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instance (Enum a) => IArray UArray a
by Mikhail Arefiev 21 Jan '12

21 Jan '12

Is there a reason why there is no instance of (Enum a) => IArray UArray a (other than that it will require OverlappingInstances and/or IncoherentInstances if e. g. UArray of Bools is used in the same code)? I have written one with the help of StackOverflow (http://stackoverflow.com/questions/8941386/) here is the source code: - UArrays.hs: http://hpaste.org/56728 - BenchmarkUArray.hs: http://hpaste.org/56729 - Makefile: http://hpaste.org/56727 Running the benchmark with +RTS -s on the UArray version shows 1,283,705,968 bytes allocated in the heap 113,304 bytes copied during GC 80,027,864 bytes maximum residency (2 sample(s)) 744,128 bytes maximum slop 155 MB total memory in use (0 MB lost due to fragmentation) versus 567,014,904 bytes allocated in the heap 962,651,888 bytes copied during GC 324,111,008 bytes maximum residency (3 sample(s)) 2,887,992 bytes maximum slop 545 MB total memory in use (0 MB lost due to fragmentation) on the boxed (Array Int Color) version. Also, boxed enum time is 1.75s and unboxed is 0.45s. The benchmark may be silly (I am not experienced in testing for Haskell performance), and I would be grateful for any tips on how to improve it to achieve more accurate measurements. Does having such thing make any sense? -- Best regards, Arefiev

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Re: [Haskell-cafe] Why were unfailable patterns removed and "fail" added to Monad?
by Michael Snoyman 20 Jan '12

20 Jan '12

On Jan 20, 2012 8:31 AM, "John Meacham" <john(a)repetae.net> wrote: > > > As expected, no warnings. But if I change this "unfailable" code above > > to the following failable version: > > > > data MyType = Foo | Bar > > > > test myType = do > > Foo <- myType > > return () > > > > I *still* get no warnings! We didn't make sure the compiler spits out > > warnings. Instead, we guaranteed that it *never* will. > > This is actually the right useful behavior. using things like > > do > Just x <- xs > Just y <- ys > return (x,y) > > will do the right thing, failing if xs or ysresults in Nothing. for > instance, in the list monad, it will create the cross product of the > non Nothing members of the two lists. a parse monad may backtrack and > try another route, the IO monad will create a useful (and > deterministic/catchable) exception pointing to the exact file and line > number of the pattern match. The do notation is the only place in > haskell that allows us to hook into the pattern matching mechanism of > the language in a general way. > > John I mention later that this is a "feature, not a bug" to some people, but I'm not one of them. The convenience of having this feature is IMO far outweighed by the cost of the runtime errors it can produce if you use the pattern matching in the wrong monad (e.g., IO, Reader, Writer...). Michael

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partial type annotations
by Nicholas Tung 20 Jan '12

20 Jan '12

Dear all, I wanted to voice support for a partial type annotations. Here's my usage scenario: I have a monad for an imperative EDSL, which has an associated expression data type, class (Monad m, Expression (ExprTyp m)) => MyDSLMonad m where data ExprTyp m :: * -> * and you write imperative EDSL code like so, my_code_block = do x <- instruction1 y <- instruction2 (x + x) ... I want the user to be able to annotate "x is an Integer". However, to do that now, one has to now add a type signature for my_code_block like so, so that the $m$ variable is in scope, my_code_block :: forall m. MyDSLMonad m => m () my_code_block = do x :: ExprTyp m Integer <- instruction1 ... If such a feature were available, one could write a nice type synonym "Expr" and use it like so, type Expr a = ExprTyp _ a my_code_block = do x :: Expr Integer <- instruction1 Suggestions for workarounds are appreciated. I created an `asExprTypeOf`, similar to Prelude's `asExprTyp`, but I don't like the syntax as much. Some previous discussion * http://www.haskell.org/pipermail/haskell/2002-April/009409.html * (a reply) http://www.haskell.org/pipermail/haskell/2002-April/009413.html * http://hackage.haskell.org/trac/haskell-prime/wiki/PartialTypeAnnotations cheers, Nicholas — https://ntung.com — 4432-nstung

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Haddock 2.8.1 and alex 3.0.1
by cwr＠netcom.co.uk 20 Jan '12

20 Jan '12

Building haddock 2.8.1 with alex 3.0.1 I get the error message: [ 7 of 33] Compiling Haddock.Lex ( dist/build/Haddock/Lex.hs, dist/build/Haddock/Lex.o ) src/Haddock/Lex.x:150:17: Couldn't match expected type `AlexInput' against inferred type `(t, t1, t2)' In the first argument of `alexScan', namely `inp' In the expression: alexScan inp sc In the expression: case alexScan inp sc of { AlexEOF -> [] AlexError _ -> error "lexical error" AlexSkip inp' _ -> go inp' sc AlexToken (inp'@(pos', _, _)) len act -> Haddock 2.8.1 builds correctly (or at least completely) with alex 2.3.5 I'm not sure if this is a bug or a feature - if it's a feature, then the dependencies for haddock 2.8.1 need updating. Will

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Not an isomorphism, but what to call it?
by Sean Leather 20 Jan '12

20 Jan '12

I have two types A and B, and I want to express that the composition of two functions f :: B -> A and g :: A -> B gives me the identity idA = f . g :: A -> A. I don't need g . f :: B -> B to be the identity on B, so I want a weaker statement than isomorphism. I understand that: (1) If I look at it from the perspective of f, then g is the right inverse or section (or split monomorphism). (2) If I look at from g, then f is the left inverse or retraction (or split epimorphism). But I just want two functions that give me an identity on one of the two types and I don't care which function's perspective I'm looking at it from. Is there a word for that? Regards, Sean

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Vector (or List) to CArray?
by Dominic Espinosa 20 Jan '12

20 Jan '12

Hello, I'm trying to use the fftw binding, and its functions operate on CArrays of Complex. My data is coming from hsndfile, so it starts out as a Vector of Double. How do I convert this data to CArray? The API functions in the CArray module don't seem to indicate how. Thanks.

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cabal-install bootstrap.sh
by cwr＠netcom.co.uk 19 Jan '12

19 Jan '12

I've just spent most of a morning trying to get bootstrap.sh from the cabal-install package to work. The trick is to use "ghc-pkg init pathname" to initialise the package file - simply adding an empty package file or directory doesn't work. Whoever is responsible for cabal-install, could you please update the README to reflect this requirement. Thanks - Will

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Is there a way to do *allocation* profiling with a breakdown by type?
by Eugene Kirpichov 19 Jan '12

19 Jan '12

Hi, I've got a program that seems to spend much of its time allocating short-lived objects, which therefore don't show up in +RTS -hy or alike. Is there a way to get a breakdown by type of the objects that are being *allocated* but not necessarily retained (disappear after gen0)? -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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Conduit versions of wai and warp?
by Erik de Castro Lopo 19 Jan '12

19 Jan '12

Hi Michael, The current versions of wai and warp: http://hackage.haskell.org/package/wai http://hackage.haskell.org/package/warp still seem to be the versions that use enumerator. Any idea when the Conduit versions might show up on Hackage? Until then, should I be grabbing these packages from Github? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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PhD program at Portland State accepting applications for Fall 2012
by Nathan Collins 19 Jan '12

19 Jan '12

Hi, Portland State University has a lot going on in functional programming. The Fall 2012 PhD program application deadline is March 1 for US students and February 1 for international students: http://cs.pdx.edu/programs/admissions FP related work at Portland: - Tim Sheard is working on the Trellys project: design and implementation (in Haskell) of a "practical" dependently typed programming language. Joint project with Aaron Stump at UIowa and Stephanie Weirich at UPenn. Here "practical" means intended for programming more than for theorem proving. Supports theorem proving, but also logically dubious features like general recursion and Type in Type. The key design issue is the interplay between safe and unsafe features. Project repo: https://code.google.com/p/trellys/ - Andrew Tolmach, Mark Jones, and James Hook are working on HASP: design and implementation (in Haskell) of a functional programming language for high-assurance systems programming. Project page: http://hasp.cs.pdx.edu/ - Sergio Antoy is working on narrowing in functional logic programming and the Curry FLP language. Intro to functional logic programming: http://web.cecs.pdx.edu/~antoy/homepage/publications/cacm/paper.pdf PAKCS Curry implementation: http://www.informatik.uni-kiel.de/~pakcs/ - Arthur Peters, a student of Sergio’s, is working on a new implementation of Curry based on a simplified graph rewriting model of functional logic computation. A paper about it and the prototype implementation are available at: http://web.cecs.pdx.edu/~amp4/vialois - Andrew Black is a co-author on a recent paper on Haskell for the Cloud: https://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/ Other FP resources in Portland: - Galois, located a half mile from the computer science department, hosts many "Tech Talks", open to the public: http://corp.galois.com/blog/ - Functional programming study group: http://pdxfunc.org/ Living in Portland: - Portland is a very popular US city, known for beer, bikes, music, and street food: http://en.wikipedia.org/wiki/Portland_Oregon (wikipedia is blacked out today) Cheers, -nathan (PhD student in programming languages)

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