningham, Professor Computer and Information Science University of Mississippi (USA)

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Instrumentation without requiring code-level changes?
by Saurabh Nanda 27 Jan '17

27 Jan '17

(another cross post from Reddit - https://www.reddit.com/r/haskell/comments/5qfd36/instrumentation_without_re… ) Is it possible to collect instrumentation data from a Hakell application without requiring the core application to change code? Here are some examples of instrumentation data: * Time taken to serve an HTTP request (not average time, but individual times, for individual requests) along with the incoming request params+headers and outgoing response length. * Time taken to make an SQL query along with the actual query (again, not average times, but individual times, for individual queries) * Time taken to fetch data from Redis along with the Redis command * Time taken to render an HTML page along with path to the HTML file Most dynamic languages (like Ruby) allow monkey-patching OR "decorating" existing functions during runtime. Most instrumentation agents (like Skylight, Newrelic), simply monkey-patch the API surfaces of known libraries to collect this data. Therefore, using these instrumentation agents is a one line code change (just include the gem, or drop the JAR, or whatever). What is the equivalent of this in Haskell? Here's what I've tried so far [1] , but it has the following disadvantages: * Requires changes to the application's cabal file to replace the non-instrumented versions of the libraries with the instrumented version (replace scotty with instrumentedscotty, in my example) * Requires the application to explicitly import the instrumented versions of the libraries (replace import Scotty with import InstrumentedScotty, in my example) * Forces all the code interacting with instrumented APIs to be run in a MonadIO environment - because of the underlying calls to getCurrentTime I'm sure there's a better way, but I couldn't get my hands on it! [1] https://gist.github.com/saurabhnanda/27592da0269bc35569ec6239e1a91b75 -- Saurabh.

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Question about STRef
by Antoine Rimlet 26 Jan '17

26 Jan '17

Hi list, I try to get the following little program (a slightly modified "Man or boy", it prints -14254067) work "as expected", that is, without consuming lots of memory: import Control.Monad.ST import Data.STRef a k x1 x2 x3 x4 x5 = do kk <- newSTRef k let b = do k <- modifySTRef kk pred >> readSTRef kk; a k b x1 x2 x3 x4 if k <= 0 then do x3' <- x3; x4' <- x4; return (x3' + x4') else do x5' <- x5; b' <- b; return (x5' + b') main = print (runST (a 22 (return 1) (return (-1)) (return (-1)) (return 1) (return 0))) I use GHC 7.8.4, and executing this program uses about 2.5 GB of memory (or about 3.5 GB with runghc). However, the "equivalent" program in OCaml only needs 4 MB (or 9 MB with the interpreter): let rec a k x1 x2 x3 x4 x5 = let kk = ref k in let rec b () = begin decr kk; a !kk b x1 x2 x3 x4 end in if k <= 0 then let v = x3 () in v + x4 () else let v = x5 () in v + b ();; print_int (a 22 (fun () -> 1) (fun () -> -1) (fun () -> -1) (fun () -> 1) (fun () -> 0));; print_newline ();; Therefore I suspect I'm doing something wrong, but I can't see what. I did try to use the strict version modifySTRef' as indicated in the manual, but with no visible improvement. Thanks, Antoine

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Request for feedback on two GHC proposals
by Mario Blažević 26 Jan '17

26 Jan '17

The time is nigh for closing the discussion on the Eval class proposal for GHC, so please pipe in if you have any interest in it, either on the practical (what's the point of this terrible idea!?) or the theoretical side (I can finally get those free theorems and I'd love unlifted products!). https://github.com/ghc-proposals/ghc-proposals/pull/27 I would also appreciate feedback on a tangentially related and wholly practical INCOMPLETE_CONTEXTS pragma proposal at https://github.com/ghc-proposals/ghc-proposals/pull/34 Thank you.

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Re: [Haskell-cafe] Question about STRef
by Johannes Waldmann 26 Jan '17

26 Jan '17

What Michael did not mention in the mail, but it's in his code: numerals defaulted to Integer, and the story changes (somewhat) if we force Int. I updated my measurements: http://www.imn.htwk-leipzig.de/~waldmann/etc/mob/ and ghc-8 looks better now - but not quite wins. > there's still some more low hanging fruit here ... for whom? What could we reasonably expect of the compiler here? It could do the strictification? the unboxing? It certainly could not replace Integer by Int, since the range of numerical values has no obvious bound. But then, all these Integers are small, but we pay quite an overhead (for represeting Int as Integer) which has increased for current ghcs? - J.

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Re: [Haskell-cafe] Question about STRef
by Johannes Waldmann 26 Jan '17

26 Jan '17

Thanks for pointing out this (Knuth's) nice test case. I should use this as an exam question ... Allocation and run-time can be reduced by replacing return (x3' + x4') with let x = (x3' + x4') in x `seq` return x And, modifySTRef' *does* help. I *do* notice a regression (for the seq-ed and primed version) ghc-8.0.2 : 2.6 GB alloc, 2.3 sec ghc-6.10.4: 2.0 GB alloc, 1.7 sec (measured on X5365 @ 3.00GHz ) Numbers are for total allocation, residency is small (it runs with +RTS -H10k -A10k but then the number of GCs goes up) - J.

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Bug-free, leak-free, battle-tested library for broadcast channels?
by Saurabh Nanda 26 Jan '17

26 Jan '17

(cross-posted from Reddit because I'm not sure of the audience overlap between haskell-cafe & reddit) I want to broadcast some instrumentation data from deep within my Haskell app. This data will have listeners in some environments (say, debug), but not others (say, production). Which library should I be using? A little searching threw two possible candidates: * http://hackage.haskell.org/package/split-channel * https://hackage.haskell.org/package/broadcast-chan But, I'm not sure if these are battle-tested. Any help would be appreciated. -- Saurabh.

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Early feedback on a GHC proposal
by Chris Smith 26 Jan '17

26 Jan '17

Anyone want to offer some early feedback on a GHC proposal I'm working on? It's called OverloadedApplication. Current version at https://github.com/cdsmith/ghc-proposals/blob/overloadedapplication/proposa… I am proposing something a bit more ambitious than what I need, because the proposal as written seems more likely to be broadly useful. The bit that I actually need is about desugaring of infix operators in the presence of RebindableSyntax. I'd love to get early thoughts from anyone who has a solid use case for the rest of the proposal. It feels like a thing that *ought* to be useful, but I'm drawing blanks at the moment.

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Re: [Haskell-cafe] A proof that the list monad is not a free monad
by David Feuer 25 Jan '17

25 Jan '17

The ConstraintKinds extension just lets you deal with kinds involving constraints. Turn it on, import the Constraint kind from somewhere (I know it's exported from GHC.Exts and also from Data.Constraint in the constraints package; I don't know if it has a nice home in base). Then you can write things like class C f a where p :: f a => proxy f -> a -> Int etc. On Jan 23, 2017 4:19 PM, "Olaf Klinke" <olf(a)aatal-apotheke.de> wrote: On a mostly unrelated note, Dan Doel elaborated [1] that the list monad is not the monad of free monoids either, even though mathematically the free monoid over x is the set of words of finite length with alphabet x. Briefly, the reason is that Haskell types are domains, not sets. Below is a proof that the free monad is not the free algebra. One approximant to free algebraic structures is to look at the free F-algebra for a functor F. This approach does not take into account any equations between the algebraic operations. {-# LANGUAGE Rank2Types,MultiParamTypeClasses,FlexibleInstances,FlexibleContexts #-} module FreeFAlg where import Control.Applicative import Control.Monad -- F-algebras (sans laws). class Functor f => Alg f a where alg :: f a -> a -- The free F-algebra over x, following Dan Doel's ideas, -- with the universal property baked in. newtype FreeAlg f x = FreeAlg (forall a. Alg f a => (x -> a) -> a) runFree :: Alg f a => FreeAlg f x -> (x -> a) -> a runFree (FreeAlg f) = f universal :: Alg f a => (x -> a) -> FreeAlg f x -> a universal = flip runFree instance Functor f => Alg f (FreeAlg f x) where alg ff = FreeAlg (\f -> alg (fmap (($f).runFree) ff)) instance Functor (FreeAlg f) where fmap h (FreeAlg fx) = FreeAlg (\f -> fx (f.h)) instance Monad (FreeAlg f) where return x = FreeAlg ($x) (FreeAlg fx) >>= k = FreeAlg (\f -> fx (\x -> runFree (k x) f)) instance Applicative (FreeAlg f) where pure = return (<*>) = ap data FreeMonad f x = Pure x | Roll (f (FreeMonad f x)) instance Functor f => Alg f (FreeMonad f x) where alg = Roll -- This looks like the same universal property... universal' :: Alg f a => (x -> a) -> FreeMonad f x -> a universal' f (Pure x) = f x universal' f (Roll ffree) = alg (fmap (universal' f) ffree) freeAlg2freeMonad :: Functor f => FreeAlg f x -> FreeMonad f x freeAlg2freeMonad = universal Pure freeMonad2freeAlg :: Functor f => FreeMonad f x -> FreeAlg f x freeMonad2freeAlg = universal' return The free monad is not the free F-algebra, because uniqueness fails. Consider the constant functor data Point a = Point instance Functor Point where fmap f Point = Point -- universal' f (Pure x) = f x -- universal' f (Roll Point) = alg Point -- but we have another function with the same type as universal': notUnique :: Alg Point a => (x -> a) -> FreeMonad Point x -> a notUnique f = const (alg Point) For classes like Monoid, which have equations between the algebraic operations (such as mappend mempty = id), we'd need the language extension that allows constraints as type parameters. I'd like to write Free Monoid x = Free (forall a. Monid a => (x -> a) -> a) but I am not experienced with the ConstraintKinds extension. Anyone help? Olaf [1] http://comonad.com/reader/2015/free-monoids-in-haskell/ _______________________________________________ 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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GHC 8.0.2 Stackage LTS8 plan
by Luke Murphy 25 Jan '17

25 Jan '17

Hi folks, Please see the following brief write up regarding what the Stackage curator team have planned for a smooth transition to GHC 8.0.2 on LTS builds. http://lwm.github.io/stackage-8.0.2/ Best, Luke

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