August 2003 - Haskell-Cafe

AW: Yet Another Monad Tutorial
by Markus.Schnell＠infineon.com 13 Aug '03

13 Aug '03

> 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

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Database interface
by Thomas L. Bevan 13 Aug '03

13 Aug '03

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Does anyone know if there is work being done on a standard Haskell database interface. I was very impressed with HaskellDB but I understand that it uses Haskell extensions not present in GHC. Tom -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.2 (GNU/Linux) iD8DBQE/OhsFYha8TWXIQwoRAhkUAKDE/z2fepe7565J5+9JmMGZxpmQvwCgre/R JokIUilP8aF37d764FYFZbY= =Zlx6 -----END PGP SIGNATURE-----

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Help with Exceptions on I/O
by Alexandre Weffort Thenorio 12 Aug '03

12 Aug '03

I have a program which creates textfiles out of other files. Since the program is runned from windows I output some text strings (Like "File created succefully") and I need to stop the program before it quits so that the user can read the line outputted to know what went on and then he can press ENTER to quit the program. I managed to do this fine if no error occurs but when a error occurs I am having problems. The code goes like that main :: IO() main =catch (do Do all the needed stuff here putStr "File created Successfully. Press RETURN to quit" dummy <- getLine --Halts the program so the user can read the above line) putStr "Exiting now..." --needed since I can't finish a do function with the "dummy<- getLine" line) (\_ -> do putStr "\nFile not found. Press RETURN (ENTER) to quit." dumb <- getLine putStr "\nExiting...") So when the program runs, if the input file is found the program writes successfull creation of file but if the file doesn't exist, after the user gives the input filename and press enter, the program creates a new line and Halts (Probably because of the getLine function) without writing out anything, then when the user press ENTER again it writes the line at the first 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 getLine that halts a program and continues when a user press any key (Instead of ENTER) and besides this is an ugly code since getLine wasn't made for that but I couldn't find anything else myself. Thank you in advance. Best Regards Alex

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RE: .. Drowning in stateful, side-effect riddled, spaghetti code? We have the answer. fdps[]c.
by Bayley, Alistair 12 Aug '03

12 Aug '03

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RE: Yet Another Monad Tutorial
by Bayley, Alistair 12 Aug '03

12 Aug '03

> From: Wolfgang Jeltsch [mailto:wolfgang@jeltsch.net] > > 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. *****************************************************************

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RE: Yet Another Monad Tutorial
by Bayley, Alistair 12 Aug '03

12 Aug '03

> -----Original Message----- > From: Jeff Newbern [mailto:jnewbern@nomaware.com] > > 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. *****************************************************************

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RE: Yet Another Monad Tutorial
by Jeff Newbern 12 Aug '03

12 Aug '03

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 <jnewbern(a)nomaware.com> Nomaware, Inc.

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Re: Yet Another Monad Tutorial
by Jeff Newbern 12 Aug '03

12 Aug '03

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 jnewbern(a)yahoo.com __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com

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RE: Yet Another Monad Tutorial
by Bayley, Alistair 12 Aug '03

12 Aug '03

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 I can't decide for myself if he's right or not. It seems to me that monadic IO functions aren't pure, but that doesn't mean that *all* monadic functions are impure, does it? Or could you argue that monadic IO functions are pure: it's just that their input includes the outside world, which means that you can never invoke an IO function more than once with the same arguments? ----------------------------------------------------------------------- > Care to explain why you think Haskell is not pure? Purely functional = you are always defining and calling pure functions. Calling a pure function with the same arguments always gives the same results. In Haskell, if monadic state is used, then this is not true (there is support from the syntax + type system to hide the state argument). So Haskell's monadic state is an implementation of state *on top of* a purely functional language. Saying that Haskell is 'purely functional' is reasonable if you want to present it to non-functional programmers. But the reality is not that simple. It's perhaps better to say that Haskell allows a clear separation of the purely functional core from the rest, which is a Good Thing. It's like saying that Erlang is a functional language. It's true that Erlang has a functional core. But the concurrent message passing built on top is very important and definitely not functional! Peter > -----Original Message----- > From: Jeff Newbern [mailto:jnewbern@yahoo.com] > Sent: 12 August 2003 10:41 > To: haskell-cafe(a)haskell.org > Subject: Yet Another Monad Tutorial > > > Hello everyone, > > Due to the scarcity of monad tutorials available (:^), I have > written one of my own. I hope that this one is both more gentle > and more comprehensive than many of the other tutorials out on > the 'net, and I am looking for feedback from experienced > Haskellers and novices alike. > > You can find the tutorial at > > http://www.nomaware.com/monads/html/ > > The tutorial begins from first principles and introduces the monad > concept, details the implementation and usage of all of the standard > monads in Haskell and the monad template library and explains the > use of monad transformers. The tutorial contains 23 individual > sections and 18 files of example code illustrating different monad > concepts. > > I hope that this tutorial is useful to many people, and I would > love to hear your experiences and suggestions for improving it. > > Thanks, > Jeff Newbern > jnewbern(a)yahoo.com ***************************************************************** 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. *****************************************************************

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Yet Another Monad Tutorial
by Jeff Newbern 12 Aug '03

12 Aug '03

Hello everyone, Due to the scarcity of monad tutorials available (:^), I have written one of my own. I hope that this one is both more gentle and more comprehensive than many of the other tutorials out on the 'net, and I am looking for feedback from experienced Haskellers and novices alike. You can find the tutorial at http://www.nomaware.com/monads/html/ The tutorial begins from first principles and introduces the monad concept, details the implementation and usage of all of the standard monads in Haskell and the monad template library and explains the use of monad transformers. The tutorial contains 23 individual sections and 18 files of example code illustrating different monad concepts. I hope that this tutorial is useful to many people, and I would love to hear your experiences and suggestions for improving it. Thanks, Jeff Newbern jnewbern(a)yahoo.com __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com

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