Re: haskell newbie seeking for advice
Jose A.Ortega Ruiz wrote: as stated in the subject, i'm a newcomer to haskell programming: i've read some tutorials and (portions of) a couple of books and am really fascinated with the language. but my haskell coding experience is limited to toy programs and short exercises. so i decided to try my hand at a small project to really learn haskell and the functional programming mindset[1], and i would appreciate to hear your opinions and comments on some issues before i start coding: - build toolchain. i'm used to the autoconfig/automake/make tools for C/C++ projects, and to ant for Java stuff. what do haskellers use? my initial thought was using a plain Makefile with ghc, but there is also hmake and maybe other tools i am missing. what would you use? [2] There's also ghc --make. -- Glynn Clements [EMAIL PROTECTED] ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Yet Another Monad Tutorial
On Wed, 13 Aug 2003 13:26:34 +0200 blaat blaat [EMAIL PROTECTED] wrote: I have a long reply, perhaps I'll post it later. Here's a more pragmatic short one. What did we lose? Well, introspection comes to mind. A monad is strange in the sense that, actually, there does not seem to be any reason to compute its value since we cannot possibly get anything out of it. In case of IO, only the haskell interpreter can. And, introspection is a nice thing to have, especially for compiler writers. Although I think the benefits in Haskell would be limited, an application might be that I could write my own promotion functions on my own programs. Lot's of issues with this paragraph, but here are the questions. What do you mean by promotion function? How would you write it in your hypothetical Haskell? How is it related to IO? How do monads and/or the IO monad in particular cause problems? Why couldn't the same be done using monads or why would it be significantly harder if it can be done? Think of the following though experiment. What if I would have a function unparse:IO a-String which gives the textual representation of the specification of the object of type IO a? Clearly, IO is not a monad anymore. Moreover, I don't think it would break the purity of Haskell. And surely, in case of the IO monad, we can think of better manners to break a program down. Keith has already pointed out the flagrant error here; which I also point out in my long reply. But again, the questions... What is unparse supposed to return? What does it have to do with IO in particular, i.e. why do I want to unparse IO computations especially? Why can't it be done with (or over) the current IO monad? What other ways do you want to breakdown a program, still resulting in understandable behavior, but can't because of the IO monad? Hmm, I am rambling. The point is, I think there is no reason to discard all alternatives to monads; I believe that central in the IO-issue is not the fact that we are using monads for IO - but that we can purely create impure programs which do the IO for us ;-), and that those do not have to be of the monadic kind. I have comments for these too, but I'll keep this short. Basically, I don't really see how what you want has all that much to do with IO in general, and I don't see monads get in the way. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Editor in Linux for Hint/Helium
Tim Stitt wrote: I'm using the Helium interpreter/compiler with the Hint interface within Linux. Unfortunately, I want an editor that allows me to jump to the row/column of the source when I receive an error. Does anyone know of a good editor in Linux to do this or the commands for Emacs etc. that can be used from within Hint to achieve it? Emacs and XEmacs allow you to go to a specified line using +number, e.g. xemacs +20 foo.txt loads foo.txt and positions the cursor on line 20. The same syntax works with emacsclient (Emacs) and gnuclient (XEmacs), which load a file into an existing XEmacs process, i.e. gnuclient +20 foo.txt Emacs requires the server-start command (e.g. in ~/.emacs) before emacsclient can connect; similarly XEmacs requires gnuserv-start. -- Glynn Clements [EMAIL PROTECTED] ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: idiom for producing comma-seperated lists?
Ketil Z. Malde wrote: Antony Courtney [EMAIL PROTECTED] writes: -- Example: format a list of strings, using a comma as a seperator: mkSepStr :: [String] - String mkSepStr xs = foldrs (\x s - x ++ , ++ s) xs t0 = mkSepStr [] -- == t1 = mkSepStr [hello]-- == hello t2 = mkSepStr [10,20,30] -- == 10, 20, 30 What do the rest of you do to solve this particular problem? Uh, concat and intersperse? Yep, that'll do the trick. Thanks for all the lightnin-fast responses. Appologies for overlooking this! -antony -- Antony Courtney Grad. Student, Dept. of Computer Science, Yale University [EMAIL PROTECTED] http://www.apocalypse.org/pub/u/antony ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Another typing question
On Tuesday 05 August 2003 15:23, Wolfgang Jeltsch wrote: There probably are ways to achieve this with Haskell's type system. The exact solution depends on what exactly you want. For just distinguishing between lists of different lengths you could use tuples like (a,a) (list of length 2) (a,a,a) (list of length 3) etc. What would be the advantage to using standard lists? In both cases I'd need explicit length checks everywhere. A better solution might be: data EmptyList element = EmptyList data TrueList list element = TrueList element (list element) A list of four Ints for example would be represented by a value of type TrueList (TrueList (TrueList (TrueList EmptyList))) Int. Isn't that just the Haskell list type by another name? You could define different types for different natural numbers: data Zero = Zero data Succ number = Succ number and do simple arithmetic with them at the type level: class Sum a b c | (a, b) - c instance Sum 0 a a instance Sum a b c = Sum (Succ a) b (Succ c) That looks interesting, I need to look into multiple parameter classes... At the moment, I cannot think of a way of defining a list type which is parameterized by such natural types. What I mean is having a type List :: * - * - * where for our above example of a list of four Ints we would have the type List (Succ (Succ (Succ (Succ Zero Int That's what I would like... What I can think of, is an association between our above list types (EmptyList and TrueList) and our natural number types by a multi-parameter class: class HasLength list length | list - length instance HasLength EmptyList Zero instance HasLength list length = HasLength (TrueList list) (Succ length) Again those multi-parameter classes... and off I go to the Haskell Web site ;-) Thanks for your comments! Konrad. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [newbie] UTF-8
Dnia pon 11. sierpnia 2003 00:49, Wolfgang Jeltsch napisa: The main problem is that you need binary I/O. Haskell 98 only provides text I/O. You don't need binary I/O for UTF-8 now; because implementations use ISO-8859-1, UTF-8 octets can be faked as characters by (chr . fromIntegral). The other point with text I/O is that under Windows the EOF character ^Z is treated specially and a conversion between Windows EOLs (^M^J) and Haskell EOLs (^J) takes place. UTF-8 preserves ASCII and doesn't use ASCII bytes for non-ASCII characters, so the situation is the same as in other encodings and text mode is usually fine. It would not be OK for UTF-16. -- __( Marcin Kowalczyk \__/ [EMAIL PROTECTED] ^^ http://qrnik.knm.org.pl/~qrczak/ ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: IO Bool - Bool
On Thu, 14 Aug 2003, Wolfgang Jeltsch wrote: On Thursday, 2003-08-14, 17:05, CEST, Kevin S. Millikin wrote: On Wednesday, August 13, 2003 11:20 PM, Tn X-10n [SMTP:[EMAIL PROTECTED] wrote: is it possible to convert IO Bool to Bool? Sure. Which Bool do you want? True? toTrue :: IO Bool - Bool toTrue x = True Or False? toFalse :: IO Bool - Bool toFalse x = False There's also boolFromIO :: IO Bool - Bool boolFromIO = boolFromIO if you want to be even less useful :) I wouldn't call these *conversion* functions because they don't look at their argument. Maybe that's not what you had in mind. Surely not. Wolfgang I'm surprise nobody has mentioned unsafePerformIO (:: IO a - a). As the name suggests, it isn't referentially transparent. Are you sure you need a function of type IO Bool - Bool? What are you trying to do? Brandon ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: idiom for producing comma-seperated lists?
Antony Courtney [EMAIL PROTECTED] writes: -- Example: format a list of strings, using a comma as a seperator: mkSepStr :: [String] - String mkSepStr xs = foldrs (\x s - x ++ , ++ s) xs t0 = mkSepStr [] -- == t1 = mkSepStr [hello]-- == hello t2 = mkSepStr [10,20,30] -- == 10, 20, 30 What do the rest of you do to solve this particular problem? Uh, concat and intersperse? Prelude concat $ List.intersperse , $ [] Prelude concat $ List.intersperse , $ [hello] hello Prelude concat $ List.intersperse , $ [10,20,30] 10, 20, 30 -kzm -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: .. Drowning in stateful, side-effect riddled, spaghetti code? We have the answer. fdps[]c.
Hey there! Stateful side-effect-laden spaghetti programming sucks, and so does the cost.. feel like your [sic] getting ripped off? ... We tear up your code and give you and your colleagues a fresh start .. - Save you a lot of money by eliminating side-effects - Develop new code in substantially reduced time - Stop customers/managers calling you on the phone about late delivery - Avoid code maintenance debt ... and more! Why keep dealing with the stress, and headaches, of mainstream programming styles? Combine pure and lazy functional programming and get on with your life today!! Come here and take a look at how we can help. http:///www.haskell.org -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: 12 August 2003 08:47 To: [EMAIL PROTECTED] Subject: .. Drowning in debt? We have the answer. fdps[]c. Hey There! Debt sucks, and so do the repayments.. feel like your getting ripped off? .. We tear up your debt and give you and your family a fresh start .. - Save you a lot of money by eliminating late fees - Settle your accounts for a substantially reduced amount - Stop creditors calling you on the phone - Avoid bankruptcy ... and more! Why keep dealing with the stress, and headaches? Combine your debt into a low interest repayment and get on with your life today!! Come here and take a look at how we can help. http://[EMAIL PROTECTED]/index.php?N=g * The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. * ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: Database interface
[EMAIL PROTECTED] wrote: If I may interject, that's precisely how a Scheme DB interface is designed. The main function is a left-fold. Not quite though: it provides for a premature termination: A major procedure: DB1:fold-left PROC INITIAL-SEED QUERY-OBJECT Premature termination sounds like a useful property. I can see two ways this could be done: keep the previous signature, and use an exception to exit early, or add a boolean return value like the scheme version: doquery :: Process - String - (a - b - IO (b,Bool)) - b - IO b Any opinions on which alternative wouuld be preferable? Tim ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[String]-[[Bool]] 2
[String] contains = ["1 0 0","1 0 0","0 1 0"] this [String] is read from a file. the file shows only: i convert it by using Lines. but i stuck wif the output. 1 0 0 1 0 0 1 0 1 if 1 then False 0 then True the expected result show be [[False,True,True],[False,True,True],[True,False,True]] My code is as followed... readF :: String - IO () readF fname = do x - (readFile fname) putStr (show(conv(words x))) i can't use lines here because change only accepts "0" or "1". but my file contains spaces. change :: String - Bool change "1" = True change "0" = False conv :: [String] - [Bool]==unable to declare [[Bool]] conv fname = [(change x)| x - (fname)] Get free gift vouchers every week with MSN Premium Vouchers ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Another typing question
On Tuesday, 2003-08-05, 15:22, CEST, Nick Name wrote: [...] Is there any way to parametrize a type by a value, rather than another type? What I would like to do is to define list of length 3 and list of length 4 as separate parametrization of the same type, such that I could write functions that accept lists (under my new typename) of any length as long as parameters have the same length. The goal is to remove the need for runtime checks all over the code. This is called dependent types and is not a feature of haskell (nor of any language that I know); there was cayenne (try a google search) but I don't believe it is still mantained. Well, there are, e.g., Ada and C++ which allow types to be parametrized by values. The point is that they obviously still use runtime checks, it's just that you don't have to code these checks explicitely. BTW, why is there no general interest for such a thing as dependent types? What negative consequences does their implementation have? I think, sometimes they could be quite handy. V. W. ;-) ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: Yet Another Monad Tutorial
Alistair, Hhhmmm. That is an interesting issue, and I am unsure how to treat it in the tutorial. I did attempt to explain the ability of the monad to isolate impure computations, but I think I need to make a better explanation of what an action is and how it is used. As for the question of whether using a monad in this way casts Haskell out of the ranks of the functionally pure, I don't know. This feels to me like a debate over personal interpretation, with decent arguments on both sides. I think the most accurate thing to say is that it enables Haskell to incorporate non-pure features without destroying the purity of the core of the language. Any suggestions for how best to present this issue to a reader who may be new to the world of FP? Should we mention it at all? Thanks, Jeff Alistair Bayley wrote: Thanks. Quite comprehensive. Peter Van Roy (the Mozart/Oz guy) said this on the PragProg list, and I didn't have the knowledge to respond. It's a point you might want to address, perhaps in this section?: http://www.nomaware.com/monads/html/laws.html#nowayout [Discussion of whether I/O monad makes Haskell impure] -- Jeff Newbern [EMAIL PROTECTED] Nomaware, Inc. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Yet Another Monad Tutorial
To many questions, not enough mail. First, thanks for all your replies. Second, I stand totally corrected on the fact that we cannot break down monads. Functions of type m a-b are called impure, see also unsafePerformIO. I am questioning (a) the exact relation between monads, monadic IO, IO in general and (b) whether the impure restriction is actually a warranted restriction, which relates to, what I find, strange evaluation behavior of the IO monad. What is the difference between putStr a, (putStr a, putStr a), putStr (putStr a), putStr (show (putStr a))? At the moment objects of type IO a _only_ get evaluted when they are the _sole_ result of a Haskell program. What kind of strange dichotomy is that? Is not the _only_ interpretation of this dichotomy that objects of type IO describe impure programs (composed of actions which are only evaluated when...)? And, if so, why can't we decompose these programs (before they get evaluated)? And, if so, is not the only relation between a monad and doing functional IO that there is a monadic manner to construct descriptions of impure programs? If so, are there also non-monadic ways which might be equally interesting? As stated by Keith, deriving show for IO is possible. In such a setting putStr (putStr a) I imagine returns putStr \a\, which I find far more orthogonal than what we have now. (Have to find the reference still). Does this boil down to an impure or a pure decomposition? From: Derek Elkins [EMAIL PROTECTED] To: blaat blaat [EMAIL PROTECTED], [EMAIL PROTECTED] pragmatic short one. What did we lose? Well, introspection comes to mind. A monad is strange in the sense that, actually, there does not seem to be any reason to compute its value since we cannot possibly get anything out of it. In case of IO, only the haskell interpreter can. And, introspection is a nice thing to have, especially for compiler writers. Although I think the benefits in Haskell would be limited, an application might be that I could write my own promotion functions on my own programs. Lot's of issues with this paragraph, but here are the questions. What do you mean by promotion function? Example of promotion: map f . map g = map (f . g) A promotion function would transform objects of type IO a into objects of type IO a while performing promotions. How would you write it in your hypothetical Haskell? How is it related to IO? How do monads and/or the IO monad in particular cause problems? Why couldn't the same be done using monads or why would it be significantly harder if it can be done? In a monadic approach, I imagine functions like unbind which now would break purity. [..] I believe that central in the IO-issue is not the fact that we are using monads for IO - but that we can purely create impure programs which do the IO for us ;-), and that those do not have to be of the monadic kind. I still stand here (but am totally willing to be corrected on this account too). Hmpf, maybe someone more enlightened should write a paper on it. Cheers, l4t3r _ MSN 8 with e-mail virus protection service: 2 months FREE* http://join.msn.com/?page=features/virus ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Yet Another Monad Tutorial
Peter G. Hancock wrote: Jeff Newbern wrote (on Tue, 12 Aug 2003 at 17:20): ? The functions exported from the IO module do not perform I/O themselves. They return I/O actions, which describe an I/O operation to be performed. The I/O actions are combined within the IO monad (in a purely functional manner) to create more complex I/O actions, resulting in the final I/O action that is the main value of the program. The result of the Haskell compiler is an executable function incorporating the main I/O action. Executing the program applies this ultimate I/O action to the outside world to produce a new state of the world. That seems to me the wrong thing to say. There is no application. Whether or not the word is put in quotes, it is something involving a function and an argument. An IO action is not a function. So then, in your view, what *is* an IO action? One conceptual model is that an IO action with type (IO a) denotes a function of type World - (World,a). Given that model, applying an IO action to the external world seems like a perfectly reasonable account of executing such an action. -antony -- Antony Courtney Grad. Student, Dept. of Computer Science, Yale University [EMAIL PROTECTED] http://www.apocalypse.org/pub/u/antony ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
AW: Yet Another Monad Tutorial
The overall message I have taken from your post is that I need to be more precise to convey the correct information and avoid confusion (or worse, misinformation). As I understood it, the tutorial should be instructive. So I would not try to be more precise at the beginning, because too much detail would lead to confusion for a beginner. Start with something like I'm now telling you some lies, which make it easier to grasp the idea of the monad, but further down the road, I'll reexplain some concepts and be more precise there. This was done by Don Knuth for the Typesetting-Books and it worked very well (for me). I hope this is a sensible idea. Markus -- Markus Schnell Infineon Technologies AG, CPR ET Tel +49 (89) 234-20875 ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: Database interface
I'll probably generalise the query function to do a fold, rathen than always accumulate a list: doquery :: Process - String - (a - b - IO b) - b - IO b If I may interject, that's precisely how a Scheme DB interface is designed. The main function is a left-fold. Not quite though: it provides for a premature termination: A major procedure: DB1:fold-left PROC INITIAL-SEED QUERY-OBJECT A QUERY-OBJECT (which in this implementation is a list of fragments that make a SQL statement, in the reverse order -- without the terminating semi-colon) is submitted to the database, using the default database connection. PROC is a procedure: SEED COL COL ... The procedure PROC takes 1+n arguments where n is the number of columns in the the table returned by the query. The procedure PROC must return two values: CONTINUE? NEW-SEED The query is executed, and the PROC is applied to each returned row in order. The first invocation of PROC receives INITIAL-SEED as its first argument. Each following invocation of PROC receives as the first argument the NEW-SEED result of the previous invocation of PROC. The CONTINUE? result of PROC is an early termination flag. If that flag is returned as #f, any further applications of PROC are skipped and DB1:fold-left finishes. The function DB1:fold-left returns NEW-SEED produced by the last invocation of PROC. If the query yielded no rows, DB1:fold-left returns the INITIAL-SEED. Thus DB1:fold-left is identical to the left fold over a sequence, modulo the early termination. There are a few minor variants of the above procedure, optimized for common particular cases: a query that is expected to return at most one row, and a query that expects to return at most one row with exactly one column. The latter query has the same interface as a lookup in a finite map. The QUERY-OBJECT is of coursed not built by hand. There is a domain-specific language (which greatly resembles the source language and makes some use of quasi-quotation) whose result is a query object. Therefore, I can write both the query and the handlers of the query result using the same syntax. The premature termination is important. Database connections and cursors are too precious resources to leave them for the garbage collector to finalize. More discussion and pointers can be found at http://srfi.schemers.org/srfi-44/mail-archive/msg00023.html The interface has been used in an industrial application. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Yet Another Monad Tutorial
I suspect that tutorials should at some point mention some definitions of monads, monoids et al --- since this is where the power (sorry, QA) comes from. I was reading an overview-for-non-specialists of category theory earlier today, and was surprised by the familiarity of much of it - based on the little Haskell I'd learned. What it didn't have, and what I'd still like to read, is an overview-for-non-specialists of monads and monoids, as this was one bit of the mathematical terminology used by Haskell that was rather opaque to me - I think of a monad as a) a sort of a self-contained something-or-other in Liebniz's philosophy, and b) a scary baddy in the ABC Warriors comic strip published by 2000AD (I'm not making this up...). Dominic ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [String] - [[Bool]]
i am new wif haskell, i would like to know how can i get a list of string and convert it to a list of a list of bool [String]- [[Bool]] for example a [String] [0 1 0 1 0 1,0 0 0 0 0 0 ,1 0 0 0 1 0] it return true if 0 and false if 1. the [String] is read from a file. map (\x - map (==0) x) $ map words [0 1 0 1 0 1,0 0 0 0 0 0 ,1 0 0 0 1 0] -- Yoshinori Tahara [EMAIL PROTECTED] ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
haskell newbie seeking for advice
hi all, as stated in the subject, i'm a newcomer to haskell programming: i've read some tutorials and (portions of) a couple of books and am really fascinated with the language. but my haskell coding experience is limited to toy programs and short exercises. so i decided to try my hand at a small project to really learn haskell and the functional programming mindset[1], and i would appreciate to hear your opinions and comments on some issues before i start coding: - build toolchain. i'm used to the autoconfig/automake/make tools for C/C++ projects, and to ant for Java stuff. what do haskellers use? my initial thought was using a plain Makefile with ghc, but there is also hmake and maybe other tools i am missing. what would you use? [2] - graphics toolkit. one of my programs will be a GUI. there seems to be a lot of choice here: gtk2hs looks nice (as used in hircules) and fudgets seems to have a pretty interesting and well-documented architecture (which covers also client/server programming: i'll be using a client/server architecture too) but its widgets are uglier and look alien in current linux desktops. i'd like to have a fudgets-gtk framework of sorts :) and then, there is htoolkit, Object I/O and whatnot... any recommendations? - TCP/IP stuff. i will have a client/server architecture with TCP/IP as the transport protocol. other than fudgets, i'm not aware of any haskell framework for IPC: am i missing something? - XML. i'll be handling XML documents, and HaXml seems to me an excellent candidate to cover this area, but, of course, i'm very interested in hearing of alternatives... thanks a lot for your help! cheers, jao Footnotes: [1] if that matters, the project will consist on a set of utilities for handling a document metadata database, based upon the OMF standard. [2] my project will consist of several executables, sharing a set of common libraries and, therefore, i plan to use (if possible) a non-flat directory layout. -- Come to think of it, there are already a million monkeys on a million typewriters, and Usenet is NOTHING like Shakespeare. - Blair Houghton. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Yet Another Monad Tutorial
Peter, Thank you for criticism. This is exactly the kind of feedback I need. The overall message I have taken from your post is that I need to be more precise to convey the correct information and avoid confusion (or worse, misinformation). I should have said that a function which performs a computation in the I/O monad must have IO as the outermost constructor in its return type. I think that is more precise (and correct :-). As for the later text concerning application of the top-level I/O action, I will need to think for a while to see how I can word this more clearly. If you (or anyone else) have ideas, I'd like to hear them. Thanks again, Jeff On Tue, 2003-08-12 at 18:24, Peter G. Hancock wrote: Jeff Newbern wrote (on Tue, 12 Aug 2003 at 17:20): (proposed revisions) In the section No Way Out: -- The IO monad is a familiar example of a one-way monad in Haskell. Because you can't escape from the IO monad, it is impossible to write a function that does a computation in the IO monad but returns a non-monadic value. I wouldn't say that, as it is inaccurate. Of course you can return a value of _some_ monadic type eg. (Maybe ...). Not only are functions of the type IO a - a impossible to create, You can quite easily write a function of type IO (IO a) - IO a, which is a special case of that type. but any function whose result type does not contain the IO type constructor is guaranteed not to use the IO monad. That's rather vague: what does it mean for a function to use a monad? Other monads, such as List and Maybe, do allow values out of the monad. So it is possible to write non-monadic functions that internally do computations in those monads. The one-way nature of the IO monad also has consequences when combining monads, a topic that is discussed in part III. -- In summary, I've only a vague idea of what you are trying to say. If you can't reformulate it more precisely, don't add the above stuff. and a little farther down: -- Some people argue that using monads to introduce non-pure features into Haskell disqualifies it from claiming to be a pure functional language. This subtle question not particularly relevant to the practical programmer is revisited in the context of the I/O monad later in the tutorial. -- That's fair enough. I don't think the question is so much subtle as religious, as we might expect from the terminology of purity. Later, in the section on the I/O monad: -- In Haskell, the top-level main function must have type IO (), so that programs are typically structured at the top level as an imperative-style sequence of I/O actions and calls to functional-style code. Revisiting the debate about the purity of Haskell (in a functional sense), it is important to note that the IO monad only simulates imperative-style I/O. That (about simulation) seems weak. A simulation isn't a vague syntactic resemblance. The functions exported from the IO module do not perform I/O themselves. They return I/O actions, which describe an I/O operation to be performed. The I/O actions are combined within the IO monad (in a purely functional manner) to create more complex I/O actions, resulting in the final I/O action that is the main value of the program. The result of the Haskell compiler is an executable function incorporating the main I/O action. Executing the program applies this ultimate I/O action to the outside world to produce a new state of the world. That seems to me the wrong thing to say. There is no application. Whether or not the word is put in quotes, it is something involving a function and an argument. An IO action is not a function. This occurs only once per execution of the program, and since the state of the world changes for each execution, the issue of purity is neatly side-stepped. -- By the program, I think you mean the IO action. I think it is right to speak of the action as something that is executed. Execution may involve (side-effect free) calculation; but execution is something essentially different from calculation, not an impure form of it. I'm sorry to sound negative -- it's just that you invited criticism. Your pages seem generally of a very high quality to me. Sorry not to be more constructive too. Peter Hancock -- Jeff Newbern [EMAIL PROTECTED] Nomaware, Inc. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Another typing question
On Tuesday, 2003-08-05, 15:22, CEST, Nick Name wrote: snip This is called dependent types and is not a feature of haskell (nor of any language that I know); there was cayenne (try a google search) but I don't believe it is still mantained. snip BTW, why is there no general interest for such a thing as dependent types? What negative consequences does their implementation have? It depends on whether you just want to write functions to take in values of such types, or whether you want to write functions to return values of such types. Doing the former uses dependant products, which would require a significant extension to GHC's guts, but not (AIUI) inherently unpleasant. Doing the latter (AIUI) essentially requires dependant sums. Basically, a family of types A_i parameterized by a value i :: I is written, for long-honored traditional reasons too complex to get into here, Sigma i::I.A_i. Now, if you have such dependant sums, there's a bad interaction with polymorphism: either you must have a type alpha which cannot appear in a family of types A_i in a dependant sum, or, for any function f :: alpha - alpha, there exists a type beta such that f cannot be aplied at either beta or Sigma i::Int.beta. Clear? The alternative, btw., is to have both dependant sums and an undecidable type-checking problem. I think, sometimes they could be quite handy. Sure. Sarcastic comment censored. Jon Cast ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: IO Bool - Bool
On Thursday 14 August 2003 06:20, Tn X-10n wrote: htmldiv style='background-color:'DIV PBRhai guys/Pis it possible to convert IO Bool to Bool?/DIV/divbr clear=allhrGet 10mb of e-mail space with a href=http://g.msn.com/8HMTENSG/2737??PS=;MSN Hotmail Extra Storage/a at only S$36 per year including GST/html It makes no sense to convert an IO Bool to a Bool. If something has type IO Bool that means it is an action which will get a Bool from the outside world. It cannot be converted into a Bool. If you want to use the action to get a Bool from the outside world you should invoke the action in somewhere in the IO part of your code, like this.. do ... boolVal - ioBoolAction ... Regards -- Adrian Hey ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Fwd: Re: Yet Another Monad Tutorial
From: Derek Elkins [EMAIL PROTECTED] blaat blaat [EMAIL PROTECTED] wrote: [...] However, given that observation, the fact that IO is a monad I find to be a _rather arbitrary design decision_; why not define a small term language which may be passed to the OS? That would be confusing to use or restrictive (unless done exceedingly well perhaps), it would be awkward to define, and it would pretty much require arbitrary design decisions (what should be included in this language? what are it's semantics? etc). Why not have (some limited) introspection on programs which we build? It would be round-about in both implementation and use to do this. Nevertheless, you can easily view the IO monad as an imperative language -embedded- in Haskell. Even when writing mostly imperative code Haskell gives you much more flexibility and protection than many imperative languages, especially the more popular ones. [...] Are we missing out on usefull stuff? No, but if we were, it'd be trivial to test/get it with an appropriate top-level runTheProgram IO computation definable in Haskell today. Trivial to test I fully agree with; however, to the missing out on usefull stuff I am not convinced. Please allow me to continue my trolling. To use the IO monad for impure calculations is, of course, a design decision. It is arbitrary to the point that I have the freedom not to agree with it; but hey, even I know the thing works ;-). The key point to my email was that doing impure IO possibly has little to do with a monadic style of programming but with the fact that we can functionally construct a description of an impure program which is evaluated _only_ when delivered as a sole result of a Haskell computation. I find that somewhat odd; but not to the point I cannot live with it ;-) About the indirectness issue. The benefit of choosing a monad is clear - the evaluation/rewrite of an action corresponds directly with an impure subroutine call. Moreover, the action can be discarded and, as you pointed out, the description of these impure subroutine calls are trivially embedded directly into Haskell - giving typesafety as a reward. It is both effective and efficient. Great Hail to the monad! Hmmm, uhm, but let's think further... What did we lose? Well, introspection comes to mind. A monad is strange in the sense that, actually, there does not seem to be any reason to compute its value since we cannot possibly get anything out of it. In case of IO, only the haskell interpreter can. And, introspection is a nice thing to have, especially for compiler writers. Although I think the benefits in Haskell would be limited, an application might be that I could write my own promotion functions on my own programs. Think of the following though experiment. What if I would have a function unparse:IO a-String which gives the textual representation of the specification of the object of type IO a? Clearly, IO is not a monad anymore. Moreover, I don't think it would break the purity of Haskell. And surely, in case of the IO monad, we can think of better manners to break a program down. Hmm, I am rambling. The point is, I think there is no reason to discard all alternatives to monads; I believe that central in the IO-issue is not the fact that we are using monads for IO - but that we can purely create impure programs which do the IO for us ;-), and that those do not have to be of the monadic kind (with all respect to the baffling ingenuity of the invention). Cheers, l4t3r _ Protect your PC - get McAfee.com VirusScan Online http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963 ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Outlook
hey there, i thought you'd like to check this out You really owe it to yourself and your family to take a look, With the money you save, put it towards a new car! --- http://btrack.iwon.com/r.pl?redir=http://[EMAIL PROTECTED]/index.asp?RefID=198478 --- for no more http://[EMAIL PROTECTED]/auto/index.htm ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Yet Another Monad Tutorial
Hello, I have put up all-in-one archives of the tutorial for people who prefer to read it offline. http://www.nomaware.com/monads/monad_tutorial.tgz and http://www.nomaware.com/monads/monad_tutorial.zip Thanks, Jeff Newbern [EMAIL PROTECTED] __ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Database interface
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 I still want to use HaskellDB (or at least the relational calculus part of it), so I was thinking of splitting it into two pieces: a library that submits SQL queries and returns the results (database interface), and a library that constructs queries with the relational calculus and generates SQL from them. Obviously the database interface must be written for each DBMS product, while the relational calculus library ought to be independent of DBMS (although it might be wise to have SQL compatibility flags for the SQL it generates. For example: Oracle 8 (what I'm using now) only supports SQL-92). Ideally, we would have the following. 1/ A high level combinator library for relational calculus built on, 2/ A standard low-level Haskell API i.e. functions like Connection - String - ( a - b - IO b) - IO b 3/ Database specific bindings. The problem is that projections and cartesian products generate new types, at least in the way it was done in HaskellDB. The form the calculus takes would need to be substantially reworked. Tom -BEGIN PGP SIGNATURE- Version: GnuPG v1.2.2 (GNU/Linux) iD8DBQE/O6DBYha8TWXIQwoRAmkCAKCwZkNpU3TjX5bj7rOZHx5DXRjPhwCgihnY sAWwEKX+5hZ1Tiu4wfSGo7Y= =ALnJ -END PGP SIGNATURE- ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: IO Bool - Bool
On Thursday, 2003-08-14, 17:05, CEST, Kevin S. Millikin wrote: On Wednesday, August 13, 2003 11:20 PM, Tn X-10n [SMTP:[EMAIL PROTECTED] wrote: is it possible to convert IO Bool to Bool? Sure. Which Bool do you want? True? toTrue :: IO Bool - Bool toTrue x = True Or False? toFalse :: IO Bool - Bool toFalse x = False I wouldn't call these *conversion* functions because they don't look at their argument. Maybe that's not what you had in mind. Surely not. Wolfgang ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Yet Another Monad Tutorial
On Tue, Aug 12, 2003 at 03:04:39PM -0400, Antony Courtney wrote: So then, in your view, what *is* an IO action? One conceptual model is that an IO action with type (IO a) denotes a function of type World - (World,a). But the IO monad is not a state monad, because other agents may change the state of the world while a Haskell program is running. It's more accurate to say that the program returns an action to be executed by the environment: data IO a = Return a | Invoke (SysCall (IO a)) data SysCall r = GetChar (Char - r) | PutChar Char r | ... In addition to the usual arguments, the system call is given a return continuation to enter on completion. (This was the model of interactive I/O used by Moggi in An Abstract View of Programming Languages, and is also used by Fudgets.) Of course the interpretation the environment places on GetChar, etc is outside of the functional world. That simple model omits exceptions. One way of adding them is data IO a = Return a | Error IOError | Invoke (SysCall (IO a)) (IOError - IO a) ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: IO Bool - Bool
On Wednesday, August 13, 2003 11:20 PM, Tn X-10n [SMTP:[EMAIL PROTECTED] wrote: is it possible to convert IO Bool to Bool? Sure. Which Bool do you want? True? toTrue :: IO Bool - Bool toTrue x = True Or False? toFalse :: IO Bool - Bool toFalse x = False Maybe that's not what you had in mind. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: Yet Another Monad Tutorial
From: Wolfgang Jeltsch [mailto:[EMAIL PROTECTED] For example, the function readFile is pure. For a specific string s the expression readFile s always yields the same result: an I/O action which searches for a file named s, reads its content and takes this content as the result of the *action* (not the expression). What about getChar? This is a function which takes no arguments, yet returns a (potentially) different value each time. I know, I know: it returns an IO action which reads a single char from the terminal and returns it. Is the IO monad the only one (currently) in which you would say that it returns an action, which is then executed by the runtime system? I would have thought that monads that are not involved in IO (e.g. State) would be pure in the sense that Van Roy was thinking. You wouldn't need to describe functions in them as returning an action. * The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. * ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: IO Bool - Bool
PBRhai guys/Pis it possible to convert IO Bool to Bool? No, it's not... and for good reason! See the discussion at http://www.haskell.org/hawiki/ThatAnnoyingIoType and http://www.haskell.org/hawiki/UsingIo In general, the HaWiki has answers to lots of newbie questions like this - you might find it useful to peruse (although you should certainly continue to ask questions here too - we're a pretty friendly bunch!). Best wishes, --KW 8-) -- Keith Wansbrough [EMAIL PROTECTED] http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: IO Bool - Bool
On Thursday, 2003-08-14, 07:20, CEST, Tn X-10n wrote: hai guys is it possible to convert IO Bool to Bool? No. The reason for introducing the IO type is to preserve the purity of Haskell, i.e., to ensure that expression evaluation doesn't depend on the state of the outside world and doesn't alter this state.*) Allowing a conversion from IO t to t would nullify this. As you might have noticed, there was (or still is) a discussion about yet another monad tutorial on The Haskell Cafe. Some of the messenges and, above all, the monad tutorial itself may help you understand how I/O in Haskell works. There is not a quick answer to your question like: This way you convert an IO Bool to a Bool. You will have to do some reading to understand the basic ideas of I/O in Haskell. They are quiet different from what you might know from other programming languages. Wolfgang *) This is at least how I would formulate it, others would probably phrase it a bit different. ;-) P.S.: Would you mind to configure your mail client such that your mails also contain a plain text variant of your message? ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Yet Another Monad Tutorial
On Wed, 13 Aug 2003 09:44:36 +0200 blaat blaat [EMAIL PROTECTED] wrote: Hmmm, I personally always thought of the IO monad as being a synonym for Program So. What is a Program? [...] For what is the interpretation of an object of type IO a? Well, it is a bit of a hack atop of Haskell, isn't it? Not for the reasons you describe below. If we return a complex type with an IO a object in it, the object cannot be evaluated, but if we return a IO object solely (program), the program get evaluated (passed to the (Haskell)-OS). However, given that observation, the fact that IO is a monad I find to be a _rather arbitrary design decision_; why not define a small term language which may be passed to the OS? That would be confusing to use or restrictive (unless done exceedingly well perhaps), it would be awkward to define, and it would pretty much require arbitrary design decisions (what should be included in this language? what are it's semantics? etc). Why not have (some limited) introspection on programs which we build? It would be round-about in both implementation and use to do this. Nevertheless, you can easily view the IO monad as an imperative language -embedded- in Haskell. Even when writing mostly imperative code Haskell gives you much more flexibility and protection than many imperative languages, especially the more popular ones. [...] Are we missing out on usefull stuff? No, but if we were, it'd be trivial to test/get it with an appropriate top-level runTheProgram IO computation definable in Haskell today. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[String]-[[Bool]]
a string is a list of Char which can be '1', '0', '/n'and spaces ' '. ["1 0 1 0 0 1 /n"]Get free gift vouchers every week with MSN Premium Vouchers ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Yet Another Monad Tutorial
Hmmm, I personally always thought of the IO monad as being a synonym for Program (or, for functions wich return an IO a, a subroutine with side-effects which returns a value of type a). We can build mini programs (like putStr), and compose them in a sequential manner (including something alike assignment). I personally use the following analogy. Someone, somewhere, sometime must have thought, Great, if I cannot do imperative programming directly, I can damned well create imperative programs and have _them_ evaluated, grumbl! And with that thought, he solved numerous problems ;-) Can we really make cheese out of it? (Dutch saying) For what is the interpretation of an object of type IO a? Well, it is a bit of a hack atop of Haskell, isn't it? If we return a complex type with an IO a object in it, the object cannot be evaluated, but if we return a IO object solely (program), the program get evaluated (passed to the (Haskell)-OS). However, given that observation, the fact that IO is a monad I find to be a _rather arbitrary design decision_; why not define a small term language which may be passed to the OS? Why not have (some limited) introspection on programs which we build? Purity will never be broken since the only way to get a program evaluated is to pass it to the Haskell-OS. Are we missing out on usefull stuff? Hehe... God, we might as well just generate impure graph rewrite rules, or plain C for that matter ;-). Cheers, l4t3r ps: Mind you, nothing wrong with that IO monad. It works, why break it. ;-) _ STOP MORE SPAM with the new MSN 8 and get 2 months FREE* http://join.msn.com/?page=features/junkmail ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: Database interface
Tim Docker writes: | Tom Pledger writes: | | This is a pretty good way to stop those nasty vague SQL row types at | the Haskell border and turn them into something respectable. Perhaps | it would even be worth constraining the extracted type to be in | DeepSeq | | doquery :: (DeepSeq v) = | Process - String - IO v - IO [v] | | Can you explain what the constraint does here? Yes. It soothes my superstitious fears about unsafeInterleaveIO. I imagined that: - doquery (like getContents) uses unsafeInterleaveIO to make the resulting list lazy, i.e. avoid grabbing all the rows at the outset, - the unsafe interleaving would *somehow* get into the extraction action, and - the current row could be overwritten while some stringv (etc.) actions on it were yet to happen, so - a good safety measure would be to force evaluation (hence DeepSeq) of each list element before overwriting the current row. But now that you ask, I think it's possible for doquery to stop the unsafeInterleaveIO from leaking into the extraction action, so the DeepSeq would only protect users from shooting themselves in the foot with weird queries like this doquery p select s from t (unsafeInterleaveIO (stringv p 1)) -- DeepSeq would allow doquery to undo the effect of unsafeInterleaveIO or this [a1, a2] - doquery p select s from t (return (stringv p 1)) s2 - a2 s1 - a1 -- DeepSeq would disallow this at compile time - Tom ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: haskell newbie seeking for advice
hi, Jose A.Ortega Ruiz wrote: hi all, as stated in the subject, i'm a newcomer to haskell programming: i've read some tutorials and (portions of) a couple of books and am really fascinated with the language. but my haskell coding experience is limited to toy programs and short exercises. so i decided to try my hand at a small project to really learn haskell and the functional programming mindset[1], and i would appreciate to hear your opinions and comments on some issues before i start coding: - build toolchain. i'm used to the autoconfig/automake/make tools for C/C++ projects, and to ant for Java stuff. what do haskellers use? my initial thought was using a plain Makefile with ghc, but there is also hmake and maybe other tools i am missing. what would you use? [2] ghc --make is also quite useful (the ghc documentation is at: haskell.org/ghc). - graphics toolkit. one of my programs will be a GUI. there seems to be a lot of choice here: gtk2hs looks nice (as used in hircules) and fudgets seems to have a pretty interesting and well-documented architecture (which covers also client/server programming: i'll be using a client/server architecture too) but its widgets are uglier and look alien in current linux desktops. i'd like to have a fudgets-gtk framework of sorts :) and then, there is htoolkit, Object I/O and whatnot... any recommendations? this is a tough one... i think most of the libraries have some quircks. there are a few recent attempts to get a good library going, but i haven't had a chance to try them (the latest as far as i know are wxHaskell and hToolkit). i have a bit of experience with fudgets and it works. it is however getting a bit old (hence the funny look, and programming it is sometimes a bit weird). it would be nice to have a new fudgets, but it seems like a rather large task. - TCP/IP stuff. i will have a client/server architecture with TCP/IP as the transport protocol. other than fudgets, i'm not aware of any haskell framework for IPC: am i missing something? ghc has a bunch of libraries for networking. they are a bit low level, but if you are used to writing networking applications in C, they are fine (and perhaps better). see the library documentation of ghc. - XML. i'll be handling XML documents, and HaXml seems to me an excellent candidate to cover this area, but, of course, i'm very interested in hearing of alternatives... never used any of those. -- == | Iavor S. Diatchki, Ph.D. student | | Department of Computer Science and Engineering | | School of OGI at OHSU | | http://www.cse.ogi.edu/~diatchki | == ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Database interface
Thomas L. Bevan writes: | Does anyone know if there is work being done on a standard Haskell | database interface. I suspect that there isn't. The pattern seems to be that someone gets an interface working well enough for some purposes, and perhaps shares it, but is too modest and/or busy to put it forward as a standard. For a row extraction mechanism, I'd vote for passing an extraction function (or IO action) to the main query function, like Tim Docker described last month. http://haskell.cs.yale.edu/pipermail/haskell-cafe/2003-July/004684.html This is a pretty good way to stop those nasty vague SQL row types at the Haskell border and turn them into something respectable. Perhaps it would even be worth constraining the extracted type to be in DeepSeq doquery :: (DeepSeq v) = Process - String - IO v - IO [v] so that the interface clearly may confine its attention to one row at a time per cursor. - Tom ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: Database interface
Tom Pledger writes: This is a pretty good way to stop those nasty vague SQL row types at the Haskell border and turn them into something respectable. Perhaps it would even be worth constraining the extracted type to be in DeepSeq doquery :: (DeepSeq v) = Process - String - IO v - IO [v] Can you explain what the constraint does here? Although I haven't touched the db interface in the meantime, when/if I do, I'll probably generalise the query function to do a fold, rathen than always accumulate a list: doquery :: Process - String - (a - b - IO b) - b - IO b Tim ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [String]-[[Bool]] 2
Tn X-10n [EMAIL PROTECTED] writes: change :: String - Bool change 1 = True change 0 = False conv :: [String] - [Bool]==unable to declare [[Bool]] Again: a String is a list of ? -kzm -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [String] - [[Bool]]
Tn X-10n [EMAIL PROTECTED] writes: i am new wif haskell, i would like to know how can i get a list of string and convert it to a list of a list of bool [String]- [[Bool]] What is the definition of a String? (I.e. what is a String a list of?) Write a function to convert one of these to the corresponding Bool. Then write a function that uses that function to convert a String to a list of Bool. And then, in a similar manner, apply it to a list of Strings to generate a list of lists of Bool. -kzm -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: Yet Another Monad Tutorial
-Original Message- From: Jeff Newbern [mailto:[EMAIL PROTECTED] Alistair, Hhhmmm. That is an interesting issue, and I am unsure how to treat it in the tutorial. I did attempt to explain the ability of the monad to isolate impure computations, but I think I need to make a better explanation of what an action is and how it is used. As for the question of whether using a monad in this way casts Haskell out of the ranks of the functionally pure, I don't know. This feels to me like a debate over personal interpretation, with decent arguments on both sides. I think the most accurate thing to say is that it enables Haskell to incorporate non-pure features without destroying the purity of the core of the language. The interesting point for me is: you can't escape from the IO monad, but you can from others. You can use non-IO functions from IO functions, but you can't use IO functions from non-IO functions. AFAICT, this is not true of all monads; as Wolfgang said, you can write a function that uses monadic values but returns a non-monadic (pure) value. You (sort of) explain this in the first two paragraphs of No way out, but things that might help are: - two example lists: of one-way monads (IO), and others (List, Maybe) - non-monadic function using monadic values ( http://www.nomaware.com/monads/html/listmonad.html#example ) Any suggestions for how best to present this issue to a reader who may be new to the world of FP? Should we mention it at all? I think you should mention it in a monad tutorial, hence the request. It will help (has helped) me understand a little more about monads. * The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. * ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: Database interface
-Original Message- From: Thomas L. Bevan [mailto:[EMAIL PROTECTED] This type of interface is fine as far as it goes, but there is only ever a loose coupling between the database and application. The HaskellDB work implemented a relational calculus that ensured that all queries were checked at compile time for validity against a particular database. HaskellDB doesn't actually type check against the database; it checks against types defined in a module, which is generated from the database. If you change the database, then you need to regenerate the module. I prefer the approach of passing an extraction function/IO action to the interface. This is the approach taken by HaSql, HaskellDB (I think, although it's so indirect I find it hard to follow), and Tim Docker's Sybase library, and it's the design I intend to use for my Oracle library (when I get around to it). I still want to use HaskellDB (or at least the relational calculus part of it), so I was thinking of splitting it into two pieces: a library that submits SQL queries and returns the results (database interface), and a library that constructs queries with the relational calculus and generates SQL from them. Obviously the database interface must be written for each DBMS product, while the relational calculus library ought to be independent of DBMS (although it might be wise to have SQL compatibility flags for the SQL it generates. For example: Oracle 8 (what I'm using now) only supports SQL-92). * The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. * ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Yet Another Monad Tutorial
Alistair, Thanks for your message regarding the debate about monads and purity. What do you think about these additions as a discussion of the issue? Does this address everything you think is needed? Is there a clearer way to explain it? Thanks, Jeff In the section No Way Out: -- The IO monad is a familiar example of a one-way monad in Haskell. Because you can't escape from the IO monad, it is impossible to write a function that does a computation in the IO monad but returns a non-monadic value. Not only are functions of the type IO a - a impossible to create, but any function whose result type does not contain the IO type constructor is guaranteed not to use the IO monad. Other monads, such as List and Maybe, do allow values out of the monad. So it is possible to write non-monadic functions that internally do computations in those monads. The one-way nature of the IO monad also has consequences when combining monads, a topic that is discussed in part III. -- and a little farther down: -- Some people argue that using monads to introduce non-pure features into Haskell disqualifies it from claiming to be a pure functional language. This subtle question not particularly relevant to the practical programmer is revisited in the context of the I/O monad later in the tutorial. -- Later, in the section on the I/O monad: -- In Haskell, the top-level main function must have type IO (), so that programs are typically structured at the top level as an imperative-style sequence of I/O actions and calls to functional-style code. Revisiting the debate about the purity of Haskell (in a functional sense), it is important to note that the IO monad only simulates imperative-style I/O. The functions exported from the IO module do not perform I/O themselves. They return I/O actions, which describe an I/O operation to be performed. The I/O actions are combined within the IO monad (in a purely functional manner) to create more complex I/O actions, resulting in the final I/O action that is the main value of the program. The result of the Haskell compiler is an executable function incorporating the main I/O action. Executing the program applies this ultimate I/O action to the outside world to produce a new state of the world. This occurs only once per execution of the program, and since the state of the world changes for each execution, the issue of purity is neatly side-stepped. -- -- Jeff Newbern [EMAIL PROTECTED] Nomaware, Inc. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: Yet Another Monad Tutorial
I think the crucial distinction is to think about is the difference between calculating a value (such as a character) compared to calculating an action (such as a computation that reads a character) --- one is wholly in your programming language while the other interacts with something outside your language (the I/O system). Many years ago we were all told it was a *good thing* to separate calculating values from *doing* I/O. Of course, you weren't actually *doing* any I/O when you *wrote* your program --- you were *calculating* (working out, writing...) your program. For most actions/computations, the properties you usually want is that A followed by (B followed by C) is the same as (A followed by B) followed by C and a unit (as in multiplication) or zero (as in addition) action. I guess the usefulness of this is quite subtle --- the Romans had no notation for zero and look what happened to them (although we still use Roman numerals for decoration). How do you know you have structured your program so that the preceding properties hold ? You show that you're working in the appropriate Monad --- remember, maths is the QA of programming. I suspect that tutorials should at some point mention some definitions of monads, monoids et al --- since this is where the power (sorry, QA) comes from. Phil On Tue, 12 Aug 2003, Bayley, Alistair wrote: Date: Tue, 12 Aug 2003 12:10:24 +0100 From: Bayley, Alistair [EMAIL PROTECTED] Subject: RE: Yet Another Monad Tutorial From: Wolfgang Jeltsch [mailto:[EMAIL PROTECTED] For example, the function readFile is pure. For a specific string s the expression readFile s always yields the same result: an I/O action which searches for a file named s, reads its content and takes this content as the result of the *action* (not the expression). What about getChar? This is a function which takes no arguments, yet returns a (potentially) different value each time. I know, I know: it returns an IO action which reads a single char from the terminal and returns it. Is the IO monad the only one (currently) in which you would say that it returns an action, which is then executed by the runtime system? I would have thought that monads that are not involved in IO (e.g. State) would be pure in the sense that Van Roy was thinking. You wouldn't need to describe functions in them as returning an action. * The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. * ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe Phil Molyneux email [EMAIL PROTECTED] tel work 020 8547 2000 x 5233 direct 020 8547 8233 home 020 8549 0045 Kingston Business School room 339 WWW http://www.kingston.ac.uk/~ku00597 Kingston University, Kingston Hill, Kingston upon Thames KT2 7LB, UK ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
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[newbie] UTF-8
Hi, We try to make a program which write on stdout the UTF-8 character corresponding to an input unicode value. And this under MacOS X, Debian linux, and Windows. It seems to be easy, but we can't have found anything valuable, on the net. Would you mind to help us? Niko. Ps: excuse my poor english. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Help with Exceptions on I/O
I have a program which creates textfiles out of other files. Since theprogram is runned from windows I output some text strings (Like "Filecreated succefully") and I need to stop the program before it quits so thatthe user can read the line outputted to know what went on and then he canpress ENTER to quit the program.I managed to do this fine if no error occurs but when a error occurs I amhaving problems.The code goes like thatmain :: IO()main =catch (do Do all the needed stuff here putStr "File created Successfully. Press RETURN toquit" dummy - getLine --Halts the program so the user canread the above line) putStr "Exiting now..." --needed since I can't finish ado function with the "dummy- getLine" line) (\_ - do putStr "\nFile notfound. Press RETURN (ENTER) to quit." dumb - getLine putStr "\nExiting...")So when the program runs, if the input file is found the program writessuccessfull creation of file but if the file doesn't exist, after the usergives the input filename and press enter, the program creates a new line andHalts (Probably because of the getLine function) without writing outanything, then when the user press ENTER again it writes the line at thefirst putStr (File not...), then writes the next putStr line under it(Exiting...) and exits. I don't know why it doesn't wirte the first line,halts and then when user press enter it writes the second and quits.Can anybody help me as I am not very familiar with exception and catches.Another question I have is: Is there any other function rather than getLinethat halts a program and continues when a user press any key (Instead ofENTER) and besides this is an ugly code since getLine wasn't made for thatbut I couldn't find anything else myself.Thank you in advance.Best RegardsAlex
