- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Exceptions
by Niklas Broberg 01 Oct '04

01 Oct '04

Alastair Reid wrote: > > 3. Can I define my own exception types? > >Sadly, no. There is only one 'exception type'. > >You can use dynamics and encode new kinds of exceptions as Strings > but that isn't very satisfactory. But not at all, allowing you to declare your own exception types is *exactly* what using dynamic exceptions is all about. No need for String encodings, we can do far better than that. =) If I have my own type of exceptions, by making that type an instance of Typeable I can use the functions 'throwDyn' and 'catchDyn' instead of the normal 'throw' and 'catch'. That way I can declare exception handlers that only exceptions of a particular type, and just re-raises any other. I could for instance write main = do ... a bunch of computations ... `catchDyn` (\e :: MyExceptionType1 -> ... handle exceptions of MyExceptionType1 ...) `catchDyn` (\e :: MyExceptionType2 -> ... handle exceptions of MyExceptionType2 ...) `catch` (\e -> ... handle exceptions of the built-in Exceptions type ...) (Note that the type annotations on the patterns are not needed if the types can be inferred properly from within the handler. Note also that the Typeable constraint can be automatically derived by ghc with -fglasgow-exts, so no difficulty there either) Check the section about dynamic exceptions at http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control.Exceptio… for more info. > > 4. Can I write code that can catch and handle multiple different > > exception types from a single block of code? > >You use standard Haskell pattern matching so, yes, you can catch multiple >kinds of exceptions. They're not different types though. But yes they are, see above. Or rather, they can be, or they can be constructors of a common user-defined type, combining common pattern matching with dynamic matching on types. =) > > 5. Is there anything different about working with exceptions in monads? > >Use raiseIO instead of raise to raise exceptions. The functions for raising exceptions are called 'throw' and 'throwIO' (and 'throwDyn'), not raise. /Niklas _________________________________________________________________ The new MSN 8: advanced junk mail protection and 2 months FREE* http://join.msn.com/?page=features/junkmail

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Re: [Haskell-cafe] Exceptions
by MR K P SCHUPKE 01 Oct '04

01 Oct '04

>But, being able to see the context in which a thunk was constructed would be >extremely useful. Yes of course... I was thinking along the lines of what is possible, rather than what is desirable. If only ghc had a -g (debug) flag like gcc that would force each funtion to push its entry onto some kind of call stack that would be returned by exceptions... Keean.

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Re: [Haskell-cafe] Exceptions
by MR K P SCHUPKE 01 Oct '04

01 Oct '04

>1. Can exceptions be used in "pure" functions (outside of monads?) Exceptions can be thrown from pure functions, and can be used exactly like in the OCaml example (but in Haskell syntax) Exceptions can only be caught in the IO monad. >2. How are these exceptions caught and handled aside from using bracket? bracket, catch, catchIO >3. Can I define my own exception types? There is a User exception type, I don't think you can define your own completely new exception type. >4. Can I write code that can catch and handle multiple different >exception types from a single block of code? Yes but see (1) only in the IO monad >5. Is there anything different about working with exceptions in monads? No. >6. Can I get a stack trace from ghc or hugs if an exception is never >caught and thus causes the program to terminate? Stack traces in lazy laguages are not very useful. Best work-around is to produce an explicit call trace by inserting prints into functions... obviously this requires the functions to be in the IOMonad. Things are still odd because of lazyness though. Keean.

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Exceptions
by John Goerzen 01 Oct '04

01 Oct '04

One of the most important features of a modern language is, to me, exceptions. I have found very little coverage of exceptions in Haskell, what what coverage there was seemed to be centered on I/O. One reason I'm asking is this: what to do when pattern matching is incomplete due to an error. For instance, this is a common pattern in my OCaml code: let queuedir_of_type t = match t with "foo" -> "/data/queue5" | "bar" -> "/data/queue3" | _ -> raise (Exception ("Invalid type " ^ t));; This is often useful when "t" is something that was originally read from a user. Python has an expressive exception mechanism, where one can catch various different exceptions from a single block of code, etc. So, I have some questions about exceptions in Haskell: 1. Can exceptions be used in "pure" functions (outside of monads?) 2. How are these exceptions caught and handled aside from using bracket? 3. Can I define my own exception types? 4. Can I write code that can catch and handle multiple different exception types from a single block of code? 5. Is there anything different about working with exceptions in monads? 6. Can I get a stack trace from ghc or hugs if an exception is never caught and thus causes the program to terminate? Thanks! -- John

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Re: [Haskell-cafe] OCaml list sees abysmal Language Shootout results]
by Greg Buchholz 30 Sep '04

30 Sep '04

Sam Mason wrote: > > You probably want some strictness annotations in there. . . Now we're getting somewhere. When I replace the tuples with my own (strict) data structure, it gets about 7.5 times faster than the original shootout example (or about 24 times faster than the version with tuples). I get another 2x speedup when I pass '+RTS -G1' to the executable. So the version below is about 15 times faster than the original using the 3MB data file from the shootout. (Now we're only about 40x slower than ocaml). Don't forget to turn on '-funbox-strict-fields' for an additional improvement. -- Compile with... -- ghc -O2 -ddump-simpl -fvia-c -funbox-strict-fields -o wc_fold2 wc_fold2.hs -- execute as... ./wc_fold2 +RTS -G1 <input.txt import IO main = do file <- getContents putStrLn $ show (foldl wc (St 0 0 0) file) data Stuple = St !Int !Int !Int deriving Show wc (St l w c) '\n' = St (l+1) w (c+1) wc (St l w c) ' ' = St l (w+1) (c+1) wc (St l w c) x = St l w (c+1) ...It still seems like it's using a lot of memory (or at least doing a lot of garbage collecting). But it it still vastly better than before. Is there any way to reduce this more? (60,000,000 bytes divided by 3,000,000 chars = 20 bytes per char). Here's the memory usage report... 61,387,752 bytes allocated in the heap 11,837,148 bytes copied during GC 99,780 bytes maximum residency (234 sample(s)) 234 collections in generation 0 ( 0.11s) 1 Mb total memory in use INIT time 0.00s ( 0.00s elapsed) MUT time 0.15s ( 0.15s elapsed) GC time 0.11s ( 0.14s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 0.26s ( 0.29s elapsed) %GC time 42.3% (48.3% elapsed) Alloc rate 409,251,680 bytes per MUT second Productivity 57.7% of total user, 51.7% of total elapsed Greg Buchholz

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RE: Simple example using Parsec (was: [Haskell-cafe] Rethinking O O idioms)
by Bayley, Alistair 30 Sep '04

30 Sep '04

Sure. Learning Parsec took me a few hours, whereas John's parsing task might be done in much less time, if it's simple enough. OTOH, there's value in being able to use Parsec, so it's a good excuse to learn. > -----Original Message----- > From: Graham Klyne [mailto:GK@ninebynine.org] > Sent: 30 September 2004 10:34 > To: Bayley, Alistair; 'John Goerzen'; haskell-cafe(a)haskell.org > Subject: Simple example using Parsec (was: [Haskell-cafe] > Rethinking OO idioms) > > At 09:01 30/09/04 +0100, Bayley, Alistair wrote: > >Ben suggested using Parsec for reading the config file. > Parsec is a fine > >library, but there is a learning curve, and you might find > it quicker to do > >the parsing yourself, if it's very simple. Your call. > > Hmmm... Parsec *can* be simple too. Here's one I did earlier: > http://www.ninebynine.org/Software/HaskellUtils/RegexParser.hs > > #g ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Simple example using Parsec (was: [Haskell-cafe] Rethinking OO idioms)
by Graham Klyne 30 Sep '04

30 Sep '04

At 09:01 30/09/04 +0100, Bayley, Alistair wrote: >Ben suggested using Parsec for reading the config file. Parsec is a fine >library, but there is a learning curve, and you might find it quicker to do >the parsing yourself, if it's very simple. Your call. Hmmm... Parsec *can* be simple too. Here's one I did earlier: http://www.ninebynine.org/Software/HaskellUtils/RegexParser.hs #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Seeking reference(s) relating to FP performance
by Graham Klyne 30 Sep '04

30 Sep '04

I've taken it as an article of faith that performance of FP language implementations has been improving quite steadily over the past few years. I'd like to assert this, but I can't find any clear evidence to support such an assertion. I note that the "about Haskell" page makes a similar assertion, but it doesn't offer any hint of supporting evidence: [[ Aren't functional programs very slow? They used to be, but the compilers have now caught up. Haskell programs run fast enough for all but the most performance-demanding applications. ]] -- http://www.haskell.org/aboutHaskell.html I'm looking for a reference -- informal will be enough -- that can give an perspective of progress in functional language implementation performance. I'm not looking for a single benchmark that shows a case of blindingly-fast functional code, but a pointer to trends of improving performance. It would also serve my purpose to have indications based on languages other than Haskell (e.g. ML and friends). Any ideas, please? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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RE: [Haskell-cafe] Rethinking OO idioms
by Bayley, Alistair 30 Sep '04

30 Sep '04

There are three pieces to this story: - read a config file into some structure - query/modify elements of that structure - write structure out to a file Create a data structure (analogous to a class with no methods) and functions to query/manipulate that structure. Nested FiniteMaps might be a good, simple, first attempt. Unlike the functions to read and write the file (which must be in the IO monad), the functions to query and manipulate the structure can (and probably should) be non-monadic. Ben suggested using Parsec for reading the config file. Parsec is a fine library, but there is a learning curve, and you might find it quicker to do the parsing yourself, if it's very simple. Your call. > ... Haskell has no mutable variables. A call like > config.setOption("main", "initpath", "/usr") in Python -- which alters > the state of the config object and returns nothing -- would be > impossible in Haskell (unless perhaps the FiniteMaps are mutable > somehow?) When you "modify" a structure, you return a new copy of it, where the bit that you wanted to change is replaced by the new value. For example, this is what happens when you call addToFM/delFromFM for FiniteMaps. This isn't necessarily as expensive as it sounds, as there's a lot of data sharing going on in Haskell programs, due to the fact that everything /is/ immutable. If it does turn out to be too expensive, then you could look at using a state monad. But assuming you need one immediately looks like premature optimisation to me. Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Rethinking OO idioms
by John Goerzen 30 Sep '04

30 Sep '04

I've worked with languages with object-oriented features for awhile now. Python and OCaml, the two with which I work the most, both have OO. One of my first projects in Haskell would be to write a Haskell version of Python's ConfigParser[1] class. I wrote[2],[3] a version of this for OCaml that works very well. In a nutshell, ConfigParser is a utility for working with sectioned configuration files in a style similar to the familiar .ini files from Windows. It has methods to read a configuration file, get/set the items that are being configured, and write a new file back out. This, then, is a fairly typical metaphor for OO programs: an instance of a class has some state that can be accessed or modified, and possibly stored and retrieved. So I am thinking about a ConfigParser for Haskell. The first thing that occured to me is that Haskell has no OO features, so I'm not sure what is the best way to handle the "class" and its various methods. The next thing that occured to me is that, unlike OCaml and Python classes, Haskell has no mutable variables. A call like config.setOption("main", "initpath", "/usr") in Python -- which alters the state of the config object and returns nothing -- would be impossible in Haskell (unless perhaps the FiniteMaps are mutable somehow?) I guess I'm having trouble translating this common OO language paradigm into the Haskell world. Thanks for any insight. -- John BTW, if I get a working ConfigParser for Haskell, I will publish it under the GPL like all the rest of my code. [1] http://www.python.org/doc/current/lib/RawConfigParser-objects.html [2] http://gopher.quux.org:70/devel/missinglib/html/ConfigParser.html [3] http://gopher.quux.org:70/devel/missinglib/html/ConfigParser.rawConfigParse…

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