- Haskell-Cafe - Haskell.org

A newbie's proud first code
by Luke Maurer 06 Nov '01

06 Nov '01

Hi! I'm glad to have run accross Haskell, as a project I've been toying with in my mind seems very well-suited for this. It involves the numeric solution of differential equations. I'm thrilled to see how easy this is, as shown in my first little code snippet: {snip} -- Euler's method for solution of first-order differential equations -- In function arguments, the initial value problem under consideration is: -- df/dt = f'(f, t) -- f(0) = f0 -- The euler function returns an infinite list of the values at each interval -- of length dt of the solution. euler :: (Num a) => (a -> a -> a) -> a -> a -> [a] euler f' dt f0 = euler' f' dt f0 0 -- t "defaults" to 0 -- This function does the actual work, sparing the user from having to supply -- a value for t (which should always be 0 in the initial call, anyway) euler' :: (Num a) => (a -> a -> a) -> a -> a -> a -> [a] euler' f' dt f t = f : euler' f' dt fnext (t + dt) where fnext = f + dt * (f' f t) {snip} (Only three lines of actual code (not counting declarations)! Consider me a convert :-) Yes, you could do it like this in Lisp, I'm sure, but this is so much more succinct and expressive ... and Lisp can't do this cool infinite list stuff, can it?) My only issue (and it is nitpicky) is that I don't really like having to declare an extra function (euler') in the same scope as euler. I tried playing with where clauses or let expressions, but I couldn't find a way to make GHC happy without introducing a bunch of extra variables (since I'd need new versions of the arguments to euler; in euler', I just used the same names. Also, nested "where" doesn't work as I'd like it to.) Is there, in fact, a more concise way to express what I'm doing? Would anyone like to comment on my style (or lack thereof), idiom, etc., so I can squelch any bad habits before they start :-) ? Thanks! Jyrinx jyrinx_list at mindspring dot com

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Re: using haskell as a shell language
by Juan Carlos Arevalo-Baeza 06 Nov '01

06 Nov '01

On Tue, 6 Nov 2001 04:32:05 +0000 (GMT), D. Tweed wrote: >[Suggest moving any followup to haskell-cafe to avoid boring people] Sure. >On Mon, 5 Nov 2001, Juan Carlos Arevalo-Baeza wrote: > >> On Tue, 6 Nov 2001 03:42:51 +0000 (GMT), D. Tweed wrote: >> >> >The name that >> >gets passed to the interpreter is `the top-level name of the script', so >> >if /home/tweed/bin/myscript.hs is actually a symlink to >> >/home/tweed/src/haskell/myscript.hs and I invoke it from /home/tweed/odd/ >> >with ../bin/myscript.hs, what gets passed is "../bin/myscript.hs" (at >> >least under bash on linux). So whilst the interpreter could in principlex >> >get to thinking it should look in /home/tweed/bin/ for imports by applying >> >"../bin/myscript.hs" to $cwd, AFAICS there's no way it can get hold of the >> >directory /home/tweed/src/haskell. When I figured this out I just decided >> >that the method that would lead to least banging of my head against wall >> >was just to absolute path any imports of non standard distribution code. >> >> Well... I still disagree. From my point of view, it could be argued the order in which directories are searched for modules, but I don't think there's any need to exclude any directories. The interpreter should search not just the current directory, but also the directory of the file where the source was found. At least. I'd even add subdirectories "./modules/" (or "./haskell.modules/" or something like that) of both of them, just for added value :) >> >> IMHO, there's no reason not to search a reasonable place for a needed piece of information. It's like Koenig lookups in C++. It just makes sense to look there, just in case. > >I entirely agree it would be great if this could be done. However it seems >that runhugs at least _doesn't_ do it, and I suspect that may be because >it technically can't. Oh, it sure can. If a file has hard-links, it just means that it can be accessed through two different paths. I'm just saying that, looking in the path that was used for accessing the file sounds like a very good (and easily implementable) idea to me. >I never use unix hardlinks myself (only symlinks), >but IIRC if you've got two or more hardlinks to a file there's no way to >find out from the filesystem what the directories of the other links to >the file are, so if you invoke via a hardlink then it's really difficult >to do automatic imports correctly. (I may be wrong about this.) Oh, yes. I understand you perfectly. But that's not a good reason not to try the one known path. Just in case. >Incidentally, I do have a setup with >symlinks as described above; it's not just hypothetical. > >> Anyway... This is not my war. Just my opinion. O:-) > >Oh, you make very good points, I just suspect that it `can't be done'. I'd >love to be proved wrong :-) Remember that I'm just advocating in favor of "looking in additional places" for the files, nothing more. That's neither technically impossible nor unreasonable. What I'm saying, if you will, is that the fact that the needle _might_ be in the haystack doesn't mean that it's not worth bothering to look in the needle box. So to speak. :) Salutaciones, JCAB email: jcab(a)roningames.com ICQ: 101728263 The Rumblings are back: http://www.JCABs-Rumblings.com

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Using the std libraries in Ghc(i)
by Jorge Adriano 04 Nov '01

04 Nov '01

How can I use the std functions (listed in the 'Standard Libraries for Haskell 98' documentation) in ghc(i)? I've read the ghc documentation, and seen the several categories (which can be added with '-package category_name') available, but I still don't know how to access functions like unfoldr, transpose etc... Thanks in advance, J.A.

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Re: URGENT: File exists
by Richard Uhtenwoldt 03 Nov '01

03 Nov '01

bracaman(a)box.sk writes: >I am trying to emulate bash in haskell, and i got a function called shell that waits fot the command and then executes it. The problem is that if the file does not exist, the program returns to Main (it gets out of the shell). The only thing i want to do is to return to the shell after an IOError. > >How can i perform such a thing? You could test if the file exists, like you said before, but that test would fail once in a while, when another process deletes the file after your test but before the file is opened. This is called a "race condition" and should be avoided when possible. The best thing is to use catch which has the type IO a -> (IOError -> IO a) -> IO a. An example (untested code) is catch (do hdl<-openFile "/home/bracaman/foo" ReadMode char<-hGetChar hdl string<-hGetLine hdl ) --we caught an exception: (\errorCode-> if isDoesNotExistError errorCode then "return to shell" else if isPermissionError errorCode then "return to shell" else --something strange happened. --re-raise the exception, which will probably return control to Main. ioError errorCode ) P.S. Instead of (do hdl<-openFile "/home/bracaman/foo" ReadMode char<-hGetChar hdl string<-hGetLine hdl ... ) you could have (do fileContentsAsABigString<-readFile "/home/bracaman/foo" ... ) but I try to stay away from readFile and getContents because they're not referentially transparent.

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Evolution of a Haskell Programmer
by Dmitry Astapov 01 Nov '01

01 Nov '01

It's a pity that such a link is not mentioned in "Haskell Bookshelf" :) http://www.willamette.edu/~fruehr/haskell/evolution.html -- Dmitry Astapov //ADEpt E-mail: adept(a)umc.com.ua GPG KeyID/fprint: F5D7639D/CA36 E6C4 815D 434D 0498 2B08 7867 4860 F5D7 639D

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HaXml, emeory usage and segmentation fault
by Dmitry Astapov 31 Oct '01

31 Oct '01

I have Hugs version February 2001, HaXml version 1.02 and this program: > module Main where > import XmlLib > > main = processXmlWith (invoices `o` tag "invoice") > > invoices = > html > [ hhead > [ htitle [ ("Invoices"!)] ], > hbody > [ customers `o` children `with` tag "customer" ] > ] > > customers = > cat > [ h2 [ ("Customer"!) ], > contracts `o` children `with` tag "contract" > ] > > contracts = > cat > [ h3 [ ("Id:"!), ("id"?)], > hpara [ ("Access:"!), keep /> txt ] `o` children `with` tag "access", > hpara [ ("Intl:"!), keep /> txt] `o` children `with` tag "inter" > ] This program can process following file: <?xml version='1.0'?> <invoice> <customer> <contract id='1'> <access>1</access> <inter>1</inter> </contract> <contract id='2'> <access>2</access> <inter>2</inter> </contract> </customer> </invoice> (I use "runhugs translate.hs invoice.xml invoice.html") Now increase amount of <customer>s to 10, and amount of <contract>s within each customer to 999. After that, "runhugs -h6000000 translate.hs invoice.xml invoice.html" dumps core :( What's the reason: bug in hugs, bug in HaXml, or my own bad programming techniques? -- Dmitry Astapov //ADEpt E-mail: adept(a)umc.com.ua GPG KeyID/fprint: F5D7639D/CA36 E6C4 815D 434D 0498 2B08 7867 4860 F5D7 639D

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how to store new values in variables?
by Anders Elfgren 31 Oct '01

31 Oct '01

Hi, its me with the dungeon master-game again.. I'm in the process of making me able to move around, which I have succeeded with. Kind of. Remember the map type Pic = [[Char]] maplevel1 = ["##############", "# #", "### #", "# #", "# # #", "#P# #", "# #m #", "##############"] Now, I've made a function called s (for south) which just moves the 'P' one step down. It looks like this s :: Pic s = remap (findPos 'P' 1) (0,1) 1 findPos just finds the positin of the P, and 0,1 tells remap to move it one step on the y-axis. The remap function looks overly complicated right now so I'm not gonna put it in here, nad I dont think it really matters. What I want to do tho, is something like this s :: Pic s = maplevel1 <- (remap (findPos 'P' 1) (0,1) 1) so that it not only returns the new Pic, but also sets the maplevel1 variable to look like it should after the move. If I do putStr (unlines s) twice, it should move down two steps.. but it doesnt. We didn't learn this, well I didn't :), in the Haskell course so if someone can just give me an example of how I should do it it'd be nice.. Thanks again. The dm2.hs can be downloaded from www.dtek.chalmers.se/~srekel/ptf and yes, the draw2 function works :) ------------------------------------------------------------------------- Anders Elfgren || AKA: Srekel and Farfar Homepage: http://hem.passagen.se/srekel || Alt email: srekel(a)icqmail.com ICQ: 4596105 || AIM: Srekel - Kaka e gott || - Cs e halva mitt liv -------------------------------------------------------------------------

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Some more news about DM-haskell..
by Anders Elfgren 31 Oct '01

31 Oct '01

Hehe sure :) If I DO finish it that is :) It should actually be close to being at the state where I just have to add different images and when to display them. Then I need to work on some kind of function to allow it to take 'w','s','n'and 'e' as input, or if I can, the arrow keys. Btw, I see that when I hit reply in Pine I only send to the person that sent me the mail, not the whole list, so I'm going to make an alias for it. For those of you that don't know, I've managed to find the problem with the help of some fellow members of this list :) On Tue, 30 Oct 2001, Andre W B Furtado wrote: > Will you release your game when it is done? It seems very interesting and I > woud like to check how it will look like... > > -- Andre PS. Andre, there's a.. um.. game I guess, for Linux called Text-mode Quake. Search for it on www.google.com and you should find something interesting :D ------------------------------------------------------------------------- Anders Elfgren || AKA: Srekel and Farfar Homepage: http://hem.passagen.se/srekel || Alt email: srekel(a)icqmail.com ICQ: 4596105 || AIM: Srekel - Kaka e gott || - Cs e halva mitt liv -------------------------------------------------------------------------

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What does this error message mean
by Anders Elfgren 30 Oct '01

30 Oct '01

Short intro (skip down for the real reason for this email) : Hi, I'm a student at Chalmers and have finished the Haskell course but actually (is IS weird, but I'm pretty sure I like it... :) ) found the language to be pretty cool and decided to try and make an ascii-based Dungeon Master type game. For anyone who doesn't know what that is : Its a game in which you walk in a dungeon with corridors in a firstperson view. These are pictures that show what it looks like when you are looking down a corridor that turns right two and respectively one "moves" ahead of the current position. (Yes, I plan to make these better looking :) ) corrl_2 = ["##############", "# \\ / #", "# \\ ____ / #", "# | } #", "# |____} #", "# / \\ #", "# / \\ #", "##############"] corrl_1 = ["##############", "# \\ / #", "# \\ ______| #", "# | | #", "# |______| #", "# / | #", "# / \\ #", "##############"] This is the whole program. draw simply prints out all three pictures (including the overhead map found below) My (current) problem lies in whatPic, which returns the error Program error: {whatPic Facing2_North (1,6)} if I type in whatPic North (1,6) or just call it through the draw2 function (which is supposed to be the main drawing function, and it actually works.. Well it did yesterday until I did something :D (sorry about pine wrapping the lines.. if you're in luck you wont notice it.) import Observe draw = putStr ((unlines corrl_2) ++ (unlines corrl_1) ++ (unlines maplevel1)) data Facing2 = North | South draw2 = putStr (unlines (whatPic North (findPos 'P' 1))) findPos :: Char -> Int -> (Int,Int) findPos obj location | head (drop (location) (unlines maplevel1)) == obj = (location `div` 14,location `mod` 15) | otherwise = findPos obj (location + 1) whatPic :: Facing2 -> (Int,Int) -> Pic whatPic facin (x,y) | corridorType facin (x,y) == (2, "rTurn1") = corrl_1 | corridorType facin (x,y) == (2, "rTurn2") = corrl_2 corridorType :: Facing2 -> (Int,Int) -> (Int, String) corridorType facing (x,y) | observe "distWall" distWall facing (x,y) (x,y) == 4 && (whatIs (findPos 'P' 1) (1,-2) == ' ') = (2, "rTurn") | otherwise = (2, "rTurn") distWall :: Facing2 -> (Int,Int) -> (Int,Int) -> Int distWall North (x,y) (newx,newy) | whatIs (x,y) (x,y-1) == '#' = abs (y - newy) | whatIs (x,y) (x,y-1) == ' ' = distWall North (x,y) (x,y-1) --distWall (North) (x,y) (newx,newy) = 2 --distWall (South) (x,y) (newx,newy) = 2 whatIs :: (Int,Int) -> (Int,Int) -> Char whatIs (x,y) (xdiff,ydiff) = head (drop (x + xdiff + y + ydiff ) (unlines maplevel1)) --[1,2,3,4,5] type Pic = [[Char]] maplevel1 = ["##############", "# #", "### #", "# #", "# # #", "# # #", "#P#m #", "##############"] corrl_2 :: Pic corrl_2 = ["##############", "# \\ / #", "# \\ ____ / #", "# | } #", "# |____} #", "# / \\ #", "# / \\ #", "##############"] corrl_1 = ["##############", "# \\ / #", "# \\ ______| #", "# | | #", "# |______| #", "# / | #", "# / \\ #", "##############"] I'm sure its something simple.. I just can't make out what could be wrong right now. Thanks for any replies :) ------------------------------------------------------------------------- Anders Elfgren || AKA: Srekel and Farfar Homepage: http://hem.passagen.se/srekel || Alt email: srekel(a)icqmail.com ICQ: 4596105 || AIM: Srekel - Kaka e gott || - Cs e halva mitt liv -------------------------------------------------------------------------

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Vim syntax files for .lhs files using \begin{code} and \end{code}
by Nicholas Nethercote 30 Oct '01

30 Oct '01

Hi, The Vim syntax file for literate Haskell programs handles code lines that begin with a '>' character fine. But in GHC much of the code is defined between "\begin{code}" and "\end{code}" pairs, and that shows up as one giant comment. Does anybody know where I might be able to obtain a Vim syntax file that handles these constructs? I made a feeble attempt myself, but Vim's syntax highlighting language defeated me. Thanks. -- Nick Nethercote njn25(a)cam.ac.uk

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