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mtl: Why there is "Monoid w" constraint in the definition of class MonadWriter?
by Petr P 10 Dec '12

10 Dec '12

The class is defined as > class (Monoid w, Monad m) => MonadWriter w m | m -> w where > ... What is the reason for the Monoid constrait? It seems superfluous to me. I recompiled the whole package without it, with no problems. Of course, the Monoid constraint is necessary for most _instances_, like in > instance (Monoid w, Monad m) => MonadWriter w (Lazy.WriterT w m) where > ... but this is a different thing - it depends on how the particular instance is implemented. I encountered the problem when I needed to define an instance where the monoidal structure is fixed (Last) and I didn't want to expose it to the user. I wanted to spare the user of of having to write Last/getLast everywhere. (I have an instance of MonadWriter independent of WriterT, its 'tell' saves values to a MVar. Functions 'listen' and 'pass' create a new temporary MVar. I can post the detail, if anybody is interested.) Would anything break by removing the constraint? I think the type class would get a bit more general this way. Thanks for help, Petr Pudlak

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cabal configure && cabal build && cabal install
by Albert Y. C. Lai 10 Dec '12

10 Dec '12

Among many programmers, and/or users who manually unpack source tarball before installing, this idiom is very common: cabal configure cabal build cabal install This idiom is an urban legend, i.e., a popular error. "cabal install" re-does the "configure" and the "build" steps, among other things. That it has not caused trouble for you so far is only because: 1. "configure" has been idempotent, so far 2. the 2nd "build" has not caused duplicate work because ghc knows when not to re-compile One day, you will run into a surprise, because you will add some flags to "configure", and then... cabal configure --prefix=/nondefault --enable-shared cabal build cabal install Whee! Your laborously entered flags are lost on deaf ears. You get the default prefix, and you get the default disable-shared. Why? Because "cabal install" re-does the "configure" step, but this time, since you give no flags to "install", you give no flags to the 2nd "configure" either. If you begin with "cabal configure", the correct idiom is: cabal configure [flags] cabal build [cabal haddock, if you want] cabal copy cabal register If you find it too long, why not just begin with: cabal install [flags] [--enable-documentation, if you want]

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Fwd: education or experience?
by Eli Frey 10 Dec '12

10 Dec '12

woops, forgot to reply all ---------- Forwarded message ---------- From: Eli Frey <eli.lee.frey(a)gmail.com> Date: Sun, Dec 9, 2012 at 4:18 PM Subject: Re: [Haskell-cafe] education or experience? To: Mike Meyer <mwm(a)mired.org> So, to put my words in perspective: I am a 23 year old who dropped out when they were 16. I only just entered industry, so it's difficult to say where I have gaps in my knowledge or what I could have gotten if I'd been to college. That being said, I can't really say I'd rather to have done it any other way. Jerzy makes a good point that you might not be the best judge of what you should learn. You arn't yet an expert, and having one help guide you could be very valuable. I certainly wish I'd had a mentor through my experience. At the same token, you might just be the most qualified person to say what you should learn, exactly because you will be drawn to what fascinates you. If you're already showing interest in Haskell, there is a great deal of material produced by the community that I'm sure will wet your apatite in the right directions When I entered the industry, I got the attention of many well-respected companies. Sure there were some places that turned me away because I didn't have a degree, but they weren't the kinds of places I'de like to work anyway. At the end of the day, my dream employer said "come back and talk with us when you have more experience," and I landed a job where I enjoy my boss (an under-considered perk, IMHO. This is worth many $ to me), am given freedom/ownership of my work, and am greatly appreciated and nurtured in my growth. I often chuckle to myself at night about my freedom from student loans, too :P. Cheers, Eli On Sun, Dec 9, 2012 at 2:44 PM, Mike Meyer <mwm(a)mired.org> wrote: > > > Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: > >Christopher Howard wrote: > >Concerning a university education, there are two approaches> > >1. I want to learn as much as possible > >2. I want to learn just enough to get a high-paying job > > There's actually a third approach ( and probably more): > > 3. I want to learn to do this job as well as possible. > > >On the other hand, approaching university from the second point of view > > > >usually does not justify the cost for the little benefit you obtain > >this > >way. Unfortunately, it seems to me that the tuition costs in the U.S. > >strongly suggest the second approach. To avoid this, I recommend to > >either go abroad or become very good and acquire a scholarship. > > That really depends on the job at in question. When I was looking for > entry level programming jobs, not having a degree meant you never got past > the hr department. Getting a degree (pretty much any degree) was required > to get the high-paying job. I'm willing to believe that's no longer the > case for programmers, because academia has consistently failed to deliver > sufficient quality programmers to meet industry needs. On the other hand > (watching my sons deal with the job market), the litmus test for "you've > got what it takes to survive in the system" is now a masters, not a > bachelors, so maybe you're wrong about that. > > The other thing to consider is what your long-term goals are. Do you want > to be a code monkey all your life? Or do you aspire to more? What are the > requirements for that "more"? Getting a degree now may well avoid doing it > later. > > Finally, with approach #3, you really need a mentor who can tell you > whether or not you're doing a competent job. You're much more likely to > find that in a university environment than trying to learn things by > yourself. Joining an open source project might get it for you. > -- > Sent from my Android tablet with K-9 Mail. Please excuse my swyping. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > On Sun, Dec 9, 2012 at 4:18 PM, Eli Frey <eli.lee.frey(a)gmail.com> wrote: > So, to put my words in perspective: I am a 23 year old who dropped out > when they were 16. I only just entered industry, so it's difficult to say > where I have gaps in my knowledge or what I could have gotten if I'd been > to college. That being said, I can't really say I'd rather to have done it > any other way. > > Jerzy makes a good point that you might not be the best judge of what you > should learn. You arn't yet an expert, and having one help guide you could > be very valuable. I certainly wish I'd had a mentor through my > experience. At the same token, you might just be the most qualified person > to say what you should learn, exactly because you will be drawn to what > fascinates you. If you're already showing interest in Haskell, there is a > great deal of material produced by the community that I'm sure will wet > your apatite in the right directions > > When I entered the industry, I got the attention of many well-respected > companies. Sure there were some places that turned me away because I > didn't have a degree, but they weren't the kinds of places I'de like to > work anyway. At the end of the day, my dream employer said "come back and > talk with us when you have more experience," and I landed a job where I > enjoy my boss (an under-considered perk, IMHO. This is worth many $ to > me), am given freedom/ownership of my work, and am greatly appreciated and > nurtured in my growth. > > I often chuckle to myself at night about my freedom from student loans, > too :P. > > Cheers, > Eli > > > > On Sun, Dec 9, 2012 at 2:44 PM, Mike Meyer <mwm(a)mired.org> wrote: > >> >> >> Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: >> >Christopher Howard wrote: >> >Concerning a university education, there are two approaches> >> >1. I want to learn as much as possible >> >2. I want to learn just enough to get a high-paying job >> >> There's actually a third approach ( and probably more): >> >> 3. I want to learn to do this job as well as possible. >> >> >On the other hand, approaching university from the second point of view >> > >> >usually does not justify the cost for the little benefit you obtain >> >this >> >way. Unfortunately, it seems to me that the tuition costs in the U.S. >> >strongly suggest the second approach. To avoid this, I recommend to >> >either go abroad or become very good and acquire a scholarship. >> >> That really depends on the job at in question. When I was looking for >> entry level programming jobs, not having a degree meant you never got past >> the hr department. Getting a degree (pretty much any degree) was required >> to get the high-paying job. I'm willing to believe that's no longer the >> case for programmers, because academia has consistently failed to deliver >> sufficient quality programmers to meet industry needs. On the other hand >> (watching my sons deal with the job market), the litmus test for "you've >> got what it takes to survive in the system" is now a masters, not a >> bachelors, so maybe you're wrong about that. >> >> The other thing to consider is what your long-term goals are. Do you want >> to be a code monkey all your life? Or do you aspire to more? What are the >> requirements for that "more"? Getting a degree now may well avoid doing it >> later. >> >> Finally, with approach #3, you really need a mentor who can tell you >> whether or not you're doing a competent job. You're much more likely to >> find that in a university environment than trying to learn things by >> yourself. Joining an open source project might get it for you. >> -- >> Sent from my Android tablet with K-9 Mail. Please excuse my swyping. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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Re: [Haskell-cafe] education or experience?
by Doug McIlroy 10 Dec '12

10 Dec '12

> > Lazy evaluation--at the heart of spreadsheets since the beginning. > Never thought of that -- nice! Unfortunately it's not literally true, because spreadsheets push recalculated values to all the variables that depend on them, rather than waiting until the dependent values are needed. But the idea that sequencing is by dependency rather than by algorithmic specification is the same. I remember playing around (before spreadsheets) with the idea for Fortran-like code and concluding that the benefit of not having to specify sequence wasn't worth the effort. What a lack of imagination! > > Pattern matching--native to string processing (e.g. COMIT, SNOBOL). > > Appeared nearly in its present form in COGENT (1965). > Hmm google gives me different cogents/ I guess you are referring to the > Reynolds one? Yes. > > Booleans as first class*--surely this is a joke. Algol 60 had them. > Not sure what you are saying (unless its about the footnote that Boole > treated bools as ints! This is new to me) I was referring to the fact that > C programmers have great difficulty thinking of bools as first class Your observation is interesting. Booleans and their utility were well known when C was designed. But since the language level was deliberately very close to real machine architecture, which rarely had native support for 1-bit quantities, the data type was deemed superfluous. (If C had begun on the IBM 7030 it probably would have have had Booleans.) I don't remember anybody predicting that the choice risked banishing Booleans from mind. The one deviation from real architecture that C did embrace was to define a representation for the result of comparison. That had another unpredicted consequence: computer architects imitated the language in hardware! (C's largely ignored bitfield capability came later and, if anything, demoted Booleans to an artifact of structs.) The joke I saw was that there was no way that Boolean values had ever been denied first-class citizenship as Strachey defined it--even if they didn't have a special syntactic identity. > > Matlab exploits them heavily (though represented as doubles). > Not sure what you are referring to I had in mind Matlab's frequent use of a Boolean array as a characteristic function describing some property of the elements of another array. Such arrays are used to control other array operations. In this usage, the characteristic array may be optimized out of existense, but it fosters higher-level thinking and concise code. In reply to your second note, Malcolm Wallace is right. I did intend to say that FP was late with lazy evaluation. My mistake was to think VisiCalc had appeared before 1970. Similarly, I believed that call by need came into our vocabulary soon after Algol 60 brought call be name and call by value to our attention as somewhat better-behaved models than Fortran's call by reference. Now I suspect that belief is wrong, too. Doug

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Exploring Programming Language Theory
by Danny Gratzer 10 Dec '12

10 Dec '12

Hello, Sorry in advance for the soft question: Recently I have been studying more about how a lazy functional language is designed and compiled and have been reading Peyton-Jones's book "implementing functional languages: an introduction"

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Re: [Haskell-cafe] education or experience?
by Doug McIlroy 10 Dec '12

10 Dec '12

> Yes... CS academics delivers less than it could/should; > and whatever this delivery is, its asymptotically sub-linear. > Some of it is to do with the not-quick-enough takeup of FP in academia, > though there are obviously many other factors as well. > http://blog.languager.org/2011/02/cs-education-is-fat-and-weak-1.html > and sequel is about this: how we are not getting over the quirks of the > past history of CS in present day teaching. Here too suggestions for > modifications/ change of emphasis are appreciated. Rusi's cogent blog post includes a list of techniques/concepts that the "unconverted" could profitably pick up from the FP community. In fact the FP community came late to some of these, just as programming languages at large came late to garbage collection. Lazy evaluation--at the heart of spreadsheets since the beginning. Pattern matching--native to string processing (e.g. COMIT, SNOBOL). Appeared nearly in its present form in COGENT (1965). Booleans as first class*--surely this is a joke. Algol 60 had them. Matlab exploits them heavily (though represented as doubles). Data orientation--COBOL fostered this outlook; see Michael Jackson. As long as Lisp ruled, FP lagged on data types. FP also deserves credit for infinite data structures (though the special case of stream processing dates way back). Doug McIlroy * It's amusing to note that "real" Booleans--the ones that Boole used--were integers. For Boole, or(a,b) = a + b - a*b.

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Re: [Haskell-cafe] education or experience?
by Doug McIlroy 10 Dec '12

10 Dec '12

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How can I avoid buffered reads?
by Leon Smith 09 Dec '12

09 Dec '12

I have some code that reads (infrequently) small amounts of data from /dev/urandom, and because this is pretty infrequent, I simply open the handle and close it every time I need some random bytes. The problem is that I recently discovered that, thanks to buffering within GHC, I was actually reading 8096 bytes when I only need 16 bytes, and thus wasting entropy. Moreover calling hSetBuffering handle NoBuffering did not change this behavior. I'm not sure if this behavior is a bug or a feature, but in any case it's unacceptable for dealing with /dev/urandom. Probably the simplest way to fix this is to write a little C helper function that will read from /dev/urandom for me, so that I have precise control over the system calls involved. But I'm curious if GHC can manage this use case correctly; I've just started digging into the GHC.IO code myself. Best, Leon {-# LANGUAGE BangPatterns, ViewPatterns #-} import Control.Applicativeimport Data.Bitsimport Data.Word(Word64)import qualified Data.ByteString as Simport qualified Data.ByteString.Lazy as Limport Data.ByteString.Internal (c2w)import qualified System.IO as IOimport qualified Data.Binary.Get as Get showHex :: Word64 -> S.ByteStringshowHex n = s where (!s,_) = S.unfoldrN 16 f n f n = Just (char (n `shiftR` 60), n `shiftL` 4) char (fromIntegral -> i) | i < 10 = (c2w '0' - 0) + i | otherwise = (c2w 'a' - 10) + i twoRandomWord64s :: IO (Word64,Word64)twoRandomWord64s = IO.withBinaryFile "/dev/urandom" IO.ReadMode $ \handle -> do IO.hSetBuffering handle IO.NoBuffering Get.runGet ((,) <$> Get.getWord64host <*> Get.getWord64host) <$> L.hGet handle 16 main = do (x,y) <- twoRandomWord64s S.hPutStrLn IO.stdout (S.append (showHex x) (showHex y)) {- Relevant part of strace: open("/dev/urandom", O_RDONLY|O_NOCTTY|O_NONBLOCK) = 3 fstat(3, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 9), ...}) = 0 ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7ffff367e528) = -1 EINVAL (Invalid argument) ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7ffff367e528) = -1 EINVAL (Invalid argument) read(3, "N\304\4\367/\26c\"\3218\237f\214yKg~i\310\r\262\"\224H\340y\n\376V?\265\344"..., 8096) = 8096 close(3) = 0 -}

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education or experience?
by Christopher Howard 09 Dec '12

09 Dec '12

I'm at something of a crossroads, and I'm hoping to get a bit of free career advice. I really enjoy programming with Haskell (and a few other exotic languages), and was hoping I could eventually make a living in that sort of field. Not rich and famous, necessarily, just enough to get by comfortably. I'm trying to decide, however; should I go back to school, finish my B.S. and pursue a Masters in CompSci? Or would the time (and money) be better spent aggressively pursuing volunteer work for companies, hoping to eventually get the experience and contacts that lead to a paying job? To be honest, I don't really want to go back to school, because I learn a lot faster (and more economically) on my own. However, I'm not sure which path is the fastest, and safest, approach to an actual paycheck. (Also, I'm something of a die-hard FOSS purist, if that affects the discussion at all.) -- frigidcode.com

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Help optimize fannkuch program
by Branimir Maksimovic 09 Dec '12

09 Dec '12

Well, playing with Haskell I have literally trasnlated my c++ program http://shootout.alioth.debian.org/u64q/program.php?test=fannkuchredux&lang=… got decent performance but not that good in comparisonwith c++ On my machine Haskell runs 52 secs while c++ 30 secs.(There is Haskell entry that is fastest but unfortunately does not runs on test machine is on par with c++http://shootout.alioth.debian.org/u64q/program.php?test=fannkuchredux&la… is something which I have missing since programsare identical.Aa with previous entries you gurus here helped a lot in both helpand learning experience.I simply love Haskell ;)I plan to contribute this program as it is much faster than current runningentry http://shootout.alioth.debian.org/u64q/program.php?test=fannkuchredux&lang=… if it is multithreaded and my is not. This is program: {-# LANGUAGE CPP, BangPatterns #-}{- The Computer Language Benchmarks Game http://shootout.alioth.debian.org/ contributed by Branimir Maksimovic -} import System.Environmentimport Text.Printfimport Data.Bits import qualified Data.Vector.Unboxed.Mutable as VMimport qualified Data.Vector.Generic.Mutable as VGimport qualified Data.Vector.Unboxed as V main = do n <- getArgs >>= readIO.head (checksum,maxflips) <- fannkuch n printf "%d\nPfannkuchen(%d) = %d\n" checksum n maxflips fannkuch n = do !perm <- V.unsafeThaw $ V.fromList [1..n] !tperm <- VG.new n !cnt <- VG.replicate n 0 let loop :: Int -> Int -> Int -> IO(Int,Int) loop !c !m !pc = do !b <- next_permutation perm n cnt if b == False then return (c,m) else do VM.unsafeCopy tperm perm !flips <- count_flips tperm 0 loop (c + (if pc .&. 1 == 0 then flips else -flips)) (max m flips) (pc+1) r <- loop 0 0 1 return r next_permutation :: VM.IOVector Int -> Int -> VM.IOVector Int-> IO(Bool)next_permutation !perm !n !cnt = do !i <- loop 1 if(i >= n) then return False else do !v <- VM.unsafeRead cnt i VM.unsafeWrite cnt i (v+1) return True where loop :: Int -> IO(Int) loop !i | i < n = do !tmp <- VM.unsafeRead perm 0 let rotate :: Int -> IO() rotate !j = if j >= i then do VM.unsafeWrite perm i tmp return () else do !v <- VM.unsafeRead perm (j+1) VM.unsafeWrite perm j v rotate (j+1) rotate 0 !v <- VM.unsafeRead cnt i if v >= i then do VM.unsafeWrite cnt i 0 loop (i+1) else return i | otherwise = return i count_flips :: VM.IOVector Int -> Int -> IO(Int)count_flips !tperm !flips = do !f <- VM.unsafeRead tperm 0 if f == 1 then return flips else do VG.reverse $ VM.unsafeSlice 0 f tperm count_flips tperm (flips+1)

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