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ErrorT vs Either
by Gracjan Polak 17 May '11

17 May '11

Hi all, A why question: Why: Control.Monad.Error Prelude> runErrorT (fail "msg") :: IO (Either String Int) Left "msg" but Control.Monad.Error Prelude> (fail "msg") :: (Either String Int) *** Exception: msg ? -- Gracjan

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>>= definition for list monad in ghc
by Michael Vanier 16 May '11

16 May '11

Usually in monad tutorials, the >>= operator for the list monad is defined as: m >>= k = concat (map k m) -- or concatMap k m but in the GHC sources it's defined as: m >>= k = foldr ((++) . k) [] m As far as I can tell, this definition is equivalent to the previous one (correct me if I'm wrong), so I was wondering why this definition was chosen instead of the other one. Does anybody know? Thanks in advance, Mike

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Problems with high-level interface to LLVM
by Николай Кудасов 16 May '11

16 May '11

Hi there, I use llvm package for building compiler for a small C-like language and it have some problems with high-level interface that package provides. I use simple scheme: [parser] -> (untyped AST) -> [analyser] -> (typed AST) -> [LLVM] -> (object code). At first, as I want to use high-level interface, typed AST must satisfy all needed class constraints. For simple types and operations this can be solved by adding neccessary constraints to the corresponding definitions. But it's not very good solution, because in general typed AST can represent high-level data structures, that would have certain representation, known when generating code, but not checking logic. Problem becomes much more complex when I want to add function calls into my language. Type class CallArgs has an instance CallArgs (IO a) (CodeGenFunction r (Value a)), which can't be given as a constraint in typed AST because of free variable r. For a long time I have had no idea of how I can add function calls into my language. The only solution I see at the moment is to reject high-level llvm interface provided by llvm library and use directly FFI bindings to C. But I can't get rid of feeling that I'm missing something or doing all stuff totally wrong. It would be great if someone, who has experience in using high-level interface to LLVM for building a compiler, could help me. Thanks in advance, Nick

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Impossible class instance?
by Emil Axelsson 16 May '11

16 May '11

Hello! At the end of this message is a program with a simple expression type, and a class `ToExpr` that generalizes expressions to arbitrary Haskell types. Every node in `Expr` is annotated with some abstract information. The program raises the following type error: test.hs:13:5: Couldn't match type `(,) a' with `(,) (a, a)' Inaccessible code in the instance declaration In the instance declaration for `ToExpr (a, b)' It seems that the mere existence of the constraint info (a,b) ~ (info a, info b) causes this error. I was hoping that this constraint would make it possible to construct the value (ia,ib) in the class instance, which is otherwise not allowed. Note: I don't want to make `info` an associated type. The idea is to make this work with any type function `info` that fulfills the above constraint. Is there any way to make this work? / Emil -------------------- {-# LANGUAGE UndecidableInstances #-} data Expr info a where Int :: info a -> Int -> Expr info a Pair :: info (a,b) -> Expr info a -> Expr info b -> Expr info (a,b) getInfo :: Expr info a -> info a getInfo (Int info _) = info getInfo (Pair info _ _) = info class ToExpr a where type Internal a toExpr :: a -> Expr info (Internal a) instance ( ToExpr a , ToExpr b , info (a,b) ~ (info a, info b) ) => ToExpr (a,b) where type Internal (a,b) = (Internal a, Internal b) toExpr (a,b) = Pair (ia,ib) (toExpr a) (toExpr b) where ia = getInfo a ib = getInfo b

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flattenend multi-level programs, in diagram form
by Adam Megacz 16 May '11

16 May '11

A few people have mentioned that the flattening process is easier to understand from example diagrams. Now it's possible to generate those diagrams automatically: http://www.cs.berkeley.edu/~megacz/garrows/flattening.examples/ - a

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Internship in Tokyo and Singapore reminder
by employment 16 May '11

16 May '11

Tsuru Capital has openings for three month interns in the offices in Tokyo and in Singapore. Positions are available immediately and for the summer. We are looking for people with moderate experience with Haskell or another functional programming language. Evidence of work written in Haskell is a big plus. More details can be found here. There is a small coding task on the site. Please send this code with your application. http://www.tsurucapital.com/en/jobs.html Any who have applied before but would like to be considered again, please feel free to send a reminder of your availability along with the code sample. Interested candidates please send resumes with code to jobs(a)tsurucapital.com . We will arrange flights and accommodation. Accommodation plus salary will be worth approximately USD18.5k for the three month period.

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cannot install base-4.3.1.0 package
by Immanuel Normann 15 May '11

15 May '11

Hi, how can I install the base-4.3.1.0 package. I tried first > cabal install base that gives me this error: Resolving dependencies... cabal: Distribution/Client/Dependency/TopDown.hs:169:37-73: Non-exhaustive patterns in lambda I googled for this error and found nothing but the simple statement in the track system http://hackage.haskell.org/trac/hackage/ticket/605: "cabal install base is not expected to work, however the error message should be better, see #597. " (but the link to #547 doesn't solve the problem either) So I downloaded base-4.3.1.0 package from hackage and run: > runhaskell Setup configure which gives this error: Setup.hs:1:0: attempting to use module `Prelude' (./Prelude.hs) which is not loaded Then I noticed that there is configure script in the base-4.3.1.0 dir, so executed this which yields this error message: config.status: error: cannot find input file: `base.buildinfo.in' Why is it so complicated to install the base package? Thanks for any help!

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how to generate source code from TH Exp?
by Stefan Kersten 15 May '11

15 May '11

hi, i was wondering if it's possible to directly generate Haskell source code from a Template Haskell `Q Exp', i.e. use TH as a kind of preprocessor? i am asking because currently the iOS port of ghc doesn't support TH and i need to generate some instances for the persistent package [1,2]. i've been toying with fmap ppr . runQ $ q but the result needs to be edited by hand quite a bit. any ideas where to start? thanks, <sk> [1] http://hackage.haskell.org/package/persistent [2] http://hackage.haskell.org/package/persistent-template

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parMap doesn't work fine
by Grigory Sarnitskiy 14 May '11

14 May '11

Hello! I've just started using parallel computations in Haskell. parMap works fine, it is so easy to use. However, parMap fails with functions returning lazy structures, e.g. tuples. This code works as expected: (parMap rpar) bm tvalues bm :: Double -> Double is some heavy function. But if I want to return list of pairs (t, bm t) it doesn't use cpu cores fine (only one is in use): (parMap rpar) (\t -> (t, bm t)) tvalues The same is valid for functions returning lists. How do I use multiple cores with functions returning tuples?

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Experimental GHC with GHCJS built in and Cabal support for JavaScript files
by Hamish Mackenzie 14 May '11

14 May '11

Blackh and I have been trying out some ideas on how to make it easier use GHCJS in your Cabal projects. We took Victor's GHCJS code and added it back into GHC (we could not easily accomplish what we wanted using the current GHC API). We set it up so that GHC outputs a .js file whenever it outputs an object file. We added code to Cabal to install the .js files (in the same place as the .hi files). At link time GHC copies all the .js files from the packages used into a .jsexe directory (with the same name and in the same location as the executable). Finally when the executable is installed we copy the .jsexe directory along with it. Only files with differing modified dates are copied to avoid slowing things down too much. I have updated my yesod-slides example to show how all this can be used. https://github.com/hamishmack/yesod-slides The README.mardown there has instructions for building GHC and Cabal with the GHCJS stuff in it. It is a bit painful to build, but the changes to yesod-slides were quite minor... * Added ghcjs-rts to the build depends * Published the yesod-slides.jsexe folder (using Yesod's staticFiles) * Added <script> tags to <head> for rts-common.js and rts-plain.js * Added Julius script to init the rts Currently GHC is hard coded to use the GHCJS Trampoline calling convention (in GHC's HscMain.hs). Hamish

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