May 2015 - Beginners - Haskell.org

Re: [Haskell-beginners] Missing C libraries on windows
by Cameron P-B 07 May '15

07 May '15

Ok, thanks everyone, It all seems to be working now. Setting the environmental variables as suggested and generally following the steps here [https://wiki.haskell.org/SDL/Windows] appears to have been all that was needed; as well as a bit of trial and error as to what was meant by the development libraries.

1 0

Missing C libraries on windows
by Cameron P-B 07 May '15

07 May '15

I'm not sure if this is the right place for this, but I've repeatedly run into issues when installing packages through cabal. It complains about missing C libraries and then explains that I can solve the problem by "installing the system package that provides this library". This has never worked for me and so I assume I'm not doing it correctly. For example, I am currently trying to install ftgl. Using --extra-include-dirs and --extra-lib-dirs to point at the lib and include of the c source folder of ftgl doesn't seem to work. Neither did putting the dll's in the path. The tutorial linked from the wiki:[ https://noamlewis.wordpress.com/2012/12/16/cabal-install-ftgl-on-windows-an… ] also doesn't work for me. And so I was wondering if anyone has had more recent success with that library and more generally if there were any suggestions as to what I might be doing wrong.

4 4

unsubscribe
by Oldman, John 07 May '15

07 May '15

unsubscribe **************************************************************************************** Note: If the reader of this message is not the intended recipient, or an employee or agent responsible for delivering this message to the intended recipient, you are hereby notified that any dissemination, distribution or copying of this communication is strictly prohibited. If you have received this communication in error, please notify us immediately by replying to the message and deleting it from your computer. Thank you. ****************************************************************************************

2 1

Unix-style command line arguments and file input?
by Vale Cofer-Shabica 05 May '15

05 May '15

Hello all, Punchline first: What's the "best practice" way of doing unix-style command line argument and file input processing in Haskell? Background: I'm using Haskell to write programs that act like well-behaved, pipe-friendly unix tools. i.e., the following are all equivalent: % ./prog file % ./prog < file % cat file | ./prog Thus far, I've done this by directly inspecting the first element of System.Environment.getArgs, which has been fine thus far. I'd also like to be able to take simple command line arguments (boolean flags and numeric parameters) and the above doesn't adapt well to that case. I'd like to do this in the idiomatic, "standard" way (a la getopt() in C). Browsing through the wiki page on command line argument parsers [1] gave me a bewildering array of options. I'm not really sure where to start, though I remember reading a blanket endorsement of optparse-applicative somewhere. Any pointers or examples that address my use-case would be much appreciated. -vale [1]: https://wiki.haskell.org/Command_line_option_parsers

5 5

Re: [Haskell-beginners] Data type
by Shishir Srivastava 05 May '15

05 May '15

Hi Brandon, Thanks for your response. My example was just a bad turnout which I conjured up using tutorials and playing with it. I was going to follow up my question with the possible practical use of why and where someone would use such a construct to wrap a function inside a new data-type. For all that matters I could have used 'length' function directly to get the same output. I appreciate that you already have given the practical example but anything more basic for beginners to highlight the usage would be helpful. Thanks, Shishir On Tue, May 5, 2015 at 4:28 PM, Shishir Srivastava < shishir.srivastava(a)gmail.com> wrote: > Thanks Matthew - that was crisp ! > > Cheers, > Shishir >

3 2

Data type
by Shishir Srivastava 05 May '15

05 May '15

Hi, Could anyone please have a look and let me know how you can create the instance of this data type ? --- data Person = Person { firstName ::String -> Int } --- I've seen this type of syntax in a lot of places in haskell code where a new data type is defined in terms of functions rather than concrete data types. I am not trying to achieve anything out of this but purely as an exercise in understanding the record syntax. As far as I understand the data type is function based and takes a 'function' instead of a value of a concrete type so how does one create an instance of this type. Thanks, Shishir

3 2

Data type
by Shishir Srivastava 05 May '15

05 May '15

Thanks Matthew - that was crisp ! Cheers, Shishir

1 0

Variable re-use
by Shishir Srivastava 03 May '15

03 May '15

Hi, Please can anyone explain how does 'a' get re-used in the code below. My understanding so far of haskell is that variables are not allowed to mutate or re-assigned. --- do a <- [1,2,3] a <- [a+1] return a [2,3,4] --- Thanks, Shishir

4 5

definition of >>= for lists
by Stanislaw Findeisen 03 May '15

03 May '15

Hi Where is the definition of (>>=) for lists? I was trying to locate it using Hoogle but no luck. :( -- http://people.eisenbits.com/~stf/ http://www.eisenbits.com/ OpenPGP: 9EC2 5620 2355 B1DC 4A8F 8C79 0EC7 C214 E5AE 3B4E

2 1

Re: [Haskell-beginners] : GCJ 2015 - Round 1A Problem - Haircut
by Gregory Guthrie 03 May '15

03 May '15

Seems to me that an estimate won't help, one has to know exactly how much to skip over. I did this: Find the lowest common multiple (lcm), and then from that the shortest cycle of services, And then skip any multiple of those from the customer queue. sim :: Problem -> [Result] sim (n, ts) = serve tSkip (servers ts) -- optimize; skip ahead over server cycles... where tsLcm = (foldl lcm $ head ts) $ tail ts tCycle = sum $ map (div tsLcm) ts tSkip = n - (div n tCycle)*tCycle (Some fixup is needed for tSkip=0, one easy one is to force an extra cycle.) > ---------------------------------------------------------------------- > Message: 1 > Date: Sat, 18 Apr 2015 10:29:11 -0400 > From: Doug McIlroy <doug(a)cs.dartmouth.edu> > To: sourabh.s.joshi(a)gmail.com > Cc: beginners(a)haskell.org > Subject: Re: [Haskell-beginners] GCJ 2015 - Round 1A Problem - Haircut > Message-ID: <201504181429.t3IETBfo025201(a)coolidge.cs.dartmouth.edu> > Content-Type: text/plain; charset=us-ascii > > > Can someone tell me how I could have avoided or fixed this? > > The trouble manifested as stack overflow on solving > > > https://code.google.com/codejam/contest/4224486/dashboard#s=p1 > > For efficiency, you'll probably need more cleverness in math than in Haskell. You can predict > an approximate start time for the Nth customer's service from the average per-customer > service time M, where 1/M = sum 1/M_k. > Starting from that estimate, one can skip over almost the entire service simulation. > > Doug McIlroy > >

2 1