January 2017 - Haskell-Cafe

Re: [Haskell-cafe] A proof that the list monad is not a free monad
by Olaf Klinke 23 Jan '17

23 Jan '17

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/

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Dropbox sdk
by Pieter Laeremans 23 Jan '17

23 Jan '17

Hi all, I've tried using the dropbox sdk package from package today. But it is a bit outdated I can't get it to compile. I 've tried using stack init, with various resolvers. One of the problems seems to be that the dependencies for the certificate package used to by dropbox-sdk, enforces base < 4.8. How should I best tackle this problem ? Should I assume that the certificate package should be replaced by something else? And just try to modify the code ? I know it's open source, so I would prefer trying out a fix, and submitting a patch request, rather than bothering the maintainers. Thanks for your help, Pieter -- Pieter Laeremans pieter(a)laeremans.org

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gloss from ghci
by Johannes Waldmann 23 Jan '17

23 Jan '17

Dear Cafe - I am trying to use gloss https://hackage.haskell.org/package/gloss from ghci. The "display" command will open a window and draw the picture on it just fine. But when I close this window by hitting ESC, ghci prints <interactive>: interrupted and exits, so I'm back at the shell from where I started ghci. This is somewhat annoying - especially since ghci's command line history does not get written, so when I re-start it, I have to re-type (work-around: before "display", close ghci, this will write history, then restart and draw. Ugly.) The gloss FAQ suggests ghci -fno-ghci-sandbox but this does not help (ghci still exits, this time, without printing the message). - J.

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A proof that the list monad is not a free monad
by Ingo Blechschmidt 23 Jan '17

23 Jan '17

Dear lovers of generalized abstract nonsense, recall that we can construct, for any functor f, the free monad over f: data Free f a = Pure a | Roll (f (Free f a)) Intuitively, Free f a is the type of f-shaped trees with leaves of type a. The join operation merely grafts trees together and doesn't perform any further computations. Values of the form (Roll _) shall be called "nontrivial" in this posting. Some of the usual monads are in fact free monads over some functor: * The Tree monad is the free monad over the functor Pair, where data Pair a = MkPair a a. The intuition as Pair-shaped trees is most strong in this example. Every parent node has a pair of children. * The Maybe monad is the free monad over the functor Unit, where data Unit a = MkUnit. In the graphical picture parent nodes have a Unit of children, so no children at all. So there are exactly two Unit-shaped trees: the tree consisting only of a leaf (corresponding to Just _) and the three consisting of a childless parent node (corresponding to Nothing). * The Identity monad is the free monad over the functor Void, where Void a is a type without constructors. * The Partiality monad is the free monad over Maybe. * The Writer [r] monad is the free monad over the functor (r,). In an (r,)-shaped tree, a parent node is decorated with a value of type r and has exactly one child node. Traversing such a path yields a list of r's and a leaf value of type a, so in total a value of type ([r],a) = Writer r a. (In particular, Free (r,) () is isomorphic to [r]. This observation led to the claim that the list monad is free. But this claim is false and indeed doesn't follow from the observation. To show that a monad m is free, one has to exhibit a monad isomorphism forall a. m a -> Free f a for some functor f. In contrast, exhibiting an isomorphism of m a with Free (f a) () is neither sufficient nor required.) Even more monads are quotients of free monads. For instance, State s is a quotient of Free (StateF s), where data StateF s a = Put s a | Get (s -> a). -- NB: This functor is itself a free functor over a type constructor -- which is sometimes called StateI [1], rendering Free (StateF s) what -- Oleg and Hiromi Ishii call a "freer monad" [2]. The quotient is exhibited by the following monad morphism, which gives semantics to the purely syntactical values of the free monad. run :: Free (StateF s) a -> State s a run (Pure x) = return x run (Roll (Put s m)) = put s >> run m run (Roll (Get k)) = get >>= run . k This point of view is the basis of the operational package by apfelmus [1] and is also talked about in an StackOverflow thread [3]. It's the main reason why free monads are useful in a programming context. So, is the list monad a free monad? Intuitively, it's not, because the join operation of the list monad (concatenation) doesn't merely graft expressions together, but flattens them [4]. Here is a proof that the list monad is not free. I'm recording it since I've been interested in a proof for quite some time, but searching for it didn't yield any results. The threads [3] and [5] came close, though. In the free monad over any functor, the result of binding a nontrivial action with any function is always nontrivial, i.e. (Roll _ >>= _) = Roll _ This can be checked directly from the definition of (>>=) for the free monad or alternatively with Reid Barton's trick of exploiting a monad morphism to Maybe, see [3]. If the list monad was isomorphic-as-a-monad to the free monad over some functor, the isomorphism would map only singleton lists [x] to values of the form (Pure _) and all other lists to nontrivial values. This is because monad isomorphisms have to commute with "return" and return x is [x] in the list monad and Pure x in the free monad. These two facts contradict each other, as can be seen with the following example: do b <- [False,True] -- not of the form (return _) if b then return 47 else [] -- The result is the singleton list [47], so of the form (return _). After applying a hypothetical isomorphism to the free monad over some functor, we'd have that the binding of a nontrivial value (the image of [False,True] under the isomorphism) with some function results in a trivial value (the image of [47], i.e. return 47). Cheers, Ingo [1] http://projects.haskell.org/operational/Documentation.html [2] http://okmij.org/ftp/Haskell/extensible/more.pdf [3] http://stackoverflow.com/questions/14641864/what-monads-can-be-expressed-as… [4] https://hackage.haskell.org/package/free-4.12.4/docs/Control-Monad-Free.html [5] https://www.reddit.com/r/haskell/comments/50zvyb/why_is_liststate_not_a_fre…

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interfacing to C library - c2hs/hsc2hs/GreenCard?
by Brian Sammon 23 Jan '17

23 Jan '17

I'd like to write a haskell app that calls a C library that I wrote. This is my first time trying to do something like this. What would people here recommend I start with? c2hs? hsc2hs? GreenCard? One of the newer tools like bindings-dsl or inline-c? It's a relatively small library -- a handful of structs and a bunch of functions that take them as input, often returning another as output, and ultimately doing some libcurl to exchange data with websites. Also, can anyone recommend some documentation to read, that's applicable to GHC 8? A lot of the stuff I found on google predates GHC 8 (and a fair amount predates GHC 7)--not sure if that's likely to be a problem.

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free monad versus operational for blackjack game
by Michael Litchard 23 Jan '17

23 Jan '17

I'm studying the code for a blackjack game: See marked answer. http://codereview.stackexchange.com/questions/46809/blackjack-in-haskell It uses the operational package, and mentions one could write equivalent code using a free monad. But my question is, for this use case is there any benefit for using a free monad?

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Possible to use Cabal-1.18 with ghc-6.12.3?
by Simon Peter Nicholls 23 Jan '17

23 Jan '17

Can I improve my setup, so that Cabal-1.18 will work better with ghc-6.12.3? I have an old project that I’m updating, which is currently compilable with 6.12.3. I’ve managed to get Cabal-1.18.1.7 and cabal-install-1.18.0.1 to build it, which gives me some handy extra tooling, but only if I pre-install all the project dependencies before updating Cabal. If I try to use a sandbox instead, the build will fail at the point where template-haskell is used: > Loading package utf8-string-0.3.7 ... <command line>: can't load .so/.DLL for: HSutf8-string-0.3.7 (libHSutf8-string-0.3.7.so: cannot open shared object file: No such file or directory) That’s a dependency in my sandbox, but this also fails if it’s installed to the default `~/.cabal`. I see a similar error if I try `runhaskell Setup.hs` instead of cabal: > Setup.hs: <command line>: can't load .so/.DLL for: HSCabal-1.18.1.7 (libHSCabal-1.18.1.7.so: cannot open shared object file: No such file or directory) So I figure Cabal-1.18 and my default 6.12.3 are not co-operating as well as they could. If I use —enable-shared in my dependency builds, I still get this error, and I also see it if I try to build statically. The template-haskell “Loading” steps always seems to fail by not finding a shared library, as if it’s stepping outside of Cabal (and my!) control. I note that installation locations are different in Cabal-1.18 (vs 1.16), but figure the GHC registration process takes care of something like `.cabal/lib/x86_64-linux-ghc-6.12.3/utf8-string-0.3.7/libHSutf8-string-0.3.7-ghc6.12.3.so`. A pre-installed parsec dependency looks like `ghc-6.12.3/lib/parsec-3.1.1/ghc-6.12.3/libHSparsec-3.1.1-ghc6.12.3.so`. Neither has the name format reported by the error. Is there something in my Cabal or my GHC setup that I can change, to get these working together? A clean fix would be great, but a bodge is fine, since this transition work is isolated in a docker container. Thanks.

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Soliciting feedback on my lens json parsing tutorial (please don't share yet!)
by Cody Goodman 22 Jan '17

22 Jan '17

Hi All, I wrote a lens json parsing tutorial here: https://github.com/codygman/concise-json-parsing-in-haskell I'd appreciate any feedback. My approach tries to use a prior understanding of Python into understanding lens enough to do something useful. Perhaps I can include links to tutorials that cover more of the theoretical foundations as well. Any criticism and/or ideas is welcome! Thanks, Cody Goodman

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Announce: Haskell Platform 8.0.2
by Gershom B 21 Jan '17

21 Jan '17

On behalf of the Haskell Platform team, I'm happy to announce the release of Haskell Platform 8.0.2 Now available at https://www.haskell.org/platform/ This includes GHC 8.0.2, cabal-install 1.24.0.2, and stack 1.3.2, all with many bugfixes and improvements since the last platform release. This platform release we've cleaned up the webpage a bit, and renamed the "minimal" distribution to the "core" distribution to highlight that it is the recommended approach (and simplified the accompanying text). A number of improvements have been made to the windows installer -- notably the /S option for silent install is now in fully working order, and we have the flags /STACK and /D (for stack and platform-ghc install paths). The installer expects no quoting of paths, even with spaces within the paths, like so: HaskellPlatform-8.0.2-full-x86_64-setup.exe /S /STACK=c:\My install path for stack\local\bin /D=c:\program files\Haskell\Platform\8.0.2 There is also an updated version of msys2 included, which includes pacman, et al. to ease the installation of libraries with more complex foreign dependencies. Changes to Contents: * As a result of transitive dependencies of platform packages, the integer-logarithms and call-stack packages have been added to the full platform. A full list of contents is available at https://www.haskell.org/platform/contents.html Thanks to all the contributors to this release, thanks to all the package and tool maintainers and authors, and a big thanks to the GHC team for all their hard work. A list of new GHC changes is available at: https://ghc.haskell.org/trac/ghc/blog/ghc-8.0.2-released A list of cabal changes is available at: https://hackage.haskell.org/package/cabal-install-1.24.0.2/changelog The new cabal documentation page is at: https://cabal.readthedocs.io/en/latest/ A list of stack changes is at: https://docs.haskellstack.org/en/stable/ChangeLog/ Happy Haskell Hacking all, Gershom (Note -- one feature we have not implemented yet for the platform but would like to is a generic linux installer that allows one to set a custom location and does not require root. Anyone who would like to volunteer to help with this, please get in touch!)

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Call for Participation: BOB 2017 (February 24, Berlin)
by Stefan Wehr 20 Jan '17

20 Jan '17

Reminder: early registration ends this Monday! ================================================================ BOB 2017 Conference "What happens if we simply use what's best?" February 24, 2017 Berlin http://bobkonf.de/2017/ Program: http://bobkonf.de/2017/program.html Registration: http://bobkonf.de/2017/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/2017/program.html The subject range of talks includes functional programming, advanced front-end development, and sophisticated uses of types. The tutorials feature introductions to Haskell, Swift, PureScript, React, QuickCheck, Agda, CRDTs and Servant. John Hughes will give the keynote talk. Registration is open online: http://bobkonf.de/2017/registration.html NOTE: The early-bird rates expire on January 23, 2017! 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/

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