- Haskell-Cafe - Haskell.org

ANNOUNCE: fix-imports 1.0.0
by Evan Laforge 31 Mar '12

31 Mar '12

I just uploaded a new version to hackage. This improves the heuristics for guessing the right module quite a bit, and adds a local config file where you can prioritize or de-prioritize packages or module prefixes. It's also a bit faster. >From the hackage description: A small standalone program to manage the import block of a haskell program. It will try to add import lines for qualified names with no corresponding import, remove unused import lines, and sort the import block according to some heuristic you can define. This only works for qualified imports! Unqualified imports are left untouched. It's most convenient if bound to an editor key. Recent major changes: version 1.0.0 Change name from FixImports to fix-imports, which is more unixy. Change ghc-pkg parsing from String to Text. It's noticeably faster. Add a more flexible system for prioritizing imports. When there are several possibilities for a module name, they are all given to a single function to decide. The config file moved from fix-imports-priority to .fix-imports and can now specify sort orders for packages and modules by prefix. Make -i includes for non-existent dirs ignored instead of causing an error.

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hint and type synonyms
by Claude Heiland-Allen 31 Mar '12

31 Mar '12

Hi all, What's the recommended way to get hint[0] to play nice with type synonyms[1]? A problem occurs with in scope type synonyms involving types not in scope. I came up with this after looking at the source[2], but it makes me feel ill: --8<-- -- hint and type synonyms don't play nice module Main where import Language.Haskell.Interpreter import Data.Typeable as T import Data.Typeable.Internal import GHC.Fingerprint.Type main = failure >> success test t = (print =<<) . runInterpreter $ do setImports [ "Prelude" ] interpret "1/5" t failure = test (as :: Rational) -- Left (WontCompile [GhcError {errMsg = "Not in scope: type constructor or class `Ratio'"}]) success = test (as :: Q) -- Right (1 % 5) newtype Q = Q Rational instance Show Q where show (Q a) = show a showsPrec n (Q a) = showsPrec n a instance Typeable Q where typeOf _ = TypeRep (Fingerprint 0 0) (T.mkTyCon "Rational") [] --8<-- Thanks, Claude [0] http://hackage.haskell.org/package/hint [1] http://www.haskell.org/onlinereport/decls.html#type-synonym-decls [2] http://hackage.haskell.org/packages/archive/hint/0.3.3.4/doc/html/src/Hint-…

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New in haskell for old-timers?
by Mats Rauhala 31 Mar '12

31 Mar '12

I ended up discussing in university with the person next to me about Haskell. He's an old-timer who's even been to conferences, but it's a long time since he's looked at Haskell. He asked me to write "what's new in Haskell", but I'm not too experienced myself. I didn't realize asking how long it's been for him, but he mentioned about new fronts in optimizing compilers and specifically mentioned a compiler that could compile to legible C. I've been following Haskell only for a couple of years, so my intuition tells me that he means `-fvia-c`, and I mentioned to him that it's about to be deprecated and replaced with the llvm back-end. I also mentioned hackage, but he hadn't heard of it. He mentioned that he'd like to use Haskell for his master's thesis, but we got a interrupted when he said what it was about. Formal something or other. I trust you fellow Haskellers have better understanding of the time frame, and can think of something interesting new features, libraries, communities etc. that's since been emerged. I ask you to give me something interesting to give him and reinstate his interest. -- Mats Rauhala MasseR

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for = flip map
by Johannes Waldmann 30 Mar '12

30 Mar '12

Good: we have mapM, and we have forM ( = flip mapM ) . Sure this is just a convenience, and indeed "forM xs $ \ x -> do ..." is quite handy, especially if "xs" is really small, and "..." is some larger expression. Bad: we have map, but we are missing: for ( = flip map ) . The function is very convenient, for the same reasons as above. I can't remember how often I typed "for = flip map" in a source file. I never put this definition in a module either, since the import statement would be longer than the definition. So, I'm all for "for" . In Data.List? In the Prelude? (Should put it right next to "map".) - J.W.

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ANN: Happstack 7
by Jeremy Shaw 30 Mar '12

30 Mar '12

We are pleased to announce the release of Happstack 7! Happstack is a fast, modern, web application framework written in Haskell. Please check out the brand new happstack.com website to read about what is new in Happstack 7, and what we are planning for Happstack 8, and what makes Happstack great! http://www.happstack.com/ - jeremy

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blog software in Haskell?
by Ketil Malde 30 Mar '12

30 Mar '12

Hi, I just upgraded my server, and set up everything again. Except wordpress, as 1) I'm not too fond of its user interface, and 2) it's a big pile of PHP, difficult to keep updated, and basically a disaster waiting to happen (and in fact, it was hacked at one point). Before I enable it again, is there any alternatives I should consider? Preferably written in Haskell, of course, but other suggestions welcome as well. -k -- If I haven't seen further, it is by standing in the footprints of giants

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Re: [Haskell-cafe] a code that cannot compile with or without NoMonomorphismRestriction
by Denis Moskvin 30 Mar '12

30 Mar '12

f :: (Show a, Ord b) => (a -> String, b -> b -> Bool) f = let commond_definitions = undefined in let f1 = id.show f2 x = (< x) in (f1, f2) > From: Ting Lei <tinlyx(a)hotmail.com> > To: <haskell-cafe(a)haskell.org> > Cc: > Date: Wed, 28 Mar 2012 23:42:30 -0700 > Subject: [Haskell-cafe] a code that cannot compile with or without > NoMonomorphismRestriction > Hi > > I have met a piece of code that cannot be compiled whether I add or remove > the NoMonomorphismRestriction flag (as of GHC 7.0.4, Haskell platform > 2011.4.0.0). > I have extracted a minimal example below: > > > > {-# LANGUAGE NoMonomorphismRestriction #-} > (f1, f2) = > let commond_definitions = undefined in > let f1 = id.show > f2 x = (< x) > in > (f1, f2) > > I needed this format because there are many shared definitions in > common_definitions for f1 and f2, and I want to keep them local. > > If I compile them with NoMonomorphismRestriction, I get: > > D:\work\test.hs:7:8: > Ambiguous type variable `a0' in the constraint: > (Show a0) arising from a use of `f1' > Possible cause: the monomorphism restriction applied to the following: > f1 :: a0 -> String (bound at D:\work\hsOcaml\test.hs:2:2) > Probable fix: give these definition(s) an explicit type signature > In the expression: f1 > In the expression: (f1, f2) > In the expression: > let > f1 = id . show > f2 x = (< x) > in (f1, f2) > D:\work\test.hs:7:12: > Ambiguous type variable `a1' in the constraint: > (Ord a1) arising from a use of `f2' > Possible cause: the monomorphism restriction applied to the following: > f2 :: a1 -> a1 -> Bool (bound at D:\work\hsOcaml\test.hs:2:6) > Probable fix: give these definition(s) an explicit type signature > In the expression: f2 > In the expression: (f1, f2) > In the expression: > let > f1 = id . show > f2 x = (< x) > in (f1, f2) > Failed, modules loaded: none. > > If I comment out > -- {-# LANGUAGE NoMonomorphismRestriction #-} > I get: > > D:\work\hsOcaml\test.hs:4:17: > Ambiguous type variable `a0' in the constraint: > (Show a0) arising from a use of `show' > Possible cause: the monomorphism restriction applied to the following: > f1 :: a0 -> String (bound at D:\work\hsOcaml\test.hs:2:2) > Probable fix: give these definition(s) an explicit type signature > or use -XNoMonomorphismRestriction > In the second argument of `(.)', namely `show' > In the expression: id . show > In an equation for `f1': f1 = id . show > D:\work\hsOcaml\test.hs:7:12: > Ambiguous type variable `a1' in the constraint: > (Ord a1) arising from a use of `f2' > Possible cause: the monomorphism restriction applied to the following: > f2 :: a1 -> a1 -> Bool (bound at D:\work\hsOcaml\test.hs:2:6) > Probable fix: give these definition(s) an explicit type signature > or use -XNoMonomorphismRestriction > In the expression: f2 > In the expression: (f1, f2) > In the expression: > let > f1 = id . show > f2 x = (< x) > in (f1, f2) > Failed, modules loaded: none. > > Can anyone show me why this does not work and how to fix it (e.g. by > adding type signature as the error message suggested)? > I tried to add type signature by couldn't figure out the right way of > doing it. > > Thanks in advance! > > Ting Denis Moskvin

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DHD >>= UHac: Register by April 1 for the DHD
by Sean Leather 30 Mar '12

30 Mar '12

DHD >>= UHac will be held from Friday, April 20, to Sunday, April 22, 2012. The Dutch HUG Day (DHD) is on the first day. The Utrecht Hackathon (UHac) follows the DHD on Friday and continues through the weekend. http://www.haskell.org/haskellwiki/DHD_UHac *Please register for the DHD by Sunday, April 1.* Our sponsor for the DHD, Ordina, will use the registration to prepare the (free!) lunch and venue. If you're coming only to UHac and not the DHD, you can register by April 19. http://www.haskell.org/haskellwiki/DHD_UHac/Register See you in Utrecht! The Organizers: Sean Leather Jurriën Stutterheim Jurriaan Hage

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How helpful is h-99 (and should we complete the missing ones)?
by Ziyao Wei 30 Mar '12

30 Mar '12

Hi all, I am new at Haskell, but I am trying to learn as much as possible. While learning, I noticed that h-99 (the 99 Haskell problems) are highly recommended on haskell.org. However, the answers to the 99 problems are not finished, and some of the answers are not really Haskell-ish. So here's my question: how useful is h-99 (are they overrated as learning tools)? I find myself solve most of them in a "from the scratch" fashion (e.g., no Monad, no Applicative, no Functor aside from List and a few Maybe). Some of them are paper-worthy, for example the prime problems. I hope some guru-level Haskeller could do away the missing few, and maybe dive deeper into the surface to produce more insights (like the knights travel page or the sieve paper, which are both beautiful). And of course, if anyone could tell me how to learn Haskell without treating it as just another functional language, I'd be very grateful:) Thanks, Ziyao

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winhugs interrupts
by Doug McIlroy 29 Mar '12

29 Mar '12

On windows I have long used hugs under cygwin, but hugs doesn't get along well with cygwin's latest terminal emulator. So I switched to winhugs. Small problem that looms big: how do you interrupt an interminable expression evaluation in winhugs? Doug

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