January 2013 - Haskell-Cafe

[Announcement] New SBV release
by Levent Erkok 02 Jan '13

02 Jan '13

I'm happy to announce v2.9 release of the Haskell SBV library: http://leventerkok.github.com/sbv/ SBV (SMT Based Verification) is a library that allows Haskell programs to take advantage of modern SMT solvers, by providing a symbolic simulation engine that can invoke 3rd party SMT solvers to prove/falsify properties about (a certain class of) Haskell programs. New in this release is the support for the CVC4 SMT solver: http://cvc4.cs.nyu.edu/web/ If you were planning to use SMT solvers before, but were worried about the not-so-commercial-friendly licenses of Yices and Z3, then this is a great opportunity: *CVC4 comes with essentially no limit on its use for research or commercial purposes*! Feedback, patches and bug reports are always welcome. -Levent.

1 0

Inference for RankNTypes
by Francesco Mazzoli 02 Jan '13

02 Jan '13

Hi list, I am a bit puzzled by the behaviour exemplified by this code: {-# LANGUAGE RankNTypes #-} one :: (forall a. a -> a) -> b -> b one f = f two = let f = flip one in f 'x' id three = (flip one :: b -> (forall a. a -> a) -> b) 'x' id four = flip one 'x' id Try to guess if this code typechecks, and if not what’s the error. While `two' and `three' are fine, GHC (7.4.1 and 7.6.1) complains about `four': Line 8: 1 error(s), 0 warning(s) Couldn't match expected type `forall a. a -> a' with actual type `a0 -> a0' In the third argument of `flip', namely `id' In the expression: flip one 'x' id In an equation for `four': four = flip one 'x' id So for some reason the quantified variable in `id' gets instantiated before it should, and I have no idea why. Any ideas? Francesco

3 8

Re: [Haskell-cafe] [Haskell-beginners] ghc and android
by Nathan Hüsken 02 Jan '13

02 Jan '13

After googling a little more, do I understand correctly that to compile haskell (with ghc) for Android, one would have to compile and run ghc itself on android (because it can not cross compile)? That seems almost impossible, I would need an android device with unusual capacity. I have heard that ghc for iphone is possible. Anyone knows how that is done? What is the difference for android? Regards, Nathan

6 6

Monaris
by Kazu Yamamoto 02 Jan '13

02 Jan '13

Happy new year from Japan. A young talented guy, @fumieval, has released Monaris, a Tetoris clone based on OpenGL. You can install it: % cabal install Monaris To my surprise, this game is implemented with free Monad. ;-) Regards, --Kazu

1 0

hsc2hs and Storable (especially) unsafe
by Evan Laforge 01 Jan '13

01 Jan '13

It so happens that on 64 bit OS X, haskell's Int is 64 bits, while C's int is 32 bits. hsc2hs's #poke does no typechecking at all, so if you have (#poke Struct, int_field) structp int -- where int :: Int Then you probably are going to corrupt memory, and in a particularly pernicious way (since chances are it won't segfault). Similarly, don't poke haskell Chars, it has to be CChar. And what's even worse, Foreign.withArrayLen gives you an Int, not a CInt! So doing the obvious thing and poking the pointer and length directly is wrong. And besides, shouldn't it be a CSize? Another trap: C++'s bool is probably one byte, haskell's Bool Storable says it's 4. It's probably not even correct to assume Double = CDouble and Float = CFloat, though it seems likely to be always true. I carefully vetted all my #pokes but I'm annoyed that hsc2hs and Foreign let me get away with this. If Storable is for marshalling types to and from C, then non-C types should not be Storable! If Storable is useful for other things (and it is!), then Foreign should define its own CStorable, that only includes C types, and hsc2hs should use that! While we're at it, fromIntegral is not a safe way to convert a haskell type to a C one. But it sure is a tempting one, because Foreign.C doesn't provide any conversion functions (except for Bool). We should have explicit conversion functions that give the choice of crashing with an error or clamping the number to the (smaller) C type's bounds. I'm not sure if anyone wants what fromIntegral does. Or maybe I shouldn't be using hsc2hs in the first place. I feel more and more that it gives haskell a bad name, being low level and error prone. I used bindings-dsl for a libgit2 binding, and at least it automates Storable instances, so it's less error-prone that way. I'd like to generate instances directly from .h files though, so I should probably check out c2hs. Back in the day when I was deciding to use hsc2hs I noticed how it lacks typechecking, but I thought that it's just a few fields, I can be careful when writing them. But one of the lessons from haskell (and static analysis in general) is that "just be careful" is going to fail you some day, probably sooner than you think. So anyway, I wrote a ForeignC module that defines CStorable that only includes C types. At first I thought I could make Storable a superclass and just re-export functions with more restritive signatures, but then it gets tricky because you wind up needing the Storable peek and poke to declare Storable instances. So I defined a completely new CStorable, and all you need to do is import ForeignC instead of Foreign, and change 'instance Storable ...' to 'instance CStorable ...'. Unfortunately that apparently means copy-pasting over the Storable-using utilities like 'with' and 'newArray'. On the plus side, I dropped it into my .hsc modules, and it found several *more* mistakes. I was thinking of putting it on hackage (along with the conversion functions), since other people might have made these same mistakes, but it's unpleasant because it duplicates so much from Foreign. Also, I only copied over the bits I'm using, so it's not complete either. Is there a better way to create safe Storable instances for C? I'd be happier with a solution that avoids the need to write Storable instances in the first place, but hsc2hs is here now, and used in a lot of places.

1 0

Haskell Summers of Code retrospective (updated for 2012)
by Gwern Branwen 01 Jan '13

01 Jan '13

The Wheel turns, and months come and pass, leaving web links that fade into 403 Forbiddens; a wind rose in NYC, whispering of the coming Winter... As is now customary for me, I've looked into how the 2012 SoCs went - the better to feed my misanthropic heart by mocking the students who failed my whimsically arbitrary and subjective standards: http://www.gwern.net/Haskell%20Summer%20of%20Code#results-2 This was not a good year. 2 students simply dropped out period, and 3 other projects turned out badly, leaving just 2 clearly successful projects for the year. Hopefully 2013 will turn out better. /r/haskell: http://www.reddit.com/r/haskell/comments/15sjur/summer_of_code_2012_retrosp… -- gwern

1 0

ANNOUNCE: graphviz-2999.15.0.0
by Ivan Lazar Miljenovic 01 Jan '13

01 Jan '13

After much prodding and poking by end-users for the past few months, I'm finally able to announce a GHC-7.6.1 compatible release of my Haskell bindings to the Graphviz graph visualisation toolkit: http://hackage.haskell.org/package/graphviz-2999.15.0.0 (It took me so long to release a version that allowed bytestring-0.10.* as I was waiting for upstream to get back to me about an inconsistency I found; in the end I decided to just try and cope with how it seems to work.) The list of changes since the last release are: * The repository is now hosted at http://hub.darcs.net/ivanm/graphviz, and using the bug-tracker there. * Updates to various `Attribute` definitions: - The list of available shapes has been expanded to take into account the new synthetic biology shapes. - The `Size` and `FontNames` attributes now have specified data types. - The optional start and end points for `Spline`s were previously the wrong way round; this has now been fixed. - Explicitly only print 2-dimensional `Point` values for `Rect` and `DPoint` (previously only 2-dimensional values where parsed, but it was possible to print a 3-dimensional value). - HTML-like labels previously disallowed empty textual label components when parsing; spotted by Daniel Hummel. * `GraphvizParams` now lets you specify whether a "cluster" is actually a cluster or a sub-graph. Requested by Gabor Greif. * Fixed an error where printing edges whose attributes contained a `ColorScheme` attribute, that attribute would stay in the state and the old color scheme would override the _parent's_ state. * Previously, some malformed attributes were accepted when being parsed as they silently became parsed as an `UnknownAttribute`. Now, attributes where the attribute name and the equal sign are successfully parsed **but** the value of the attribute is _not_ successfully parse will throw an unrecoverable error. **Note**: this does mean that some Dot graphs that are accepted by Graphviz (as they separate tokenizing from parsing; as such something like `"0.1"` is successfully accepted as an integer, specifically `0`) are no longer accepted when parsing them in. * Miscellaneous parsing improvements: - Whilst not specified anywhere, Graphviz seems to treat empty quotes as values for attributes expecting a number as their default; as such, this is now taken into account when parsing. - `DPoint` values can now parse an optional `+` prefix. - Whitespaces after commas in HSV colors are now accepted. * Error messages from parsing are improved to help you track down where the parsing error occurred (in that it's easier to find which attribute failed to parse, etc.). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

2 2

A big hurray for lambda-case (and all the other good stuff)
by Ben Franksen 01 Jan '13

01 Jan '13

Hi Everyone just wanted to drop by to say how much I like the new lambda case extension. I use it all the time and I just *love* how it relieves me from conjuring up dummy variables, which makes teh code not only esier to write but also to read. A big, huge thank you to the ghc developers. This has been sooooo long on my wish list. Also much appreciated and long awaited: tuple sections (though I use them not quite as often). Both should *definitely* go into Haskell'13. Of course, thank you also for all the other beautiful stuff in ghc-7.6.1, especially PolyKinds, DataKinds etc. GHC is just simply amazing. You guys RULE THE WORLD! Cheers -- Ben Franksen () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

15 18

Mancunian Haskellers out there?
by Alfredo Di Napoli 01 Jan '13

01 Jan '13

Morning Cafe, I've been living in Manchester for 1 month now and I'm wondering if some on you are from the Greater Manchester area, so that we could chat about out beloved language in front of a glass of ale / tea :P Happy new year to everyone! A.

5 6

Backtracking in HXT
by Mateusz Kowalczyk 01 Jan '13

01 Jan '13

Hello, I'm not sure if this is the right place to ask this but here goes. I'm currently working with HXT and I quite like it. There's one issue I can't solve (at least now without some dirty, dirty hacking) and I'm sure it's fairly simple. Consider the following mark-up <a href="example.com/somelink.html">Hello One!</a> <a href="example.com/anotherlink.html">Hello Two!</a> Now, what I'm trying to achieve is to get the href based on the text. I can test for what the text is by traversing <a>s, then using the getChildren >>> getText arrow. What I can't figure out is how to check the text and then get the href as by the time I get to the text, I'll further down the tree! I imagine it would look something along the lines of (getChildren >>> getText <?*> "Hello Two") `guards` getAttrValue "href". <?*> is just a random operator I made up for illustration purposes that works as a predicate over arrows. Is this the right approach? Is there a built-in that already does what I want? It seems like a common task... Mateusz Kowalczyk

2 3