RE: [Haskell-cafe] +RTS -M800M
| Is there any chance there is a bug in recent GHCs (or perhaps Linux?) | affecting this? I'm fairly sure I've lately seen Haskell programs | consuming significantly larger amounts of memory than they should be | allowed to, and I can try to isolate a test case if it is of | interest. New bugs are always possible! And in the last few months, Simon has been doing fairly major surgery on parts of the runtime system, as part of getting multiprocessor-GHC to play nicely. So if you can find a test case where GHC is generating code that uses a lot more memory than it used it, it would be very valuable to us. Thanks! Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Using MonadError within other Monads
On 12/19/05, Andrew Pimlott [EMAIL PROTECTED] wrote: [It is best to post questions only to haskell-cafe.] Roger. On Mon, Dec 19, 2005 at 03:53:53PM +1300, Karl Grapone wrote: I'm having trouble making use of MonadError within another Monad, in this case IO. snip Looking at the signature of f, f :: IO (Either String String) Either String String is just an ordinary value produced in the IO monad--the monad structure of IO and Either String are completely independent. snip The outer do is a computation in the IO monad, the inner do is a computation in the Either String monad, and the net effect is an IO computation returning an Either String computation--which is just what the type signature says. I had to change your code in 3 other places to make it type-check; hopefully you can now find them. ;-) No problem, with a little shove in the right direction :). Not entirely sure why I didn't arrive at it myself. I'm using StateT successfully elsewhere and was overeager in using similar methods which, as you point out below, is a different situation. I still find monadic signatures a little confusing, I believe ghci was, at some point, deriving types for f or g that had MonadError in place of Either String, so I'll have to think carefully about why it is so different from the ErrorT type you show below. When people speak of nesting monads, they often mean using monad transformers. If you were using the ErrorT monad transformers, your signature would look like f :: ErrorT String IO String You might want to try rewriting your code that way, but I would suggest making it work with the current type signatures first. At one point I was playing with types like 'Either String (IO String)', which to me seems very similar to the ErrorT type. Intuitively they don't seem like they'd be correct... I feel like determining that there is an error involves interaction with the outside world, consequently the error part should be inside the IO monad. Now that you've cleared up the simpler case for me I'll try out the ErrorT case. Thanks for the help. Karl ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
Am Sonntag, 18. Dezember 2005 17:25 schrieb Daniel Carrera: [...] This is a real problem for Haskell. I expect that a lot of people try Haskell and give up because they can't even write the simplest function. Hello Daniel, honestly, I have to say that during my years with Haskell, this seems to be the first time that I see somebody trying to enter a Haskell program via the command line of an interactive environment. So I suppose that it's not a lot of people who experience the problems you do. It's hard not to be put off by this. I love the theory behind Haskell, but the practice of it seems to be a real problem. Did you have a look at the Hugs or GHCi documentation? I'm very sure, it will tell you that what you may enter at the command line are (merely) expressions to evaluate and special commands like :t for displaying the type of an expression, and that the Haskell declarations (normally) have to be read from a file. I hope someone will show me how to make this program work. Even better, I hope someone will fix the compilers and interpreters if they need fixing, or fix the documentation if that's what needs fixing. I suppose that neither the compilers and interpreters nor the documentation need(s) fixing, although I might be wrong with the documentation. Best, Daniel. Best wishes, Wolfgang ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
Am Sonntag, 18. Dezember 2005 17:42 schrieb Daniel Carrera: Lemmih wrote: GHC is a compiler. If you want to compile to a binary then you must define a function called 'main'. Otherwise just load the file in ghci (`ghci fac.hs`). I would expect GHC to be able to compile a program with a function that is not called 'main'. I wouldn't expect it to print anything when run, but I would expect it to compile. So you want a program that doesn't contain a main variable to be compiled into an executable which does nothing? This doesn't seem to make much sense. In addition, Haskells requirement of a main variable is nothing new. Take C, C++, Java, PASCAL. They all require you to somehow specify a main program/function/method. `ghci fac.hs` doesn't give any errors. I don't understand why loading the file like that is ok but typing the entries is not. Because the interactive environments are about evaluating expressions based on declarations which were made in Haskell source files, not about creating new declarations. Expressions and declarations are just two different things. I think, Cale explained why it might be a bad idea to allow entering declarations at the command line. This might be a problem for people exporing the language. Hmm, don't know. Best, Daniel. Best wishes, Wolfgang ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] First steps in Haskell
Daniel is right, by definition. He is a new user. He had difficulty. That much is incontrovertible. While he may seem unusual, perhaps he is only unusual in that he's told us about his experience rather than trying Perl instead. For which, much thanks, Daniel! In any case, I think it'd be great if he, or anyone else, started a Wiki page on very first steps in Haskell, along the lines he suggests. Maybe one exists already? Once it does, it'd be good to point to it from haskell.org. Simon | -Original Message- | From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of | Wolfgang Jeltsch | Sent: 19 December 2005 09:50 | To: haskell-cafe@haskell.org | Subject: Re: [Haskell-cafe] First steps in Haskell | | Am Sonntag, 18. Dezember 2005 17:25 schrieb Daniel Carrera: | [...] | | This is a real problem for Haskell. I expect that a lot of people try | Haskell and give up because they can't even write the simplest function. | | Hello Daniel, | | honestly, I have to say that during my years with Haskell, this seems to be | the first time that I see somebody trying to enter a Haskell program via the | command line of an interactive environment. So I suppose that it's not a | lot of people who experience the problems you do. | | It's hard not to be put off by this. I love the theory behind Haskell, | but the practice of it seems to be a real problem. | | Did you have a look at the Hugs or GHCi documentation? I'm very sure, it will | tell you that what you may enter at the command line are (merely) expressions | to evaluate and special commands like :t for displaying the type of an | expression, and that the Haskell declarations (normally) have to be read from | a file. | | I hope someone will show me how to make this program work. Even better, | I hope someone will fix the compilers and interpreters if they need | fixing, or fix the documentation if that's what needs fixing. | | I suppose that neither the compilers and interpreters nor the documentation | need(s) fixing, although I might be wrong with the documentation. | | Best, | Daniel. | | Best wishes, | Wolfgang | ___ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
Am Sonntag, 18. Dezember 2005 18:04 schrieb Daniel Carrera: Joel Koerwer wrote: Then after you've played with you creation a bit, check out http://haskell.org/learning.html http://haskell.org/learning.html Thank you. I did find that page, and it was very easy to find. There's a link on the Haskell Homepage to this page. The problem is that the content of that page, and its links, didn't show me how to write a Haskell program (like you did). If you want to know how to feed, for example, Hugs with your Haskell program, you might have to have a look at some Hugs documentation. Remember that different Haskell implementations like Hugs, GHCi etc. may have different ways of reading Haskell programs. So it's advisable to have a look at some documentation which is specifically about the Haskell system you use. On the Learning Haskell page you can click, for example, on the Hugs link, then on Documentation and then on The Hugs 98 User's Guide or The Hugs 98 User Manual. Best, Daniel. Best wishes, Wolfgang ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
Wolfgang Jeltsch wrote: In addition, Haskells requirement of a main variable is nothing new. Certainly nothing new. I just wish that the documentation I read had told me about it. `ghci fac.hs` doesn't give any errors. I don't understand why loading the file like that is ok but typing the entries is not. Because the interactive environments are about evaluating expressions based on declarations which were made in Haskell source files, not about creating new declarations. Expressions and declarations are just two different things. I think, Cale explained why it might be a bad idea to allow entering declarations at the command line. Ok. I think I understand. I'm sure this will make more sense later. This might be a problem for people exporing the language. Hmm, don't know. I think that usability is important. I'm a big fan of usability. You don't want to give artificial barriers to people who are just starting to explore the language. Those are the people most likely to turn away. Make the first step as simple as possible. The mini-tutorial I posted to the list would be enough. It's little more than a paragraph, and it gets the user past that first step. Best, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/I am not over-weight, I am under-tall. / ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
Wolfgang Jeltsch wrote: The problem is that the content of that page, and its links, didn't show me how to write a Haskell program (like you did). If you want to know how to feed, for example, Hugs with your Haskell program, you might have to have a look at some Hugs documentation. Remember that different Haskell implementations like Hugs, GHCi etc. may have different ways of reading Haskell programs. remember? This is the first time I hear this, and I'm bewildered. I expect two C compilers to be able to compile a correct C program. Certainly a hello world program. How is it possible that two Haskell implementations disagree on how to read a Haskell program? (unless it's an advanced program with very unique compiler features - which is bad). On the Learning Haskell page you can click, for example, on the Hugs link, then on Documentation and then on The Hugs 98 User's Guide or The Hugs 98 User Manual. I doubt a lot of people who didn't already know about Haskell would think to click on the manual for the *compiler* instead of the link that says Haskell Tutorial. Best, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/I am not over-weight, I am under-tall. / ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
Am Sonntag, 18. Dezember 2005 18:02 schrieb Daniel Carrera: Chris Kuklewicz wrote: Almost everything is explained under http://www.haskell.org/ghc/docs/6.4.1/html/users_guide/ghci.html Ok. How would a visitor to the Haskell site find this document? The point is that the visitor should know that he/she might need a document about GHCi if he/she wants to use GHCi. A introductionary document about Haskell might not explain a specific Haskell system. If you read a book which is about C++ in general, it won't tell you how to use the GNU C++ compiler or Microsoft Visual C++. If you know that you need some GHC(i) documentation, it shouldn't be much of a problem to find it. Go to the GHC homepage, click on Documentation and you will find The User's Guide whose short description tells you that it covers GHCi. If this is the correct document for a beginner to start with Haskell, perhaps the site should be updated to point to it instead of the documents it currently points to. At least, the site currently points to the GHC homepage. I find some usability problems in the documentation section. Which documentation section do you mean? Think of usability in terms of barriers. If you have low barriers, a lot of people will have enough motivation to cross them and get started with Haskell. If the barriers are very high, only the most intent and motivated users will get started. Barriers are bad. Maybe, some barriers could be lowered but I don't think that the barriers are currently very high. What do others think? Consider some barriers for a user who wants to learn Haskell: * There's no way for a new user to figure out how to successfully run the simplest Haskell program. There is! The Hugs 98 User's Guide and The GHC User's Guide. * The first tutorial listed requires the user to give up some personal information before getting the tutorial. That's bad, of course. These are very significant barriers. Concerning the latter one, I agree with you. Sure, it's not all bad. For example, Haskell has a friendly community (low barrier). :-) But the barriers that exist are a problem because they hit the person who is trying to take the very very first step. If you can make that *fist* step easier, more people will take it. What do you mean with *fist* step? :-) :-) [...] Best, Daniel. Best wishes, Wolfgang ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Int vs Integer
Am Montag, 19. Dezember 2005 01:10 schrieb Jared Updike: Int is for bounded values -2**32 to 2**32 (I think... maybe 2**-31 and 2**31 or less if it's boxed?) based on the underlying machine representation. Not really true. As far as I remember, the Haskell Report just says that Int covers at least the integers from -2^27 to 2^27 - 1. The range for Int can be higher than the range just given. For 32-bit machines it will probably be -2^31 to 2^31 - 1 but there's no guarantee for that. [...] Jared. Best wishes, Wolfgang ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
On Mon, Dec 19, 2005 at 10:14:07AM +, Daniel Carrera wrote: Wolfgang Jeltsch wrote: If you want to know how to feed, for example, Hugs with your Haskell program, you might have to have a look at some Hugs documentation. Remember that different Haskell implementations like Hugs, GHCi etc. may have different ways of reading Haskell programs. remember? This is the first time I hear this, and I'm bewildered. I expect two C compilers to be able to compile a correct C program. Certainly a hello world program. How is it possible that two Haskell implementations disagree on how to read a Haskell program? (unless it's an advanced program with very unique compiler features - which is bad). I think what Wolfgang meant was that different Haskell implementations may have: - different executable names, so you have to invoke them differently - different options - different style of work etc... Of course, all of them should accept all Haskell 98 programs. Best regards Tomasz -- I am searching for a programmer who is good at least in some of [Haskell, ML, C++, Linux, FreeBSD, math] for work in Warsaw, Poland ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
Wolfgang Jeltsch wrote: The point is that the visitor should know that he/she might need a document about GHCi if he/she wants to use GHCi. A introductionary document about Haskell might not explain a specific Haskell system. If you read a book which is about C++ in general, it won't tell you how to use the GNU C++ compiler or Microsoft Visual C++. I just Googled for Introduction to C. The first link was: http://www.le.ac.uk/cc/tutorials/c/ It includes a brief section on both MS Visual C++ and the Unix CC. Maybe I've just been lucky in reading all the right tutorials until now :) but every time I learn a new language, the intro tutorial tells me how to get Hello World running. I have never looked for a compiler tutorial. I guess that Haskell is unique among interpreted languages in that there are two compilers and they work different. I wouldn't expect a huge and elaborate description of Hugs or GHCI on a tutorial. I'd expect a brief If you have Hugs do xyz and if you have GHC do abc. If you know that you need some GHC(i) documentation, it shouldn't be much of a problem to find it. But you see, I didn't. I thought I needed Haskell documentation, so that's what I looked for. At least, the site currently points to the GHC homepage. Yes. I thought it was for the purpose of installing it. Since I installed it with 'apt-get install ghc' I didn't think to click there. I find some usability problems in the documentation section. Which documentation section do you mean? The web page titled Learning Haskell. Maybe, some barriers could be lowered but I don't think that the barriers are currently very high. What do others think? To be fair, the barriers are not as high as they could be. For example, the Haskell community is friendly and helpful and that's a low barrier. It took no time between my posting a question and getting a good answer. So in the end it took me no more than a day to write Hello World. I do suggest that the Learning Haskell page could be improved with a brief (couple of paragraph) tutorial to get someone through Hello world. Or perhaps update the tutorials to say that. I wrote a suggestion on another post. * There's no way for a new user to figure out how to successfully run the simplest Haskell program. There is! The Hugs 98 User's Guide and The GHC User's Guide. Okay, I stand corrected. I rephrase the concern as The links that say Learn Haskell don't show you how to run a simple Haskell program. * The first tutorial listed requires the user to give up some personal information before getting the tutorial. That's bad, of course. These are very significant barriers. Concerning the latter one, I agree with you. :) But the barriers that exist are a problem because they hit the person who is trying to take the very very first step. If you can make that *fist* step easier, more people will take it. What do you mean with *fist* step? :-) :-) Heh... typing accuracy is over-rated. Cheers, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/I am not over-weight, I am under-tall. / ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
Tomasz Zielonka wrote: I think what Wolfgang meant was that different Haskell implementations may have: - different executable names, so you have to invoke them differently - different options - different style of work etc... Of course, all of them should accept all Haskell 98 programs. Ah. Thanks. Cheers, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/I am not over-weight, I am under-tall. / ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
Simon P-J: Daniel is right, by definition. He is a new user. He had difficulty. That much is incontrovertible. While he may seem unusual, perhaps he is only unusual in that he's told us about his experience rather than trying Perl instead. For which, much thanks, Daniel! Actually, I have sometimes wished that the various interactive Haskell interfaces had the possibility to enter also declarations interactively, as Daniel originally asked for. Lisp interpreters often support this to some extent, so it is not out of line for a newcomer to expect it also for Haskell. I often use hugs as a calculator, and sometimes it would be very convenient to be able to make one or a few short declarations while making some interactive calculations. It can be quite tedious to create and edit a source code file on the side and then load it, when all you want is a short declaration or two. Would it be that complex to have an interactive interface enter a declarations mode when it sees a declaration coming, and then let it create, compile and load a temporary module when the declaration is finished? Björn Lisper ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] First steps in Haskell
| Actually, I have sometimes wished that the various interactive Haskell | interfaces had the possibility to enter also declarations interactively GHCi does. ghci let f x = hello ghci f True True But there's no editor -- it's strictly a one-line definition Simon | -Original Message- | From: Bjorn Lisper [mailto:[EMAIL PROTECTED] | Sent: 19 December 2005 13:06 | To: Simon Peyton-Jones | Cc: [EMAIL PROTECTED]; haskell-cafe@haskell.org | Subject: Re: [Haskell-cafe] First steps in Haskell | | Simon P-J: | Daniel is right, by definition. He is a new user. He had difficulty. | That much is incontrovertible. | | While he may seem unusual, perhaps he is only unusual in that he's told | us about his experience rather than trying Perl instead. For which, | much thanks, Daniel! | | Actually, I have sometimes wished that the various interactive Haskell | interfaces had the possibility to enter also declarations interactively, as | Daniel originally asked for. Lisp interpreters often support this to some | extent, so it is not out of line for a newcomer to expect it also for | Haskell. I often use hugs as a calculator, and sometimes it would be very | convenient to be able to make one or a few short declarations while making | some interactive calculations. It can be quite tedious to create and edit a | source code file on the side and then load it, when all you want is a short | declaration or two. Would it be that complex to have an interactive | interface enter a declarations mode when it sees a declaration coming, and | then let it create, compile and load a temporary module when the declaration | is finished? | | Björn Lisper ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
Am Montag, 19. Dezember 2005 12:13 schrieb Daniel Carrera: [...] I guess that Haskell is unique among interpreted languages Haskell is not an interpreted language. There are Haskell interpreters, there are Haskell compilers. in that there are two compilers and they work different. The basic principles of the interactive environments (Hugs and GHCi) are the same, and so are the basic principles of the compilers (GHC, nhc98, ...). Of course, they differ in the details. But so do different C compilers. [...] What do you mean with *fist* step? :-) :-) Heh... typing accuracy is over-rated. Well, I couldn't resist... ;-) Cheers, Daniel. Best wishes, Wolfgang ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
Simon: Me: | Actually, I have sometimes wished that the various interactive Haskell | interfaces had the possibility to enter also declarations interactively GHCi does. Ah, I see! Does it open a let-environment with a local definition? ghci let f x = hello ghci f True True Hmm, an interesting semantics for f given the declaration :-) But there's no editor -- it's strictly a one-line definition Which is useful enough in many situations. I'll try it out... Björn ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] XML Schema for Haskell parsing?
GCCXML defines some XML schema for the results of a C(++) program parsing. Does there exist any agreed-upon XML schema to represent the results of Haskell program parsing? Thanks. -- Dimitry Golubovsky Anywhere on the Web ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Wiki resources for Re: First steps in Haskell
Simon Peyton-Jones [EMAIL PROTECTED] writes: In any case, I think it'd be great if he, or anyone else, started a Wiki page on very first steps in Haskell, along the lines he suggests. Maybe one exists already? Once it does, it'd be good to point to it from haskell.org. There's both HaskellNewbie[1] and HaskellDemo[2]. It would be nice if they were linked from the front page of haskell.org. Has there been any discussion about keeping the haskell.org pages in a darcs repository for easy local editing? http://www.haskell.org/hawiki/HaskellNewbie http://www.haskell.org/hawiki/HaskellDemo -- Shae Matijs Erisson - http://www.ScannedInAvian.com/ - Sockmonster once said: You could switch out the unicycles for badgers, and the game would be the same. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] comprehension generators from IO [a]'s ?
[reposted from haskell mailing list where I got no bites :) ] Folks, I'm new to using monads, and I'd like some help improving a function definition that I wrote and which works, but for which I think there should be a clearer way to write it. What I'm after is something like: -- (psuedo-code) [(b,c,d) | b - getDirectoryContents a_dir, c - getDirectoryContents (a_dir ++ / ++ b), d - getDirectoryContents (a_dir ++ / ++ b ++ / ++ c) ], ie. where the generators feed from IO actions instead of lists, but I gather this comprehension style isn't supported, which is too bad because it's really easy to read. (Is this what was meant by monad comprehensions that I've heard reference to?) Here's how I actually wrote it, using nested folds: import System.IO -- Load directory entries 3 levels deep under a_dir, as list of tuples (b,c,d) load3DirLevels :: FilePath - IO [(String,String,String)] load3DirLevels a_dir = do bs - getDirectoryContents a_dir foldM (\tups b - do cs - getDirectoryContents (a_dir ++ / ++ b) foldM (\tups' c - do ds - getDirectoryContents (a_dir ++ / ++ b ++ / ++ c) foldM (\tups'' d - do return $ (b, c, d) : tups'' ) tups' ds ) tups cs ) [] bs This function isn't so clear at a glance, and yet what it's doing seems like a pretty common thing to want to do: are there any library functions for monads (or IO in particular) that make this sort of thing easier, or should I to try and write my own function? Looks not too difficult to write but I think I might miss something important if I didn't ask first... How would you do it? Thanks Steve ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Using MonadError within other Monads
On Mon, Dec 19, 2005 at 10:21:36PM +1300, Karl Grapone wrote: I still find monadic signatures a little confusing, I believe ghci was, at some point, deriving types for f or g that had MonadError in place of Either String, so I'll have to think carefully about why it is so different from the ErrorT type you show below. You used throwError, which is a MonadError method, so it's not surprising to get a message about MonadError. Regarding ErrorT... At one point I was playing with types like 'Either String (IO String)', which to me seems very similar to the ErrorT type. Actually, your type IO (Either String String) is the same as ErrorT String IO String except for the newtype wrapper. The big difference in use is that the ErrorT String IO Monad instance combines the monad structures of IO and Either. So you won't need nested do's, and a few other things will have to change (and you might have to think a bit about mapErrs). Intuitively they don't seem like they'd be correct... I feel like determining that there is an error involves interaction with the outside world, consequently the error part should be inside the IO monad. You're correct about that--Either String (IO String) is not a monad. Andrew ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Help with division
Hello all, Playing around with Haskell... I'm writing a 'choose' function: --//-- fac :: Int - Int fac 0 = 1 fac n = n*fac(n-1) choose :: Int - Int - Int choose n k = fac(n) / (fac(k) * fac(n-k)) --//-- I'm having problems with the last line. --//-- Prelude :l test.hs Compiling Main ( test.hs, interpreted ) test.hs:13:20: No instance for (Fractional Int) arising from use of `/' at test.hs:13:20 Probable fix: add an instance declaration for (Fractional Int) In the definition of `choose': choose n k = (fac (n)) / ((fac (k)) * (fac (n - k))) Failed, modules loaded: none. --//-- The problem is that it doesn't like the / sign. For example, if I replace the / by * the function is compiled just fine. Does anyone know what I'm missing? Cheers, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/I am not over-weight, I am under-tall. / ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] comprehension generators from IO [a]'s ?
Okay...that works. Sweet.
gdc x = ListT $ getDirectoryContents x
get3levels top = runListT $ do
b - gdc top
c - gdc $ top++('/':b)
d - gdc $ top++('/':b)++('/':c)
return (b,c,d)
I feel bound to point out http://haskell.org/hawiki/ListTDoneRight which
has more to say about the details of ListT
Andrew Pimlott wrote:
On Mon, Dec 19, 2005 at 01:42:19PM -0600, Steve Harris wrote:
What I'm after is something like:
-- (psuedo-code)
[(b,c,d) |
b - getDirectoryContents a_dir,
c - getDirectoryContents (a_dir ++ / ++ b),
d - getDirectoryContents (a_dir ++ / ++ b ++ / ++ c) ],
Check out Control.Monad.List.ListT, which combines list non-determinism
with another monad. Eg,
ListT (getDirectoryContents a_dir) :: ListT IO FilePath
Andrew
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] First steps in Haskell
On Monday 19 December 2005 12:13, Daniel Carrera wrote: Wolfgang Jeltsch wrote: * There's no way for a new user to figure out how to successfully run the simplest Haskell program. There is! The Hugs 98 User's Guide and The GHC User's Guide. Okay, I stand corrected. I rephrase the concern as The links that say Learn Haskell don't show you how to run a simple Haskell program. Hello, very interesting exchange, this. It appears you are experiencing some (mild) culture shock here. It is typical for people in the Haskell community to view things in a rather principled way. A language tutorial is supposed to introduce /the language/. If you want to know how to compile or execute a Haskell program, well then, look at the appropriate tutorial on the /implementation/. At first this may appear like deliberately creating hurdles, but it isn't, it's merely the way many (though not all) Haskell people tend to think. They take it for granted that a new user is at least educated enough to be aware of the difference between the language itself, and its concrete implementation in the form of an interpretation or a compilation system. , Now although it appears you very well know the difference, you just aren't used to it being shoved in your face ;-) Well, another quirk is that adherence to what is custom (e.g. in other language communities) is not viewed as an important value among lambda folks, at least not if there is some general guiding principle that stands against it (in this case: the language itself is not to be confused with its concrete implementation). Once SPJ famously called it wairing the hair shirt, refering to the unusual way in which Haskell evolved in an extremely principled way (purely functional, lazy) which, for instance, during the first years made programming IO quite a challenge... until monadic IO was invented, so we(*) could finally do it *right*. Although I like this attitude very much in principle (pun intended), I can see that for newcomers this /can/ be a hurdle and I would recommend to tone it down a bit and compromise where it doesn't really do any harm -- such as explaining how to load and execute a Haskell program (using Hugs for instance) in a beginner's tutorial on (yes) Haskell The Language Itself ;-) Cheers, Ben (*) Figuratively speaking. Personally, I didn't know anything about Haskell back then. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Help with division
Greg Buchholz wrote: Integral division is spelled div... Thanks, that worked. The course material on the website uses a /. See the second link from the top, English notes. On page 5, right at the bottom. This course material teaches Gofer which it claims is essentially a subset of Haskell. I guess that division must be one of the differences between Gofer and Haskell. Cheers, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/I am not over-weight, I am under-tall. / ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Int vs Integer
On Monday 19 December 2005 11:26, Wolfgang Jeltsch wrote: Am Montag, 19. Dezember 2005 01:10 schrieb Jared Updike: Int is for bounded values -2**32 to 2**32 (I think... maybe 2**-31 and 2**31 or less if it's boxed?) based on the underlying machine representation. Not really true. As far as I remember, the Haskell Report just says that Int covers at least the integers from -2^27 to 2^27 - 1. The range for Int can be higher than the range just given. For 32-bit machines it will probably be -2^31 to 2^31 - 1 but there's no guarantee for that. http://www.haskell.org/onlinelibrary/basic.html 6.4 Numbers ... The finite-precision integer type Int covers at least the range [ - 2^29, 2^29 - 1]. As Int is an instance of the Bounded class, maxBound and minBound can be used to determine the exact Int range defined by an implementation. ... Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] comprehension generators from IO [a]'s ?
On 12/19/05, Chris Kuklewicz [EMAIL PROTECTED] wrote:
Okay...that works. Sweet.
gdc x = ListT $ getDirectoryContents x
get3levels top = runListT $ do
b - gdc top
c - gdc $ top++('/':b)
d - gdc $ top++('/':b)++('/':c)
return (b,c,d)
Yeah, that's awesome: just as readable as the comprehension
psuedo-code if not more-so. I knew some monad wizards would step out
with something nice.
Thanks for spelling out what Andrew was getting at also, it would have
been lost on me. I haven't yet ventured into using any monads but IO
yet, maybe ListT would be a good place to start.
I feel bound to point out http://haskell.org/hawiki/ListTDoneRight which
has more to say about the details of ListT
OK. Thanks to you both Chris and Andrew.
Steve
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: First steps in Haskell
OOn Mon, 19 Dec 2005 10:50:00 +0100, Wolfgang Jeltsch [EMAIL PROTECTED] wrote: Am Sonntag, 18. Dezember 2005 17:25 schrieb Daniel Carrera: [...] This is a real problem for Haskell. I expect that a lot of people try Haskell and give up because they can't even write the simplest function. honestly, I have to say that during my years with Haskell, this seems to be the first time that I see somebody trying to enter a Haskell program via the command line of an interactive environment. So I suppose that it's not a lot of people who experience the problems you do. I sympathise with Daniel's problems. I have hit many of them, as well. As Daniel said, they are not complete road blocks, merely bumps in the learning curve. Part of the problem may be that much of Haskell's tutorial material is focused on introducing functional programming (which is a new concept to the target audience), coupled with the fact that IO and imperative programming is a relatively advanced concept. That means that writing standalone programs (as well as being somewhat compiler-dependent in its details) takes a while to be discussed. Example: main is first mentioned in A Gentle Introduction to Haskell in chapter 7.1. That's a *lot* of reading before you get to write Hello, world... I suppose that neither the compilers and interpreters nor the documentation need(s) fixing, although I might be wrong with the documentation. I suspect that the reference documentation is fine, and the tutorials are great, given what they are trying to do. But there is scope for a very prominent Getting going document. Daniel's 40-second intro to Haskell looks good. Expand it a little with a few lines showing tow to build and compile a Hello, world program in GHC and Hugs (with a disclaimer that the code itself is magic for now, you'll understand when you read about monads and the IO monad) to help people who need to get over the but how do I write a *program* hump, and you're pretty much there. Paul. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
