- Haskell-Cafe - Haskell.org

Wishful thinking: a text editor that expands function applications into function definitions
by Duane Johnson 03 Apr '09

03 Apr '09

So I was thinking about a "killer feature" for a text editor. Wouldn't it be neat if you could expand function calls into their definitions, in-place? For example, suppose we have "minus" defined like so, somewhere in another file: > minus (a, b, c) (x, y, z) = (a - x, b - y, c - z) Later, we make use of the function in our current context: > let p1 = (1, 2, 3) > p2 = (4, 5, 6) > in p1 `minus` p2 By telling the editor to "expand" the minus, we get a temporary replacing of the above with: > (1 - 4, 2 - 5, 3 - 6) Another example: > parse s = map readLine ls And supposing that readLine is defined somewhere else, moving the cursor to readLine in the line above and "expanding" becomes: > parse s = map (\line -> words $ dropWhile (== ' ') line) This is all pretty standard for the kinds of things we do in Haskell to work it out by hand, but is there any reason the parser couldn't do this? I think it would be even harder to do automatically in any other language. Maybe it's already been attempted or done? Curious, Duane Johnson

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Problem with prepose.lhs and ghc6.10.1
by oleg＠okmij.org 03 Apr '09

03 Apr '09

> ../haskell/prepose.lhs:707:0: Parse error in pattern > which is pointing at: > normalize a :: M s a = M (mod a (modulus (undefined :: s))) The code indeed used lexically scoped type variables -- which GHC at that time implemented differently. Incidentally, on the above line, M s a is the type annotation on the result of (normalize a) rather than on the argument 'a'. Therefore, putting a parentheses like (a :: M s a) is wrong. Since the implementation of local type variables in GHC changed over the years, it's best to get rid of them. Here is the same line in the Hugs version of the code > normalize :: (Modular s a, Integral a) => a -> M s a > normalize a = r a __ > where > r :: (Modular s a, Integral a) => a -> s -> M s a > r a s = M (mod a (modulus s)) Of course the signature for normalize can be omitted. Hugs did not support lexically scoped type variables then (and probably doesn't support now). Today I would have written this definition simpler: > normalize :: (Modular s a, Integral a) => a -> M s a > normalize a = r > where r = M (mod a (modulus (tcar2 r))) > > tcar2:: f s x -> s > tcar2 = undefined (the function tcar2 is used in a few other places in the Hugs code, for a similar purpose).

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ANN: hledger 0.4 released
by Simon Michael 03 Apr '09

03 Apr '09

Dear all, I have released hledger 0.4 on hackage. There is also a new website at http://hledger.org , with screenshots (textual!), a demo (will it survive!?), and docs (not too many!) Release notes are at http://hledger.org/NEWS , and bravely pasted below. In case you forgot: hledger is a text-mode double-entry accounting tool. It reads a plain text journal file describing your transactions and generates precise activity and balance reports. I use it every day to track money and time, and as the basis for client invoices and tax returns. hledger is a partial clone, in haskell, of John Wiegley's excellent ledger. I wrote it because I did not want to hack on c++ and because haskell seemed a good fit. Please cabal update and cabal install hledger and give it a whirl. Install with the "-f happs" flag to enable the new happstack-based web interface. I am sm on the #ledger channel on freenode, and I welcome your feedback, especially if you notice a problem. Best - Simon ------------------------------------------------- 2009/04/03 hledger 0.4 released Changes: * new "web" command serves reports in a web browser (install with -f happs to build this) * make the vty-based curses ui a cabal build option, which will be ignored on MS windows * drop the --options-anywhere flag, that is now the default * patterns now use not: and desc: prefixes instead of ^ and ^^ * patterns are now case-insensitive, like ledger * !include directives are now relative to the including file (Tim Docker) * "Y2009" default year directives are now supported, allowing m/d dates in ledger * individual transactions now have a cleared status * unbalanced entries now cause a proper warning * balance report now passes all ledger compatibility tests * balance report now shows subtotals by default, like ledger 3 * balance report shows the final zero total when -E is used * balance report hides the final total when --no-total is used * --depth affects print and register reports (aggregating with a reporting interval, filtering otherwise) * register report sorts transactions by date * register report shows zero-amount transactions when -E is used * provide more convenient timelog querying when invoked as "hours" * multi-day timelog sessions are split at midnight * unterminated timelog sessions are now counted. Accurate time reports at last! * the test command gives better --verbose output * --version gives more detailed version numbers including patchlevel for dev builds * new make targets include: ghci, haddocktest, doctest, unittest, view-api-docs * a doctest-style framework for functional/shell tests has been added * performance has decreased slightly: || hledger-0.3 | hledger-0.4 | ledger-0.3 ==============================++========== -f sample.ledger balance || 0.02 | 0.01 | 0.07 -f sample1000.ledger balance || 1.02 | 1.39 | 0.53 -f sample10000.ledger balance || 12.72 | 14.97 | 4.63 Contributors: * Simon Michael * Tim Docker * HAppS, happstack and testpack developers Stats: * Known errors: 0 * Commits: 132 * Committers: 2 * Tests: 56 * Non-test code lines: 2600 * Days since release: 75

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Wrappers, API's for Web Apps - Google Summer of Code
by Hrushikesh Tilak 03 Apr '09

03 Apr '09

Hi, I am interested in participating in this year's Google Summer of Code. One of my proposals is going to be to write and extend existing Haskell wrappers and API's for web services. Some of the popular web services that I use are Google Maps, Flickr, digg, reddit, Facebook and twitter, Quick google searches reveal that great haskell tools already exist for twitter, while there is an ongoing project for a Facebook API by Jeremy Shaw. I couldn't find any haskell projects for the other tools. I find this project interesting primarily because I use these tools and would like to have haskell interfaces to them. Along with tools like Happstack, these interfaces would extend the power of haskell for developing Web 2.0 apps and mashups. I would like to enquire about the community's opinions and interests in such a project. I would also appreciate any suggestions either pertaining to any other web services. I have created a ticket at http://hackage.haskell.org/trac/summer-of-code/ticket/1578 Thanks, Hrushikesh Tilak

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Loading C object files into ghci
by Murray 03 Apr '09

03 Apr '09

Hi Cafe, A quick question: Is there a way to load a C object file associated with FFI imports into ghci after invocation (or in a .ghci file)? I've checked the docs as carefully as I can and can't find anything. Naming the file on the command line works of course but I was hoping for something I could keep in a per-directory .ghci file so that emacs and yi interactive modes work properly. I know I can (setq haskell-program-name "ghci /long/path/to/my/project/build/dist/.../blah.o") for emacs but I think it's hard-coded in yi. Sorry if this is obvious, or a FAQ. Murray. <murray(a)sonology.net>

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Missing dependency?
by Lyle Kopnicky 03 Apr '09

03 Apr '09

Hi folks, Since the time package is not included in ghc-6.10.2, I installed it via cabal. Then I tried to configure my project, and it says that the dependency is missing. Mysterious. Can anyone explain? lwk@lwk-desktop:~/devel/vintage-basic$ ghc-pkg list /usr/local/lib/ghc-6.10.2/./package.conf: Cabal-1.6.0.3, HUnit-1.2.0.3, QuickCheck-1.2.0.0, array-0.2.0.0, base-3.0.3.1, base-4.1.0.0, bytestring-0.9.1.4, containers-0.2.0.1, directory-1.0.0.3, (dph-base-0.3), (dph-par-0.3), (dph-prim-interface-0.3), (dph-prim-par-0.3), (dph-prim-seq-0.3), (dph-seq-0.3), editline-0.2.1.0, filepath-1.1.0.2, (ghc-6.10.2), ghc-prim-0.1.0.0, haddock-2.4.2, haskell-src-1.0.1.3, haskell98-1.0.1.0, hpc-0.5.0.3, html-1.0.1.2, integer-0.1.0.1, mtl-1.1.0.2, network-2.2.1, old-locale-1.0.0.1, old-time-1.0.0.2, packedstring-0.1.0.1, parallel-1.1.0.1, parsec-2.1.0.1, pretty-1.0.1.0, process-1.0.1.1, random-1.0.0.1, regex-base-0.72.0.2, regex-compat-0.71.0.1, regex-posix-0.72.0.3, rts-1.0, stm-2.1.1.2, syb-0.1.0.1, template-haskell-2.3.0.1, unix-2.3.2.0, xhtml-3000.2.0.1 /home/lwk/.ghc/i386-linux-6.10.2/package.conf: HTTP-4000.0.4, time-1.1.2.3, zlib-0.5.0.0 lwk@lwk-desktop:~/devel/vintage-basic$ runhaskell Setup.hs configure Configuring vintage-basic-1.0.1... Setup.hs: At least the following dependencies are missing: time >=1.1 lwk@lwk-desktop:~/devel/vintage-basic$ Thanks, Lyle

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Type families and kind signatures
by Louis Wasserman 03 Apr '09

03 Apr '09

The following module does not compile, and I can't figure out why: {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE KindSignatures #-} module Foo where import Control.Monad import Data.Maybe class Key k where type Map k :: * -> * empty :: Map k v look :: k -> Map k v -> Maybe v update :: k -> (Maybe v -> Maybe v) -> Map k v -> Map k v instance (Key k1, Key k2) => Key (k1, k2) where type Map (k1, k2) v = Map k1 (Map k2 v) empty = empty update (k1, k2) f = update k1 (update k2 f . fromMaybe empty) look (k1, k2) = look k1 >=> look k2 The compile fails with Foo.hs:16:1: Number of parameters must match family declaration; expected 1 In the type synonym instance declaration for `Map' In the instance declaration for `Key (k1, k2)' Is this a bug with type synonym families? Is there something silly I'm missing? Louis Wasserman wasserman.louis(a)gmail.com

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Issue with IsFunction/Vspace in GHC 6.10.1
by Jacques Carette 02 Apr '09

02 Apr '09

I was playing with some of Oleg's code (at end for convenience). After minor adjustments for ghc 6.10.1, it still didn't work. The error message is quite puzzling too, as it suggests adding exactly the constraint which is present... Any ideas? Jacques -- Oleg's definition of a vector space class, based on IsFunction and -- TypeCast. See http://okmij.org/ftp/Haskell/isFunction.lhs -- for the January 2003 message, which works in GHC 6.2.1 and 6.4 -- code below *works* in 6.8.1 AFAIK {-# LANGUAGE EmptyDataDecls, MultiParamTypeClasses, FunctionalDependencies #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-} module Q where class Vspace a v | v -> a where (<+>) :: v -> v -> v (*>) :: a -> v -> v instance (IsFunction v f, Vspace' f a v) => Vspace a v where (<+>) = doplus (undefined::f) (*>) = dostar (undefined::f) class Vspace' f a v | f v -> a where doplus :: f -> v -> v -> v dostar :: f -> a -> v -> v instance Num a => Vspace' HFalse a a where doplus _ = (+) dostar _ = (*) -- etc. No problem. instance (IsFunction v f, Vspace' f a v, Vspace a v) => Vspace' HTrue a (c->v) where doplus _ f g = \x -> f x <+> g x dostar _ a f x = a *> (f x) test1 = (1::Int) <+> 2 test2 = ((\x -> x <+> (10::Int)) <+> (\x -> x <+> (10::Int))) 1 test3 = ((\x y -> x <+> y) <+> (\x y -> (x <+> y) <+> x)) (1::Int) (2::Int) test4 = ((\x y -> x <+> y) <+> (\x y -> ((2 *> x) <+> (3 *> y)))) (1::Int) (2::Int) data HTrue data HFalse class IsFunction a b | a -> b instance TypeCast f HTrue => IsFunction (x->y) f instance TypeCast f HFalse => IsFunction a f -- literally lifted from the HList library class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x

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Retargeting the ghc code generator
by Vasili I. Galchin 02 Apr '09

02 Apr '09

Hello, Is there any project to retarget the GHC code generator to generate Common Intermediate Language(CLI) in order to run on Mono or .NET? I assume that Mondrian did precisely that. Thanks, Vasili

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Looking for practical examples of Zippers
by Gü?nther Schmidt 02 Apr '09

02 Apr '09

Hi, my quest for data structures continues. Lately I came across "Zippers". Can anybody point be to some useful examples? Günther

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