June 2009 - Haskell-Cafe

Confusion on the third monad law when using lambda abstractions
by Jon Strait 20 Jun '09

20 Jun '09

Hi everyone, I use and love Haskell, but I just have this nagging concern, that maybe someone can help me reason about. If I'm missing something completely obvious here and making the wrong assumptions, please be gentle. :) I'm reading the third (bind associativity) law for monads in this form: m >>= (\x -> k x >>= h) = (m >>= k) >>= h Now considering the definition of liftM2: liftM2 f m1 m2 = m1 >>= (\x1 -> m2 >>= (\x2 -> return (f x1 x2))) Isn't this liftM2 definition in the same form as the LHS of the third law equation, with (\x2 -> return (f x1 x2)) being the h function? Comparing this definition with the third law equation, the equation doesn't work because on the RHS equivalent; the x1 argument would be lost. So, why wasn't I finding a mention of a qualification that states that the third law only applies as long as the function in the h position doesn't reference arguments bound from previous 'binds'? It took going all the way back to Philip Wadler's 1992 paper, 'Monads for functional programming' to find reassurance: "The scope of variable x includes h on the left but excludes h on the right, so this law is valid only when x does not appear free in h." I'm also thinking of the Maybe monad, where Nothing >>= \x -> Just (x + 1) >>= \y -> return (y + 2) evaluates to Nothing after the first monadic bind and doesn't evaluate the rest of the expression. However, Nothing >>= Just . (+ 1) >>= return . (+ 2) should evaluate through the first and second monadic bind, evaluating to Nothing each time, of course. For the Maybe monad, both expressions give the same result, but if there were another monad defined like, data MadMaybe a = Nothing | Perhaps | Just a instance Monad MadMaybe where (Just x) >>= k = k x Nothing >>= _ = Perhaps Perhaps >>= _ = Nothing - then the two previous expressions run in the MadMaybe monad would evaluate to different values. Since the first of the previous expressions evaluates like the LHS of the third law equation above and the second expression evaluates like the RHS, the expressions should be equivalent, but they are not. Does this put into question the third monad law's relevance to Haskell monads, or is it that MadMaybe shouldn't be made a monad because it violates the third law? Thanks for any insight. Jon

9 12

hs-dotnet users?
by Günther Schmidt 20 Jun '09

20 Jun '09

Hi all, I'm just touching base with Sigbjorns hs-dotnet package. Are there other users of this package out there who would like to share their experience? Günther

1 0

Use MySQL from Haskell
by Maciej Podgurski 20 Jun '09

20 Jun '09

Hello, I'm trying to use MySQL from Haskell but it seems impossible for me to install one of the MySQL packages on my Windows XP machine. First I tired to install hsql-mysql-1.7.1 on GHC 6.10.3 but installing haskelldb-hsql failed with a hidden package error. So I added the old-time package to the cabal file but there still was a compile error (something due to the change from Exception to Exception e => e). So I switched to GHC 6.8.3 and tried it again. Now it says: Configuring hsql-mysql-1.7.1... Warning: 'extra-lib-dirs: /usr/lib/mysql' directory does not exist. Warning: 'include-dirs: /usr/include/mysql' directory does not exist. Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package process-1.0.0.1 requires filepath-1.1.0.0 package directory-1.0.0.1 requires filepath-1.1.0.0 package Cabal-1.6.0.3 requires filepath-1.1.0.2 Setup: Missing dependency on a foreign library: * Missing C library: mysqlclient This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. There's no Haskell package mysqlclient and I don't know how to install a C library in Haskell. So I switched to HDBC-2.1.1 and got the next compile error: Building convertible-1.0.5... Data/Convertible/Instances/Num.hs:671:0: warning: no newline at end of file [...] [5 of 8] Compiling Data.Convertible.Instances.C ( Data/Convertible/Instances/C.hs, dist\build/Data/C onvertible/Instances/C.o ) [6 of 8] Compiling Data.Convertible.Instances.Time ( Data/Convertible/Instances/Time.hs, dist\build/ Data/Convertible/Instances/Time.o ) Data/Convertible/Instances/Time.hs:64:0: Duplicate instance declarations: instance Typeable NominalDiffTime -- Defined at Data/Convertible/Instances/Time.hs:(64,0)-(65,42) instance Typeable NominalDiffTime -- Defined in time-1.1.3:Data.Time.Clock.UTC Data/Convertible/Instances/Time.hs:67:0: Duplicate instance declarations: instance Typeable UTCTime -- Defined at Data/Convertible/Instances/Time.hs:(67,0)-(68,34) instance Typeable UTCTime -- Defined in time-1.1.3:Data.Time.Clock.UTC So please help me, what GHC/package configuration will I need to use MySQL from my Haskell programs on Windows? I really like Haskell but all those "broken" packages are really discouraging. :( Best wishes, Maciej

6 9

Installing agda through cabal
by Paulo J. Matos 19 Jun '09

19 Jun '09

Hi all, I am trying to install agda through cabal but I get this: $ cabal install alex Resolving dependencies... [1 of 1] Compiling Main ( /tmp/alex-2.3.116333/alex-2.3.1/Setup.lhs, /tmp/alex-2.3.116333/alex-2.3.1/dist/setup/Main.o ) /tmp/alex-2.3.116333/alex-2.3.1/Setup.lhs:6:51: Warning: In the use of `buildVerbose' (imported from Distribution.Simple.Setup): Deprecated: "Use buildVerbosity instead" /tmp/alex-2.3.116333/alex-2.3.1/Setup.lhs:7:51: Warning: In the use of `defaultUserHooks' (imported from Distribution.Simple): Deprecated: "Use simpleUserHooks or autoconfUserHooks, unless you need Cabal-1.2 compatibility in which case you must stick with defaultUserHooks" Linking /tmp/alex-2.3.116333/alex-2.3.1/dist/setup/setup ... Warning: defaultUserHooks in Setup script is deprecated. Configuring alex-2.3.1... Preprocessing executables for alex-2.3.1... Building alex-2.3.1... [ 1 of 16] Compiling Set ( src/Set.hs, dist/build/alex/alex-tmp/Set.o ) [ 2 of 16] Compiling DFS ( src/DFS.hs, dist/build/alex/alex-tmp/DFS.o ) [ 3 of 16] Compiling Sort ( src/Sort.hs, dist/build/alex/alex-tmp/Sort.o ) [ 4 of 16] Compiling CharSet ( src/CharSet.hs, dist/build/alex/alex-tmp/CharSet.o ) [ 5 of 16] Compiling Map ( src/Map.hs, dist/build/alex/alex-tmp/Map.o ) [ 6 of 16] Compiling Paths_alex ( dist/build/autogen/Paths_alex.hs, dist/build/alex/alex-tmp/Paths_alex.o ) [ 7 of 16] Compiling Util ( src/Util.hs, dist/build/alex/alex-tmp/Util.o ) [ 8 of 16] Compiling AbsSyn ( src/AbsSyn.hs, dist/build/alex/alex-tmp/AbsSyn.o ) [ 9 of 16] Compiling ParseMonad ( src/ParseMonad.hs, dist/build/alex/alex-tmp/ParseMonad.o ) [10 of 16] Compiling Scan ( dist/build/alex/alex-tmp/Scan.hs, dist/build/alex/alex-tmp/Scan.o ) [11 of 16] Compiling Output ( src/Output.hs, dist/build/alex/alex-tmp/Output.o ) [12 of 16] Compiling Info ( src/Info.hs, dist/build/alex/alex-tmp/Info.o ) [13 of 16] Compiling NFA ( src/NFA.hs, dist/build/alex/alex-tmp/NFA.o ) [14 of 16] Compiling DFA ( src/DFA.hs, dist/build/alex/alex-tmp/DFA.o ) [15 of 16] Compiling Parser ( dist/build/alex/alex-tmp/Parser.hs, dist/build/alex/alex-tmp/Parser.o ) [16 of 16] Compiling Main ( src/Main.hs, dist/build/alex/alex-tmp/Main.o ) Linking dist/build/alex/alex ... Installing executable(s) in /home/pmatos/.cabal/bin pmatos@drserver ~ $ cabal install agda Resolving dependencies... Configuring Agda-2.2.2... cabal: alex version >=2.0.1 && <3 is required but it could not be found. cabal: Error: some packages failed to install: Agda-2.2.2 failed during the configure step. The exception was: exit: ExitFailure 1 As you can see, I had just finished installing alex 2.3.1, so why does cabal still request alex >=2.0.1 && <3? Cheers, Paulo Matos

4 5

duplicate definition for symbol
by Henk-Jan van Tuyl 19 Jun '09

19 Jun '09

L.S., I am trying to run a program in GHCi, but I get a meesage that an object file is loaded twice; it appears that two different versions of package "process" are loaded, see the session text below. How can I solve this? GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :load "Main.lhs" [1 of 1] Compiling Main ( Main.lhs, interpreted ) Ok, modules loaded: Main. *Main> :main Loading package syb ... linking ... done. Loading package base-3.0.3.0 ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.0 ... linking ... done. Loading package bytestring-0.9.1.4 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package filepath-1.1.0.1 ... linking ... done. Loading package Win32-2.2.0.0 ... linking ... done. Loading package directory-1.0.0.2 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package time-1.1.2.2 ... linking ... done. Loading package stm-2.1.1.2 ... linking ... done. Loading package split-0.1.1 ... linking ... done. Loading package HUnit-1.2.0.3 ... linking ... done. Loading package QuickCheck-1.2.0.0 ... linking ... done. Loading package process-1.0.1.1 ... linking ... done. Loading package haskell98 ... linking ... done. Loading package mtl-1.1.0.2 ... linking ... done. Loading package parsec-2.1.0.1 ... linking ... done. Loading package network-2.2.0.1 ... linking ... done. Loading package hslogger-1.0.7 ... linking ... done. Loading package process-1.0.1.0 ... GHCi runtime linker: fatal error: I found a duplicate definition for symbol _runInteractiveProcess whilst processing object file C:\Programs\ghc\ghc-6.10.1\process-1.0.1.0\HSprocess-1.0.1.0.o This could be caused by: * Loading two different object files which export the same symbol * Specifying the same object file twice on the GHCi command line * An incorrect `package.conf' entry, causing some object to be loaded twice. GHCi cannot safely continue in this situation. Exiting now. Sorry. -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

3 2

Running a "sub-process" which dies with the main program
by Deniz Dogan 19 Jun '09

19 Jun '09

Hi I couldn't come up with a better subject than this one, so anyways... I have a small program which spawns a subprocess. However, when I hit C-c, the subprocess won't die, instead it will just keep running until it's done or until I kill it. I've looked around in System.Process for something suitable for my needs, but I can't seem to find it. Any ideas? -- Deniz Dogan

3 8

Could someone give me a sample about haskelldb?
by Magicloud Magiclouds 19 Jun '09

19 Jun '09

Hi, I am learning it following the very few documents on its site. Well, I failed, with the import modules, I still cannot compile it. The error is on "T.*". 6 import Database.HaskellDB.HDBC.SQLite3 7 import Database.HaskellDB 8 import Database.HaskellDB.DBSpec 9 import Database.HaskellDB.DBSpec.PPHelpers 10 import Database.HaskellDB.Query 48 q = do 49 t <- table $ Table "notes" [] 50 restrict ( t!T.done .<>. False ) 51 r <- project ( T.subject << t!T.subject ) 52 order [ desc r T.priority 53 , desc r T.dt ] 54 return r -- 竹密岂妨流水过山高哪阻野云飞

2 2

FFI question
by Vasili I. Galchin 19 Jun '09

19 Jun '09

Hello, I would like to write bindings from Haskell to Java. Is this possible? If so, where are examples? Vasili

1 0

Runtime strictness analysis for polymorphic HOFs?
by Paul Chiusano 19 Jun '09

19 Jun '09

Hello, I was recently trying to figure out if there was a way, at runtime, to do better strictness analysis for polymorphic HOFs, for which the strictness of some arguments might depend on the strictness of the strictness of function types that are passed as arguments [1]. As an example, consider foldl. The 'seed' parameter of foldl can be made strict as long as the binary function used for the fold is strict in its arguments. Unfortunately, because strictness analysis is done statically, Haskell can't assume anything about the strictness of the binary function - assuming we only compile one instance of foldl, it must be the most conservative version possible, and that means making the seed parameter lazy. :-( I started thinking about ways you could to a check at runtime for this sort of thing, something to the effect of asking foldl, before heap-allocating a thunk for the seed parameter, whether that parameter could be made strict. foldl could then inspect other arguments that have been supplied, and based on these arguments, evaluate or go ahead with creating a thunk the seed parameter. It's a runtime cost, sure, but would it be more than the cost of having to do an additional heap allocation? In any case a small runtime cost might be worth it if the analysis becomes more uniform. So, my question is: does doing this sort of runtime analysis seem like a horrible idea? Is it even possible? Has anyone tried this in the past? (So far I haven't found anything, but would love references if people have them.) Note that I'm not suggesting Haskell should do anything like this. I'm playing around with the ideas because I'm interesting in creating a lazy language and I was hoping to have strictness analysis be very predictable and uniform, something the programmer can count on and use to simply reason about space usage ... which might be hopelessly unrealistic goal! I guess the more general question is - is "perfect" strictness analysis (however that is defined) possible, if we're willing to incur some runtime cost? What would that look like? Best, Paul [1]: More background on my thinking here - a bit half-baked, so bear with me! http://pchiusano.blogspot.com/2009/06/perfect-strictness-analysis-part-1.ht… http://pchiusano.blogspot.com/2009/06/perfect-strictness-analysis-part-2.ht…

9 14

how to #include files within parsec ... without unsafePerformIO?
by Leonard Siebeneicher 19 Jun '09

19 Jun '09

Hi Neil, Hi Daniel. Thank you for your help, using parsec 3.0.0 and liftIO was the solution. After installing new parsec I have had to use import Text.Parsec ... instead of ... import Text.ParserCombinators.Parsec ... to get it work. It seems Text.ParserCombinators.Parsec refers to old Parsec 2 Have a nice week--end, Leonard

1 0