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Embedding version info in executables

haskell-cafe - Fri, 07/11/2014 - 10:26am
What are existing solutions for embedding version info (git revision, build date/time, versions of dependencies) in Haskell programs? Roman _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe< at >
Categories: Offsite Discussion

Edward Z. Yang: Type classes: confluence, coherence and global uniqueness

Planet Haskell - Fri, 07/11/2014 - 10:07am

Today, I'd like to talk about some of the core design principles behind type classes, a wildly successful feature in Haskell. The discussion here is closely motivated by the work we are doing at MSRC to support type classes in Backpack. While I was doing background reading, I was flummoxed to discover widespread misuse of the terms "confluence" and "coherence" with respect to type classes. So in this blog post, I want to settle the distinction, and propose a new term, "global uniqueness of instances" for the property which people have been colloquially referred to as confluence and coherence.

Let's start with the definitions of the two terms. Confluence is a property that comes from term-rewriting: a set of instances is confluent if, no matter what order constraint solving is performed, GHC will terminate with a canonical set of constraints that must be satisfied for any given use of a type class. In other words, confluence says that we won't conclude that a program doesn't type check just because we swapped in a different constraint solving algorithm.

Confluence's closely related twin is coherence (defined in the paper "Type classes: exploring the design space"). This property states that every different valid typing derivation of a program leads to a resulting program that has the same dynamic semantics. Why could differing typing derivations result in different dynamic semantics? The answer is that context reduction, which picks out type class instances, elaborates into concrete choices of dictionaries in the generated code. Confluence is a prerequisite for coherence, since one can hardly talk about the dynamic semantics of a program that doesn't type check.

So, what is it that people often refer to when they compare Scala type classes to Haskell type classes? I am going to refer to this as global uniqueness of instances, defining to say: in a fully compiled program, for any type, there is at most one instance resolution for a given type class. Languages with local type class instances such as Scala generally do not have this property, and this assumption is a very convenient one when building abstractions like sets.

So, what properties does GHC enforce, in practice? In the absence of any type system extensions, GHC's employs a set of rules to ensure that type class resolution is confluent and coherent. Intuitively, it achieves this by having a very simple constraint solving algorithm (generate wanted constraints and solve wanted constraints) and then requiring the set of instances to be nonoverlapping, ensuring there is only ever one way to solve a wanted constraint. Overlap is a more stringent restriction than either confluence or coherence, and via the OverlappingInstances and IncoherentInstances, GHC allows a user to relax this restriction "if they know what they're doing."

Surprisingly, however, GHC does not enforce global uniqueness of instances. Imported instances are not checked for overlap until we attempt to use them for instance resolution. Consider the following program:

-- T.hs data T = T -- A.hs import T instance Eq T where -- B.hs import T instance Eq T where -- C.hs import A import B

When compiled with one-shot compilation, C will not report overlapping instances unless we actually attempt to use the Eq instance in C. This is by design: ensuring that there are no overlapping instances eagerly requires eagerly reading all the interface files a module may depend on.

We might summarize these three properties in the following manner. Culturally, the Haskell community expects global uniqueness of instances to hold: the implicit global database of instances should be confluent and coherent. GHC, however, does not enforce uniqueness of instances: instead, it merely guarantees that the subset of the instance database it uses when it compiles any given module is confluent and coherent. GHC does do some tests when an instance is declared to see if it would result in overlap with visible instances, but the check is by no means perfect; truly, type-class constraint resolution has the final word. One mitigating factor is that in the absence of orphan instances, GHC is guaranteed to eagerly notice when the instance database has overlap (assuming that the instance declaration checks actually worked...)

Clearly, the fact that GHC's lazy behavior is surprising to most Haskellers means that the lazy check is mostly good enough: a user is likely to discover overlapping instances one way or another. However, it is relatively simple to construct example programs which violate global uniqueness of instances in an observable way:

-- A.hs module A where data U = X | Y deriving (Eq, Show) -- B.hs module B where import Data.Set import A instance Ord U where compare X X = EQ compare X Y = LT compare Y X = GT compare Y Y = EQ ins :: U -> Set U -> Set U ins = insert -- C.hs module C where import Data.Set import A instance Ord U where compare X X = EQ compare X Y = GT compare Y X = LT compare Y Y = EQ ins' :: U -> Set U -> Set U ins' = insert -- D.hs module Main where import Data.Set import A import B import C test :: Set U test = ins' X $ ins X $ ins Y $ empty main :: IO () main = print test -- OUTPUT $ ghc -Wall -XSafe -fforce-recomp --make D.hs [1 of 4] Compiling A ( A.hs, A.o ) [2 of 4] Compiling B ( B.hs, B.o ) B.hs:5:10: Warning: Orphan instance: instance [safe] Ord U [3 of 4] Compiling C ( C.hs, C.o ) C.hs:5:10: Warning: Orphan instance: instance [safe] Ord U [4 of 4] Compiling Main ( D.hs, D.o ) Linking D ... $ ./D fromList [X,Y,X]

Locally, all type class resolution was coherent: in the subset of instances each module had visible, type class resolution could be done unambiguously. Furthermore, the types of ins and ins' discharge type class resolution, so that in D when the database is now overlapping, no resolution occurs, so the error is never found.

It is easy to dismiss this example as an implementation wart in GHC, and continue pretending that global uniqueness of instances holds. However, the problem with global uniqueness of instances is that they are inherently nonmodular: you might find yourself unable to compose two components because they accidentally defined the same type class instance, even though these instances are plumbed deep in the implementation details of the components. This is a big problem for Backpack, or really any module system, whose mantra of separate modular development seeks to guarantee that linking will succeed if the library writer and the application writer develop to a common signature.

Categories: Offsite Blogs

Dominic Orchard: Automatic SIMD Vectorization for Haskell and ICFP 2013

Planet Haskell - Fri, 07/11/2014 - 9:00am

I had a great time at ICFP 2013 this year where I presented my paper “Automatic SIMD Vectorization for Haskell”, which was joint work with Leaf Petersen and Neal Glew of Intel Labs. The full paper and slides are available online. Our paper details the vectorization process in the Intel Labs Haskell Research Compiler (HRC) which gets decent speedups on numerical code (between 2-7x on 256-bit vector registers). It was nice to be able to talk about HRC and share the results. Paul (Hai) Liu also gave a talk at the Haskell Symposium which has more details about HRC than the vectorization paper (see the paper here with Neal Glew, Leaf Petersen, and Todd Anderson). Hopefully there will be a public release of HRC in future.  

Still more to do

It’s been exciting to see the performance gains in compiled functional code over the last few years, and its encouraging to see that there is still much more we can do and explore. HRC outperforms GHC on roughly 50% of the benchmarks, showing some interesting trade-offs going on in the two compilers. HRC is particularly good at compiling high-throughput numerical code, thanks to various strictness/unboxing optimisations (and the vectorizer), but there is still more to be done.

Don’t throw away information about your programs

One thing I emphasized in my talk was the importance of keeping, not throwing away, the information encoded in our programs as we progress through the compiler stack. In the HRC vectorizer project, Haskell’s Data.Vector library was modified to distinguish between mutable array operations and “initializing writes”, a property which then gets encoded directly in HRC’s intermediate representation. This makes vectorization discovery much easier. We aim to preserve as much effect information around as possible in the IR from the original Haskell source.

This connected nicely with something Ben Lippmeier emphasised in his Haskell Symposium paper this year (“Data Flow Fusion with Series Expressions in Haskell“, joint with Manuel Chakravarty, Gabriele Keller and Amos Robinson). They provide a combinator library for first-order non-recursive dataflow computations which is guaranteed to be optimised using flow fusion (outperforming current stream fusion techniques). The important point Ben made is that, if your program fits the pattern, this optimisation is guaranteed. As well as being good for the compiler, this provides an obvious cost model for the user (no more games trying to coax the compiler into optimising in a particular way).

This is something that I have explored in the Ypnos array language, where the syntax is restricted to give (fairly strong) language invariants that guarantee parallelism and various optimisations, without undecidable analyses. The idea is to make static as much effect and coeffect (context dependence) information as possible. In Ypnos, this was so successful that I was able to encode the Ypnos’ language invariant of no out-of-bounds array access directly in Haskell’s type system (shown in the DSL’11 paper; this concept was also discussed briefly in my short language design essay).

This is a big selling point for DSLs in general: restrict a language such that various program properties are statically decidable, facilitating verification and optimisation.

Ypnos has actually had some more development in the past year, so if things progress further, there may be some new results to report on. I hope to be posting again soon about more research, including the ongoing work with Tomas Petricek on coeffect systems, and various other things I have been playing with. – D

Categories: Offsite Blogs

OCL 2014: Submission Deadline Extended by One Week

General haskell list - Fri, 07/11/2014 - 6:25am
(Apologies for duplicates) ************************************************************** ** Submission Deadline Extended to July 18th, 2014 ** ************************************************************** CALL FOR PAPERS 14th International Workshop on OCL and Textual Modeling Applications and Case Studies (OCL 2014) Co-located with ACM/IEEE 17th International Conference on Model Driven Engineering Languages and Systems (MODELS 2014) September 30, 2014, VALENCIA, SPAIN Modeling started out with UML and its precursors as a graphical notation. Such visual representations enable direct intuitive capturing of reality, but some of their features are difficult to formalize and lack the level of precision required to create complete and unambiguous specifications. Limitations of the graphical notations encouraged the development of text-based modeling languages that
Categories: Incoming News

OCL 2014: Submission Deadline Extended by One Week

haskell-cafe - Fri, 07/11/2014 - 6:25am
(Apologies for duplicates) ************************************************************** ** Submission Deadline Extended to July 18th, 2014 ** ************************************************************** CALL FOR PAPERS 14th International Workshop on OCL and Textual Modeling Applications and Case Studies (OCL 2014) Co-located with ACM/IEEE 17th International Conference on Model Driven Engineering Languages and Systems (MODELS 2014) September 30, 2014, VALENCIA, SPAIN Modeling started out with UML and its precursors as a graphical notation. Such visual representations enable direct intuitive capturing of reality, but some of their features are difficult to formalize and lack the level of precision required to create complete and unambiguous specifications. Limitations of the graphical notations encouraged the development of text-based modeling languages that
Categories: Offsite Discussion

GHC-7.8.3 is out!

Haskell on Reddit - Fri, 07/11/2014 - 6:18am
Categories: Incoming News

ANN: FFI bindings to cuBLAS and cuSPARSE

haskell-cafe - Fri, 07/11/2014 - 6:00am
I have written FFI bindings to the cuBLAS and cuSPARSE libraries, which are CUDA libraries for executing linear algebra computations on the GPU. It's a relatively straightforward translation of the C API. It's slightly novel in that I use language-c and Template Haskell to parse the C headers and create the FFI declarations, avoiding the boilerplate that may otherwise be necessary, even using a preprocessor such as c2hs. I've done a similar thing with a subset of the MAGMA GPU library. It's less polished, and the installation process is more unforgiving, so I haven't put it up on Hackage. Finally, I've written a library which abstracts the immutable API of hmatrix and provides a pure, hmatrix-like interface for cuBLAS/MAGMA, enabling simultaneous development of linear algebra programs using either hmatrix or the above GPU bindings as backends. Additionally, I have written "medium-level" mutable and immutable interfac
Categories: Offsite Discussion

Haskell beginner looking for direction

Haskell on Reddit - Fri, 07/11/2014 - 3:38am

Disclaimer: I think I've been doing functional programming for just over half a month.

I've been learning Haskell through "Learn You a Haskell" and messing around on some different webpages, getting a closer look at some packages (the numeric probability package from Hackage, for example) while reading about monads a bit. I want to continue my learning but in a more focused manner (and I'd like to take a break from LYAH; I feel that it has been better serving as a reference than something to "read" at this point).

I think one of the best ways to do this (and please tell me if you disagree) is to get a good outline of topics to learn and learn them in order. I have heard Real World Haskell is outdated in some ways but still a useful tool. Would you recommend this or any other tools? If I choose to read Real World Haskell, which sections should I skip and read somewhere else (or at least read with caution)?

Background: I am a mathematics undergrad student in my 4th year. My interests are in machine learning, statistics, and information theory. I have a background in basic measure theory (from a probabilistic perspective) but not category theory as well as mathematical statistics. I can program in Python and Java. Haskell interests me for a number of reasons, including the fact that it is so mathematically oriented, it seems to have great potential uses in data science/machine learning/AI (this is a big one, and if anyone has any links to things related to this, I'd love to see them), its use is becoming more widespread (and thus potentially viable in a number of new industry settings?), and honestly, it just feels nice to use.

submitted by Knux-
[link] [10 comments]
Categories: Incoming News

Any tips for newbies on designing ASTs for existing languages?

Haskell on Reddit - Fri, 07/11/2014 - 3:10am

I've heard that Haskell is very good at parsing and there are some good libraries for this (parsec, attoparsec, uu-parsinglib and so on).

Anyway, in the process "input -> Lexing -> Parsing" I haven't reached parsing stage. I'm stuck on designing the Token type and AST type. Any tips, rules of thumbs, mind patterns, little and concrete examples, tutorials for this type of intuition?

My goal is implement basic parsers and prettyprinters for ActionScript, C++ and Java, to be able to translate code between them. I cannot use existing parsing libraries, because they are just too broad. I need only subsets of these languages to be able to intertranslate. I assume that correcty designed AST is larger part of parsing problem, so I ask here.

submitted by danbst
[link] [9 comments]
Categories: Incoming News

What's the performance bottleneck in this prime sieve function?

Haskell on Reddit - Fri, 07/11/2014 - 2:01am

Learning haskell, I wrote this simple function that's supposed to return a list of prime numbers up to n. It works for low enough n, but above 5000 or so, it becomes noticeably slower to begin, which is far lower than a prime sieve has any right to slow down. What am I missing?

import Data.List sieve :: (Integral a) => a -> [a] sieve n | n < 2 = [] | n == 2 = [2] | otherwise = comb [2..n] where comb xxs@(x:xs) | x^2 > last xs = xxs | otherwise = x : comb (xs \\ [2*x, 3*x .. n]) comb [] = [] submitted by ZankerH
[link] [21 comments]
Categories: Incoming News

I just built my first app in haskell (a CHIP-8 cpu emulator). Code review and/or tips?

Haskell on Reddit - Fri, 07/11/2014 - 12:01am

Hey guys,

I started working on an emulator for the old CHIP-8 CPU as a project to learn haskell. Although it probably wasn't the best choice of project due to how basic it is, I've definitely gotten hooked on haskell.

I've done a few clojure projects in the past, so I'm familiar with the functional programming basics, but I haven't experimented much with haskell's abstractions (apart from basic monads).

If anyone can offer any improvements or tips, i'd be greatly appreciated. I'm planning on doing plenty more haskell projects, so I'd like to nip any bad practices in the bud.

You can find the project here:

Thanks in advance!

submitted by tominated
[link] [3 comments]
Categories: Incoming News

Tutorial for reader and write monad?

Haskell on Reddit - Thu, 07/10/2014 - 9:24pm

Hey everyone. I've just started out with Haskell, and I just can't seem to understand how the reader and writer monads work. Could someone point me to some place where they're explained well? Thanks!

submitted by Bollu
[link] [15 comments]
Categories: Incoming News

Meetup - Calling all haskellers from Croatia

haskell-cafe - Thu, 07/10/2014 - 8:07pm
We've opened a meetup group for all haskellers from Croatia and someone suggested we should post info about it here as it might reach more interested people that way. Here's the link to the meetup group (please join): Sorry for the spam if this doesn't apply to you. -Deni _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe< at >
Categories: Offsite Discussion

[ANN](and feedback request) unagi-chan: Fast and scalable concurrent queues for x86, with a Chan-like API

haskell-cafe - Thu, 07/10/2014 - 7:39pm
I'm happy to finally release unagi-chan, an implementation of high-performance concurrent FIFO queues that have an API very similar to Control.Concurrent.Chan. You can see benchmarks and documentation here: If you have a moment (especially if you're on a non-x86 architecture) please take 10 minutes and run the tests with: $ cabal configure --enable-tests $ cabal build $ time ./dist/build/test/test Thanks to Ryan Newton for helping answer some low-level questions about his atomic-primops package which provides the CAS and fetch-and-add implementations that are the core of unagi-chan. REQUEST FOR FEEDBACK: would anyone be interested in any functionality like the following: - concurrent builder for Text, something like: new :: IO (InTextChan , Lazy.Text) write :: String -> InTextChan -> IO () - something similar for ByteString (is there a class for types convertable to ByteString?) - concurrent Text and/or ByteSt
Categories: Offsite Discussion

PEPM 2015: Call for papers

General haskell list - Thu, 07/10/2014 - 6:43am
PEPM 2015 Paper Submission Deadline: September 12 (FIRM) Note: deadline is significantly earlier than previous years. Hope to see you in Mumbai, India! ----------------------------- C A L L F O R P A P E R S ----------------------------- ======= PEPM 2015 =========== ACM SIGPLAN 2015 WORKSHOP ON PARTIAL EVALUATION AND PROGRAM MANIPULATION Tue-Wed, January 13-14, 2015, Mumbai, India, co-located with POPL'15 Sponsored by ACM SIGPLAN SCOPE The PEPM Symposium/Workshop series aims at bringing together researchers and practitioners working in the areas of program manipulation, partial evaluation, and program generation. PEPM focuses on techniques, theory, tools, and applications of analysis and manipulation of programs. The 2015 PEPM workshop will be based on a broad interpretation of semantics-based program manipulation and continue last years' successful effort to expand t
Categories: Incoming News

New gtk2hs 0.12.4 release

gtk2hs - Wed, 11/21/2012 - 12:56pm

Thanks to John Lato and Duncan Coutts for the latest bugfix release! The latest packages should be buildable on GHC 7.6, and the cairo package should behave a bit nicer in ghci on Windows. Thanks to all!


Categories: Incoming News