December 2007 - Haskell-Cafe

Why does this blow the stack?
by Justin Bailey 26 Dec '07

26 Dec '07

Given this function: dropTest n = head . drop n $ [1..] I get a stack overflow when n is greater than ~ 550,000 . Is that inevitable behavior for large n? Is there a better way to do it? Justin

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Fwd: Re: [Haskell-cafe] Functions are first class values in C
by Cristian Baboi 26 Dec '07

26 Dec '07

On Sat, 22 Dec 2007 16:48:51 +0200, Peter Verswyvelen <bf3(a)telenet.be> wrote: > Cristian Baboi wrote >> Lazy constant in C: >> int C1 (){ return 7; } > > Not really, this is not lazy, since it always recomputes the value "7". > > To have "lazy" values in C you would have to do something like: > > struct ValueInt > { > int IsComputed; > union > { > int Value; > struct > { > int (*ComputeValue)(void *args); > void* Args; > }; > }; > }; > > int GetLazyInt (ValueInt* v) > { > if( !v->IsComputed ) > { > v->Value = v->ComputeValue(v->Args); > v->IsComputed = true; > } > return v->Value; > } > > But this of course, is totally useless in C and very bulky. It's also > impossible to know when to call freemem on the Args (hence garbage > collection in FP), when *not* to use lazy values but strict values > instead (hence strictness analysis in FP), etc... I know FP have automatic garbage collection. I know FP compilers use strictness analysis. In C++ one can isolate memory management in constructors and destructors. There are C compilers that are also able to do some optimizations. > I must say I had the same opinion as you had for many many years. I > always thought "functions as first class values" where just function > pointers, so what is it these Haskell/FP guys are so excited about? But > if you dig deeper, you'll see the magic... Notice you will have to give > yourself some time; it is very hard to get out of the imperative blob. > E.g. I'm still being sucked into the imperative blob after my first year > of Haskell hacking :) > PS: As I'm relatively new to Haskell, don't take the above C code too > seriously; it certainly will not reflect the way a real Haskell system > works. I am new to Haskell, but not new to declarative programming. I programmed in Prolog for several years, and I tryed LISP, but I don't liked the LISP syntax. I don't take my C example seriously either. The thing is I think that for a language to have "first-class" functions, it must be "homoiconic" if I understand the terms correctly. Have you tryed to write a Haskell program that manipulate Haskell programs ? Please don't tell me that Haskell compiler is written in Haskell, because there are C compilers written in C.

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Trouble with types
by Konstantin Vladimirov 25 Dec '07

25 Dec '07

Colleagues! The essence of trouble may be given by code: [haskell] module TypeTrouble where class FirstClass a where firstFunction :: (SecondClass b) => a -> b class SecondClass a where secondFunction :: a -> Double data FirstData = FirstData Double data SecondData = SecondData Double instance SecondClass SecondData where secondFunction (SecondData d) = d instance FirstClass FirstData where firstFunction (FirstData d) = SecondData d [/haskell] I need, the firstFunction of FirstClass types to return a value of a SecondClass type. For example SecondData for FirstData, but for some FirstClass ThirdData, some SecondClass FourthData, etc. GHC 6.8.1 produces an error: [quote] typetrouble.hs:17:31: Couldn't match expected type `b' against inferred type `SecondData' `b' is a rigid type variable bound by the type signature for `firstFunction' at typetrouble.hs:4:31 In the expression: SecondData d In the definition of `firstFunction': firstFunction (FirstData d) = SecondData d In the definition for method `firstFunction' [/quote] How can I make this idea work? -- With best regards, Konstantin

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Peyton Jones' "Beautiful Concurrency" .. i.e. Software Transactional Memory ...
by Galchin Vasili 24 Dec '07

24 Dec '07

Hello, My brain is a "out to lunch". I have read the paper "Beautiful Concurrency" (as well as a bunch of "gaming" papers regarding multi cores). I am playing with the "Account" example in the paper. In the paper, the alias "type Account = TVar Int" is used. I want to actually apply the function "withdraw" to an example "Account" parameter. I keep getting a type check error. Can someone give me a concrete example of > withdraw ...... ?? Kind regards, Vasya

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FW: Treating command-line arguments as a Haskell expression
by Simon Peyton-Jones 24 Dec '07

24 Dec '07

Would someone familiar with the command-line-parsing libraries care to help Krassimir? Thanks Simon -----Original Message----- From: Krassimir Krustev [mailto:krassimir.krustev@gmail.com] Sent: 23 December 2007 11:38 To: Simon Peyton-Jones Subject: Treating command-line arguments as a Haskell expression Dear Dr. Jones: I've been pulling my hair over the Christmas Holidays trying to figure the following: I want to call a function from within Haskell module so that the name of this function corresponds to my first command-line argument and the rest rest of the command-line arguments are would become themselves the argument to this function. Coundn't figure this out. It is my first week with Haskell. Yours, Krassimir

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Re: [Haskell-cafe] Functions are first class values in C
by Miguel Mitrofanov 24 Dec '07

24 Dec '07

> It is true, however, that preprocessing used to be (in pre-standard > days) separate from the language. This has not been true for decades. Well, I've seen cpp to be used as a preprocessor not for C sources but for something else.

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Fwd: Re: [Haskell-cafe] Functions are first class values in C
by Cristian Baboi 24 Dec '07

24 Dec '07

------- Forwarded message ------- From: "Cristian Baboi" <cristian.baboi(a)gmail.com> To: "Jules Bean" <jules(a)jellybean.co.uk> Cc: Subject: Re: [Haskell-cafe] Functions are first class values in C Date: Sat, 22 Dec 2007 15:58:50 +0200 On Sat, 22 Dec 2007 14:09:13 +0200, Jules Bean <jules(a)jellybean.co.uk> wrote: > Cristian Baboi wrote: >> Let me show you an example to prove it. > That's not C. > That's the C preprocessor, which is a textual substitution macro > language. Well, the preprocessor is part of the language in a way. These two come together. > Macros certainly aren't first class (you can't pass a macro to a > function, only its expansion). In Haskell I cannot pass a function to a function, only its expansion. > C does support function pointers, which are something like first class > functions. The main things C lacks which people associate with true > first-class function is: > The ability to construct anonymous/local functions. If you look at the example you will see I've done that. > The ability to capture local variables and return a function with some > variables bound. If I can construct "anonymous" functions and "constants", I can construct functions with some variables bound. > The ability to write type-safe functions with polymorphic arguments. I didn't know this must be a property of first-class functions. C is staticaly typed, so type errors will be detected. Haskell is just C with some syntactic sugar :-)

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Fwd: Re: [Haskell-cafe] more on the "Beautiful Concurrency" function "withdraw" application ...
by Dean Herington 24 Dec '07

24 Dec '07

>Date: Mon, 24 Dec 2007 01:44:16 -0500 >To: "Galchin Vasili" <vigalchin(a)gmail.com>, haskell-cafe(a)haskell.org >From: Dean Herington <heringtonlacey(a)mindspring.com> >Subject: Re: [Haskell-cafe] more on the "Beautiful Concurrency" >function "withdraw" application ... >Cc: >Bcc: >X-Attachments: > >At 12:19 AM -0600 12/24/07, Galchin Vasili wrote: >>module Main where >>import Control.Concurrent.STM >>import Control.Concurrent >>import System.Random >> >>type Account = TVar Int >> >>transfer :: Account -> Account -> Int -> IO () >>transfer from to amount >> = atomically (do {deposit to amount; >> withdraw from amount}) >> >>deposit :: Account -> Int -> STM () >>deposit acc amount = withdraw acc (- amount) >> >>withdraw :: Account -> Int -> STM () >>withdraw acc amount >> = do { bal <- readTVar acc; >> writeTVar acc (bal - amount) } >> >> >> >>When I try >> >>withdraw (TVar 10) .. >> >>I get "Not in scope: data constructor 'TVar'. OK .. "Tvar" is a >>class and not a Haskell data type ... so now what? > >You first need to create an account: > > > acct <- newTVar 100 Sorry, that would be: > acct <- atomically (newTVar 100) >Then you can withdraw from it: > > > atomically (withdraw acct 10) > >Dean

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more on the "Beautiful Concurrency" function "withdraw" application ...
by Galchin Vasili 24 Dec '07

24 Dec '07

module Main where import Control.Concurrent.STM import Control.Concurrent import System.Random type Account = TVar Int transfer :: Account -> Account -> Int -> IO () transfer from to amount = atomically (do {deposit to amount; withdraw from amount}) deposit :: Account -> Int -> STM () deposit acc amount = withdraw acc (- amount) withdraw :: Account -> Int -> STM () withdraw acc amount = do { bal <- readTVar acc; writeTVar acc (bal - amount) } When I try withdraw (TVar 10) .. I get "Not in scope: data constructor 'TVar'. OK .. "Tvar" is a class and not a Haskell data type ... so now what? Vasya

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Re: ANNOUNCE: GHC version 6.8.2
by Judah Jacobson 24 Dec '07

24 Dec '07

I neglected to CC the below email to haskell-cafe; apologies if anyone gets this twice. ---------- Forwarded message ---------- From: Judah Jacobson <judah.jacobson(a)gmail.com> Date: Dec 21, 2007 2:12 PM Subject: Re: [Haskell-cafe] Re: ANNOUNCE: GHC version 6.8.2 To: John Dorsey <haskell(a)colquitt.org> On Dec 21, 2007 12:48 PM, John Dorsey <haskell(a)colquitt.org> wrote: > On a related topic, I've been trying to build 6.8.2 on Leopard lately. > I've been running up against the infamous OS X readline issues. I know > some builders here have hacked past it, but I'm looking for a good > workaround... ideally one that works without changes outside the GHC > build area (besides installing a real readline). > > Here's what I noticed before I started drowning in the build platform. > (I'm no gnu-configure expert nor GHC insider.) > > I can get gnu-readline installed from Macports, no problem. > > The top-level configure in GHC doesn't respond to my various attempts: > > o using --with-readline-libraries and --with-readline-includes > (Although it looks like the libraries/readline/configure script > might recognize these, I can't get an option to pass through.) Actually, this is supposed to work. When running the top-level ghc configure, you should be able to just say ./configure --with-readline-libraries=/opt/local/lib --with-readline-includes=/opt/local/include and have those arguments automatically passed to the readline configure script. I just checked that this works on my machine (Tiger x86) when building 6.8.2 against MacPorts' readline. If it doesn't work for you, what errors are you getting? Also, a couple of other generic questions: - Are you on x86 or PPC? (The latter is not working right yet with Leopard.) - How are you bootstrapping the build of 6.8.2: are you using a pre-built binary of ghc-6.8.1, or some other way? Best, -Judah

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