RE: Summary of GHC 6.6 test suite failures on MSYS/MinGW
Dont worry about Maessen_hashtab. It fails core Lint, and its on my list to look at. (Its been failing for some time) S From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Brian Smith Sent: 13 September 2006 18:23 To: glasgow-haskell-bugs@haskell.org Subject: Summary of GHC 6.6 test suite failures on MSYS/MinGW I finished re-running the test suite with all the necessary libraries installed. Here are the tests that are failing: make stage=2 TESTS=TH_exn TH_fail cabal01 cabal02 conc023 conc037 conc049 conc056 ffi012 ghcpkg04 maessen_hashtab readwrite002 tcfail115 tcfail140 utf8_002 utf8_003 utf8_004 utf8_005 Of particular concern is maessen_hashtab because it segfaults during the test. Below is a brief description of the result of each test. For many of these tests, I do not yet know enough to fix them. But, hopefully this report will be helpful to somebody else. Regards, Brian * maessen-hashtab This test segfaults, dereferencing a null pointer. This happens every way (normal,opt,optasm,prof,profasm,ghci,threaded1,threaded2) it gets run. The error message printed is too long to repeat here. I have no idea what the problem is. * utf8_002, utf8_003, utf8_004, utf8_005: These tests all fail the same way, outputing: utf8_002.hs:2:0: lexical error at end of input instead of: utf8_002.hs:2:0: lexical error (UTF-8 decoding error) * readwrite002 This program complains to stderr: readwrite002.exe: readwrite002.inout: hWaitForInput: invalid argument (Invalid argument) and to stdout: ### Caught EOF ### Caught EOF ### Caught EOF * TH_exn(normal) TH_fail(normal) These two fail because the GHC indents the pretty-printed code in the error message differently than expected. * conc023(ghci) This test only fails when run via GHCi. This test runs out of memory. The error message to stderr is: conc023: failed to create OS thread: Not enough storage is available to process this command. When I came back to the computer I saw that Windows was warning me that I was out of virtual memory and that it was increasing my page file size. I have 1GB of RAM which I think should be sufficient to run the testsuite successfully. * conc037(threaded2) This test prints the output in a different order than expected, but I don't know enough about how Control.Concurrent is supposed to work to know if the problem is in GHC or in the test. Please note that I am running the tests on a dual core (Core Duo) processor: ! newThread started ! mainThread ! newThread back again ! 1 sec later ! ! shutting down --- 1,6 ! mainThread ! newThread started ! newThread back again ! 1 sec later ! ! shutting down * ffi012(ghci) This happens only when run via GHCi. The following error message is issued instead of the expected output: During interactive linking, GHCi couldn't find the following symbol: Main_d1d4 This may be due to you not asking GHCi to load extra object files, archives or DLLs needed by your current session. Restart GHCi, specifying the missing library using the -L/path/to/object/dir and -lmissinglibname flags, or simply by naming the relevant files on the GHCi command line. Alternatively, this link failure might indicate a bug in GHCi. If you suspect the latter, please send a bug report to: glasgow-haskell-bugs@haskell.org ghcpkg04(normal) An error message different than the expected. I didn't investigate this one too closely: ! ! ghcpkg04.hs:1:0: ! Failed to load interface for `A': ! it was found in multiple packages: testpkg-1.2.3.4 newtestpkg-2.0 --- 1,4 ! ! ghcpkg04.hs:1:0: ! Failed to load interface for `A': ! Use -v to see a list of the files searched for. * tcfail115, tcfail140 The error messages output by GHC are very different than the expected ones. No clue as to why though. * cabal01, cabal02 These fail because Cabal won't work when the GHC executable is actually a shell script on Windows. * process001, process002, prog001, prog002, prog003 These fail because they depend on executables (ls, rm) that are not available on Windows. ___ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
Re: [GHC] #900: SPECIALISE broken
#900: SPECIALISE broken ---+ Reporter: simonmar | Owner: Type: bug | Status: closed Priority: normal| Milestone: 6.6 Component: Compiler |Version: 6.5 Severity: normal| Resolution: fixed Keywords:| Os: Unknown Difficulty: Unknown | Architecture: Unknown ---+ Changes (by simonpj): * resolution: = fixed * status: new = closed Comment: Fixed. Test is tc212.hs Simon -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/900 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler___ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
threadDelay not ending
I've got some unit test code that forks off test processes using the 'system' function and then delays using 'threadDelay' to synchronize with the test process. This has worked fine until I upgraded to 6.4.2, now some of the 'threadDelay' calls never return - it's like they are stuck in limbo. Any ideas? What would have changed between 6.4 and 6.4.2 that would cause this behavior? Thanks, Rich ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: threadDelay not ending
What is your environment? My project (which is about 70% Haskell) makes extensive use of threadDelay. I've not seen this behavior with 6.4.2. My environment is Linux using a recent 2.6 kernel. For obvious reasons I need to know whether there is a threadDelay issue here that is preparing to bite me. Seth Kurtzberg On Mon, September 18, 2006 7:23 am, Rich Fought wrote: I've got some unit test code that forks off test processes using the 'system' function and then delays using 'threadDelay' to synchronize with the test process. This has worked fine until I upgraded to 6.4.2, now some of the 'threadDelay' calls never return - it's like they are stuck in limbo. Any ideas? What would have changed between 6.4 and 6.4.2 that would cause this behavior? Thanks, Rich ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users -- Seth Kurtzberg [EMAIL PROTECTED] Software Engineer Specializing in Reliability and Security ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: threadDelay not ending
I am running on Red Hat Enterprise Linux with the latest RH 2.6 kernel. This is very bizarre and I am having a hard time figuring out what is going on. I don't see any issues in the project code itself, just my unit tests. Rich Seth Kurtzberg wrote: What is your environment? My project (which is about 70% Haskell) makes extensive use of threadDelay. I've not seen this behavior with 6.4.2. My environment is Linux using a recent 2.6 kernel. For obvious reasons I need to know whether there is a threadDelay issue here that is preparing to bite me. Seth Kurtzberg On Mon, September 18, 2006 7:23 am, Rich Fought wrote: I've got some unit test code that forks off test processes using the 'system' function and then delays using 'threadDelay' to synchronize with the test process. This has worked fine until I upgraded to 6.4.2, now some of the 'threadDelay' calls never return - it's like they are stuck in limbo. Any ideas? What would have changed between 6.4 and 6.4.2 that would cause this behavior? Thanks, Rich ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
RE: ANNOUNCE: GHC 6.6 Release Candidate
Excellent example. It's very hard to give good error messages for impredicative polymorphism. I've tried to improve this one a bit. (Test is tcfail165.hs) Simon | -Original Message- | From: [EMAIL PROTECTED] [mailto:glasgow-haskell-users- | [EMAIL PROTECTED] On Behalf Of Tomasz Zielonka | Sent: 01 September 2006 19:55 | To: Simon Marlow | Cc: glasgow-haskell-users@haskell.org | Subject: Re: ANNOUNCE: GHC 6.6 Release Candidate | | On Fri, Sep 01, 2006 at 11:03:09AM +0100, Simon Marlow wrote: | Please test as much as possible, bugs are much cheaper if we find them | before the release! | | I was playing with impredicativity, when I got this strange error | message: | | Prelude :l Imp | [1 of 1] Compiling Imp ( Imp.hs, interpreted ) | | Imp.hs:15:17: | Couldn't match expected type `forall a. (Show a) = a - String' |against inferred type `a - String' | Expected type: forall a1. (Show a1) = a1 - String | Inferred type: forall a1. (Show a1) = a1 - String | In the second argument of `putMVar', namely | `(show :: forall a. (Show a) = a - String)' | In the expression: | putMVar var (show :: forall a. (Show a) = a - String) | Failed, modules loaded: none. | | I am still trying to understand this extension, so my code probably | makes not much sense, but it's alarming that the compiler cannot unify | two types that are even equal. Maybe the bug is in the error message? | | Here is the code: | | module Imp where | | import Control.Concurrent | | main = do | var - newEmptyMVar :: IO (MVar (forall a. Show a = a - String)) | let thread x = do | forkIO $ sequence_ $ repeat $ do | f - takeMVar var | putStrLn (f x) | threadDelay 10 | thread (1 :: Integer) | thread abcdef | putMVar var (show :: forall a. Show a = a - String) | threadDelay 1000 | | I am using ghc-6.5.20060831 with -fglasgow-exts | | Best regards | Tomasz | ___ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: threadDelay not ending
On Mon, 18 Sep 2006 11:56:21 -0500 Rich Fought [EMAIL PROTECTED] wrote: I am running on Red Hat Enterprise Linux with the latest RH 2.6 kernel. This is very bizarre and I am having a hard time figuring out what is going on. I don't see any issues in the project code itself, just my unit tests. Rich, I'm having trouble coming up with even a credible theory of what might be happening. I can't come up with any theory, no matter how far fetched, that would account for things working with 6.4.1 and not working with 6.4.2. If there is anything I can do to help you narrow this down, let me know. Seth Rich Seth Kurtzberg wrote: What is your environment? My project (which is about 70% Haskell) makes extensive use of threadDelay. I've not seen this behavior with 6.4.2. My environment is Linux using a recent 2.6 kernel. For obvious reasons I need to know whether there is a threadDelay issue here that is preparing to bite me. Seth Kurtzberg On Mon, September 18, 2006 7:23 am, Rich Fought wrote: I've got some unit test code that forks off test processes using the 'system' function and then delays using 'threadDelay' to synchronize with the test process. This has worked fine until I upgraded to 6.4.2, now some of the 'threadDelay' calls never return - it's like they are stuck in limbo. Any ideas? What would have changed between 6.4 and 6.4.2 that would cause this behavior? Thanks, Rich ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Change Data.Bits.rotate to rotate Integer (unbounded) types
Welcome back! Since Data.Bits is not defined in the Haskell 1998 standard, are we free to change the implementation of Data.Bits? if we are free to change the implementation of Data.Bits, would it be all right to change the operation of rotate, rotateL and rotateR over unbounded types (to my knowledge, currently only Integer)? I would like to change rotate, rotateL and rotateR to actually rotate (not shift) Integers. -Pete ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
[Haskell] Haskell Weekly News: September 18, 2006
--- Haskell Weekly News http://haskell.org/haskellwiki/HWN Issue 41 - September 18, 2006 --- Welcome to issue 41 of HWN, a weekly newsletter covering developments in the Haskell community. Each week, new editions are posted to [1]the Haskell mailing list as well as to [2]the Haskell Sequence and [3]Planet Haskell. [4]RSS is also available, and headlines appear on [5]haskell.org. The 2006 Haskell Workshop was held today in Portland, Oregon. Thanks to Edward Kmett for a report on the event. 1. http://www.haskell.org/mailman/listinfo/haskell 2. http://sequence.complete.org/ 3. http://planet.haskell.org/ 4. http://sequence.complete.org/node/feed 5. http://haskell.org/ Announcements * Haskell98 Termination Analyser . Stephan Swidersk [6]announced the integration of an automatic Haskell98 termination analyzer in the termination tool AProVE. The tool accepts full Haskell as specified in the Haskell 98 Report and is available through our web interface. [7]More 6. http://article.gmane.org/gmane.comp.lang.haskell.general/14193 7. http://aprove.informatik.rwth-aachen.de/ * Free theorems . Janis Voigtlaender [8]announced that Sascha Boehme has done a project to implement the Reynolds/Wadler algorithm generating theorems from polymorphic types, plus simplifications and postprocessings for such free theorems. [9]More info 8. http://article.gmane.org/gmane.comp.lang.haskell.general/14160 9. http://haskell.as9x.info/ * Haddock/GHC SoC . David Waern [10]announced a short status report of the Port Haddock to use GHC Summer of Code project. The GHC modifications, are finished and will be included in the GHC head repository soon. 10. http://article.gmane.org/gmane.comp.lang.haskell.general/14149 * AutoForms release 0.2 . Mads Lindstr?m [11]released AutoForms 0.2, a library to ease the creation of GUIs. It does this by using generic programming (SYB) to construct GUI components. [12]More info 11. http://article.gmane.org/gmane.comp.lang.haskell.general/14148 12. http://autoforms.sourceforge.net/ * HSPClientside 0.2 . Joel Bj?rnson [13]announced a new version of HSPClientside (0.2) ,developed as a GSoC project during this summer. HSPClientside is a Haskell Server Pages library for generating JavaScript code. [14]More info 13. http://article.gmane.org/gmane.comp.lang.haskell.general/14133 14. http://darcs.haskell.org/SoC/hsp.clientside/ * SOE implementation based on Gtk2Hs . Duncan Coutts [15]Due to popular demand the new SOE implementation based on Gtk2Hs is [16]available. The rendering quality is better than the original HGL version. [17]Here's a side-by-side comparison 15. http://article.gmane.org/gmane.comp.lang.haskell.general/14132 16. file://localhost/home/dons/dons/src/hwn/now 17. http://haskell.org/~duncan/gtk2hs/SOE-cairo.png * The experimental GHCi debugger . Pepe [18]announced the results of his SoC project, the experimental Haskell debugger. [19]More details 18. http://article.gmane.org/gmane.comp.lang.haskell.general/14131 19. http://haskell.org/haskellwiki/GHC/GHCiDebugger * SmallCheck . Colin Runciman [20]released a prototype tool that is similar in spirit, and in some of its workings, to QuickCheck. SmallCheck is, though, based on exhaustive testing in a bounded space of test values. [21]More info 20. http://article.gmane.org/gmane.comp.lang.haskell.general/14129 21. http://www.cs.york.ac.uk/fp/smallcheck0.0.tar * Frisby: composable, linear time parser for arbitrary PEG grammers . John Meacham [22]released Frisby, an implementation of the 'packrat' parsing algorithm, which parse PEG grammars and have a number of very useful qualities, they are a generalization of regexes in a sense that can parse everything in LL(k), LR(k), and more, including things that require unlimited lookahead, all in guaranteed linear time. [23]More information 22. http://article.gmane.org/gmane.comp.lang.haskell.general/14128 23. http://repetae.net/computer/frisby/ * HaskellNet . Jun Mukai [24]published a status report on the state of his SoC project, HaskellNet 24. http://article.gmane.org/gmane.comp.lang.haskell.general/14126 * GHC's new support engineer . Simon Marlow [25]announced that GHC now has a full-time support engineer, Ian Lynagh (aka Igloo on IRC). He'll be helping with all aspects of GHC, especially release management, bug diagnosis and tracking, documentation, packaging, and supporting other GHC hackers. Welcome Ian! 25.
Re: [Haskell] Haskell Weekly News: September 18, 2006
On 18/09/06, Donald Bruce Stewart [EMAIL PROTECTED] wrote: * SOE implementation based on Gtk2Hs . Duncan Coutts [15]Due to popular demand the new SOE implementation based on Gtk2Hs is [16]available. The rendering quality is better than the original HGL version. [17]Here's a side-by-side comparison 15. http://article.gmane.org/gmane.comp.lang.haskell.general/14132 16. file://localhost/home/dons/dons/src/hwn/now 17. http://haskell.org/~duncan/gtk2hs/SOE-cairo.png Somewhat unsurprisingly, link 16 didn't work for me. -- -David House, [EMAIL PROTECTED] ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
[Haskell-cafe] Problems interpreting
Hi, I'm a student and I have to do a program with Haskell. This is the first
time I use this languaje, and I'm having problems with the indentation. I
want to check if this function is correct, but when I try to make the GHCi
interpret it, I get line 18:parse error (possibly incorrect indentation)
The function is:
-- Replaces a wildcard in a list with the list given as the third argument
substitute :: Eq a = a - [a] - [a] - [a]
substitute e l1 l2= [check_elem c | c - l1] where
check_elem::Eq a = a - [a]
check_elem x =
if x == e then return l2
-- Tries to match two lists. If they match, the result consists of the
sublist
-- bound to the wildcard in the pattern list.
(line 18) match :: Eq a = a - [a] - [a] - Maybe [a]
match _ _ _ = Nothing
{- TO BE WRITTEN -}
Thank you for your attention!
--
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Re: [Haskell-cafe] Problems interpreting
On Mon, Sep 18, 2006 at 02:51:34AM -0700, Carajillu wrote: Hi, I'm a student and I have to do a program with Haskell. This is the first time I use this languaje, and I'm having problems with the indentation. I want to check if this function is correct, but when I try to make the GHCi interpret it, I get line 18:parse error (possibly incorrect indentation) The function is: ... if x == e then return l2 I did not understand what you are trying to do, anyway the error message is due to the expression above, and not to indentation. The Haskell if constructions is: if ... then ... else, and else cannot be missing. Moreover, return is not a way to return a value. It is a special function that works with the monad class. Actually it is one of the two methods of the monad class, and it inserts a value into a monad. As far as your code goes, I'd suggest you to read some tutorials, like The Gentle Introduction, or Hasekll for C Programmers. Have a look here: http://haskell.org/haskellwiki/Learning_Haskell and here http://haskell.org/haskellwiki/Books_and_tutorials Hope this helps. Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
Carajillu [EMAIL PROTECTED] writes: I get line 18:parse error (possibly incorrect indentation) ..which is a bit misleading, as the problem is on the preceeding line of code. if x == e then return l2 And if x /= e? What is check_elem then?¹ -- Tries to match two lists. If they match, the result consists of the sublist -- bound to the wildcard in the pattern list. (line 18) -k ¹ If you really don't want to provide that option, you could perhaps do: check_elem x | x == e = return l2 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
On Mon, Sep 18, 2006 at 12:54:34PM +0200, Albert Crespi wrote: Thank you very much for your reply! As I said, it is my first experience with Haskell, I have been programming in Java and C for some years, and I find this language very different from them. Anyway I'll try to fix the function with the information that you gave me. Thanks again! You're welcome. By the way, this is what the comments say you are trying to do: -- Replaces a wildcard in a list with the list given as the third argument substitute :: Eq a = a - [a] - [a] - [a] substitute e l1 l2= [c | c - check_elem l1] where check_elem [] = l1 check_elem (x:xs) = if x == e then (l2 ++ xs) else check_elem xs This is the result: *Main substitute 1 [1,2,3] [] [2,3] *Main substitute 1 [1,2,3] [7,8,9] [7,8,9,2,3] *Main Have fun with Haskell. Ciao Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
Wow! I'm starting to love this languaje, and the people who uses it!:) Andrea Rossato wrote: On Mon, Sep 18, 2006 at 12:54:34PM +0200, Albert Crespi wrote: Thank you very much for your reply! As I said, it is my first experience with Haskell, I have been programming in Java and C for some years, and I find this language very different from them. Anyway I'll try to fix the function with the information that you gave me. Thanks again! You're welcome. By the way, this is what the comments say you are trying to do: -- Replaces a wildcard in a list with the list given as the third argument substitute :: Eq a = a - [a] - [a] - [a] substitute e l1 l2= [c | c - check_elem l1] where check_elem [] = l1 check_elem (x:xs) = if x == e then (l2 ++ xs) else check_elem xs This is the result: *Main substitute 1 [1,2,3] [] [2,3] *Main substitute 1 [1,2,3] [7,8,9] [7,8,9,2,3] *Main Have fun with Haskell. Ciao Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- View this message in context: http://www.nabble.com/Problems-interpreting-tf2290155.html#a6361815 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
On Mon, Sep 18, 2006 at 04:16:55AM -0700, Carajillu wrote: Wow! I'm starting to love this languaje, and the people who uses it!:) You spoke too early. My code had a bug, a huge one... this is the right one: -- Replaces a wildcard in a list with the list given as the third argument substitute :: Eq a = a - [a] - [a] - [a] substitute e l1 l2= [c | c - check_elem l1] where check_elem [] = l1 check_elem (x:xs) = if x == e then (l2 ++ xs) else [x] ++ check_elem xs Ciao, Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
Hi check_elem (x:xs) = if x == e then (l2 ++ xs) else [x] ++ check_elem xs Why not: check_elem (x:xs) = if x == e then (l2 ++ xs) else x : check_elem xs Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
On Mon, Sep 18, 2006 at 12:25:21PM +0100, Neil Mitchell wrote: Why not: check_elem (x:xs) = if x == e then (l2 ++ xs) else x : check_elem xs Thanks Thank you! Lists are my personal nightmare...;-) Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Problems interpreting
Andrea Rossato [EMAIL PROTECTED] writes: On Mon, Sep 18, 2006 at 04:16:55AM -0700, Carajillu wrote: Wow! I'm starting to love this languaje, and the people who uses it!:) You spoke too early. My code had a bug, a huge one... this is the right one: -- Replaces a wildcard in a list with the list given as the third argument substitute :: Eq a = a - [a] - [a] - [a] substitute e l1 l2= [c | c - check_elem l1] where check_elem [] = l1 check_elem (x:xs) = if x == e then (l2 ++ xs) else [x] ++ check_elem xs I think it's nicer to do it like this: substitute e l l' = concat (map subst_elem l) where subst_elem x | x == e = l' | otherwise = [x] since subst_elem has a more straightforward meaning than check_elem, and the concatenation is handled by a well known standard function. Also, it would usually be more useful to have the argument to replace /with/ before the argument to replace /in/, so that (substitute '*' wurble) is a function that replaces all the '*'s in it's argument with wurbles. And if you do that, you can write it like this: subst e l' = concat . map subst_elem where subst_elem x | x == e = l' | otherwise = [x] -- Jón Fairbairn [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why is type 'b' forced to be type 'm a' and not possibly 'm a - m a' (Anatoly Zaretsky)
Message: 6
Date: Fri, 15 Sep 2006 18:36:35 +0300
From: Anatoly Zaretsky [EMAIL PROTECTED]
Subject: Re: [Haskell-cafe] Why is type 'b' forced to be type 'm a'
and not possibly 'm a - m a'
To: Vivian McPhail [EMAIL PROTECTED]
Cc: Haskell Cafe haskell-cafe@haskell.org
Message-ID:
[EMAIL PROTECTED]
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
On 9/15/06, Vivian McPhail [EMAIL PROTECTED] wrote:
class Forkable a where
fork :: String - a - a - a
...
{-
instance (Monad m, Forkable (m a), Forkable b) = Forkable (m a - b)
where
fork n a1 a2 a = do
a' - a
fork n (a1 $ return a') (a2 $ return a')
-}
Let's do manual type checking.
First, fork :: Forkable a = String - a - a - a
So for Forkable (m a - b)
fork :: String - (m a - b) - (m a - b) - m a - b
Then
fork n a1 a2 a :: b
But you define it as
fork n a1 a2 a = do {...}
So it should be of type Monad t = t a, not just any `b'.
Instead, you can define
instance (Monad m, Forkable (m b)) = Forkable (m a - m b) where
...
Well, I can partially instantiate what I am trying to achieve by enumerating
cases. Note that when the first type is a monadic type the computation gets
evaluated and then forked, but when the first type is a function it merely
gets passed. My problem is that there are a very large number of possible
cases. So in the case Forkable (m a - b), a number of instances of which I
can implement (e.g. Forkable (m a - m a - m a), Forkable ((m a - m a) -
m a), and Forkable (m a - (m a - m a) - m a)), I don't see why 'b' should
necessarily typecheck to 't t1'.
What I would like to be able to do is differentiate between Forkable (m a -
b) and Forkable (function type - b).
By the way, the following code typechecks and runs correctly, my problem is
that enumerating all possible types requires five factorial (120) different
instances, and to a lazy functional programmer who can 'see' the pattern it
seems that there must be a nicer way of achieving my end.
\begin{code}
instance (Monad m, Forkable (m a)) = Forkable (m a - m a) where
fork n a1 a2 a = do
a' - a
fork n (a1 $ return a') (a2 $ return a')
instance (Monad m, Forkable (m a)) = Forkable (m a - m a - m a) where
fork n a1 a2 a b = do
a' - a
fork n (a1 $ return a') (a2 $ return a') b
instance (Monad m, Forkable (m a)) = Forkable ((m a - m a) - m a) where
fork n a1 a2 a = do
fork n (a1 a) (a2 a)
instance (Monad m, Forkable (m a)) = Forkable (m a - (m a - m a) - m a)
where
fork n a1 a2 a b = do
a' - a
fork n (a1 $ return a') (a2 $ return a') b
\end{code}
Note that to compile it you also need -fallow-undecidable-instances
and -fallow-overlapping-instances.
--
Tolik
Thanks for your help so far!
Cheers,
Vivian
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Re: [Haskell-cafe] Problems interpreting
Hello Carajillu,
Monday, September 18, 2006, 1:51:34 PM, you wrote:
Hi, I'm a student and I have to do a program with Haskell. This is the first
time I use this languaje, and I'm having problems with the indentation. I
want to check if this function is correct, but when I try to make the GHCi
interpret it, I get line 18:parse error (possibly incorrect indentation)
i want to add to Andrea's answer:
The function is:
-- Replaces a wildcard in a list with the list given as the third argument
substitute :: Eq a = a - [a] - [a] - [a]
substitute e l1 l2= [check_elem c | c - l1] where
check_elem::Eq a = a - [a]
check_elem x =
if x == e then return l2
Haskell functions are like mathematical ones - first, you don't need
to use 'return' to denote result - just write it. second, you should
provide results for any possible input values, otherwise function will
just fail on return values for which you don't provided result. 'if'
construct ensures this by forcing to write both 'then' and 'else'
parts. so, your function, probably, should return l2 if comparison
succeeds, and x otherwise:
check_elem x =
if x == e then l2 else x
next problem is that x and l2 had different types and type-checking
will not allow you to write this. It's right - you can't construct
list, whose some elements are, for example,
-- i've got a break to rescue a bat that was flied into our house :)
Int and some - [Int]: [1,[0,0,0],3,4] is impossible
if your need to replace all elements of l1 that are equal to e,
with _several_ list elements, i.e.
substitute 2 [1,2,3,4] [0,0,0] = [1,0,0,0,3,4]
you should use the following technique:
replace all elements of l1 with either l2 or [x] (where x is original
list element). result of such operation:
substitute1 2 [1,2,3,4] [0,0,0] = [[1],[[0,0,0],[3],[4]]
i.e. now all elements are lists, although some have just one element.
then you should use concat operation to transform this list into what
you need:
concat [[1],[[0,0,0],[3],[4]] = [1,0,0,0,3,4]
i think that you are interested to write the complete solution yourself.
just note that there is concatMap function that will allow you to
further simplify the code
-- Tries to match two lists. If they match, the result consists of the
sublist
-- bound to the wildcard in the pattern list.
(line 18) match :: Eq a = a - [a] - [a] - Maybe [a]
match _ _ _ = Nothing
{- TO BE WRITTEN -}
this definition is perfectly ok :)
--
Best regards,
Bulatmailto:[EMAIL PROTECTED]
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Re: [Haskell-cafe] Re: Problems interpreting
On Mon, Sep 18, 2006 at 12:42:59PM +0100, Jón Fairbairn wrote: And if you do that, you can write it like this: subst e l' = concat . map subst_elem where subst_elem x | x == e = l' | otherwise = [x] Pretty. Just to many keystrokes. This should take two keystrokes less, probably: subst e l [] = [] subst e l (x:xs) = if x == e then l ++ xs else x : subst e l xs ;-) andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
Finally I took Andrea's solution check_elem (x:xs) = if x == e then (l2 ++ xs) else [x] ++ check_elem xs I think it's easy to understand for me ( in my noob level), than the recursive one. I'm testing it and it's working really well. The other solutions are a little complicated for me, but I'm still trying to undestand them. Thanks! Andrea Rossato wrote: On Mon, Sep 18, 2006 at 12:25:21PM +0100, Neil Mitchell wrote: Why not: check_elem (x:xs) = if x == e then (l2 ++ xs) else x : check_elem xs Thanks Thank you! Lists are my personal nightmare...;-) Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- View this message in context: http://www.nabble.com/Problems-interpreting-tf2290155.html#a6362822 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
Not a good solution, it just substitutes the first occurrence of the item in the list. I'll try the others Carajillu wrote: Finally I took Andrea's solution check_elem (x:xs) = if x == e then (l2 ++ xs) else [x] ++ check_elem xs I think it's easy to understand for me ( in my noob level), than the recursive one. I'm testing it and it's working really well. The other solutions are a little complicated for me, but I'm still trying to undestand them. Thanks! Andrea Rossato wrote: On Mon, Sep 18, 2006 at 12:25:21PM +0100, Neil Mitchell wrote: Why not: check_elem (x:xs) = if x == e then (l2 ++ xs) else x : check_elem xs Thanks Thank you! Lists are my personal nightmare...;-) Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- View this message in context: http://www.nabble.com/Problems-interpreting-tf2290155.html#a6362912 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] Problems interpreting
Hello Andrea, Monday, September 18, 2006, 3:22:43 PM, you wrote: substitute e l1 l2= [c | c - check_elem l1] why not just substitute e l1 l2= check_elem l1 ? :) where check_elem [] = l1 should be where check_elem [] = [] otherwise you just append second (backup? :) copy of original list to result :) -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
On Mon, Sep 18, 2006 at 05:42:47AM -0700, Carajillu wrote: Not a good solution, it just substitutes the first occurrence of the item in the list. I'll try the others I did not get this point. You must take Jón's approach with map. andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
On Mon, Sep 18, 2006 at 02:52:45PM +0200, Andrea Rossato wrote: On Mon, Sep 18, 2006 at 05:42:47AM -0700, Carajillu wrote: Not a good solution, it just substitutes the first occurrence of the item in the list. I'll try the others I did not get this point. You must take Jón's approach with map. or use this line in mine: check_elem (x:xs) = if x == e then l2 ++ check_elem xs else x : check_elem xs that is to say, you must check also the tail after the matched element. andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
On Mon, Sep 18, 2006 at 05:35:47AM -0700, Carajillu wrote: I'm testing it and it's working really well. The other solutions are a little complicated for me, but I'm still trying to undestand them. Jón's approach (the last version of his message), usually cleaner and more concise, is called point-free and is quite common in functional programming. It can be a bit confusing to newcomers, though, since part of the function's arguments do not appear explicitly in the expressions' body (as the list to be matched and modified in your example). You can read something more about this style here: http://haskell.org/haskellwiki/Pointfree Hope this helps. Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
Definitely I'll take this solution, I'm reading about Pointfree, I think it's not that dificult to understand. And moreover it's the simpliest way to write code. Jón Fairbairn-2 wrote: Andrea Rossato [EMAIL PROTECTED] writes: On Mon, Sep 18, 2006 at 04:16:55AM -0700, Carajillu wrote: Wow! I'm starting to love this languaje, and the people who uses it!:) You spoke too early. My code had a bug, a huge one... this is the right one: -- Replaces a wildcard in a list with the list given as the third argument substitute :: Eq a = a - [a] - [a] - [a] substitute e l1 l2= [c | c - check_elem l1] where check_elem [] = l1 check_elem (x:xs) = if x == e then (l2 ++ xs) else [x] ++ check_elem xs I think it's nicer to do it like this: substitute e l l' = concat (map subst_elem l) where subst_elem x | x == e = l' | otherwise = [x] since subst_elem has a more straightforward meaning than check_elem, and the concatenation is handled by a well known standard function. Also, it would usually be more useful to have the argument to replace /with/ before the argument to replace /in/, so that (substitute '*' wurble) is a function that replaces all the '*'s in it's argument with wurbles. And if you do that, you can write it like this: subst e l' = concat . map subst_elem where subst_elem x | x == e = l' | otherwise = [x] -- Jón Fairbairn [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- View this message in context: http://www.nabble.com/Problems-interpreting-tf2290155.html#a6363827 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] Re: Problems interpreting
Hello Andrea, Monday, September 18, 2006, 4:23:21 PM, you wrote: subst e l' = concat . map subst_elem where subst_elem x | x == e = l' | otherwise = [x] Pretty. Just to many keystrokes. This should take two keystrokes less, probably: subst e l [] = [] subst e l (x:xs) = if x == e then l ++ xs else x : subst e l xs but the goal is not keystrokes itself but easy of understanding. for me, first solution looks rather idiomatic and intuitively understandable. second solution requires more time to got it, but seems easier for novices that are not yet captured higher-level Haskell idioms. i just want to said that it will be easier to read it if you split it into several lines: subst e l [] = [] subst e l (x:xs) = if x == e then l ++ xs else x : subst e l xs or subst e l [] = [] subst e l (x:xs) | x==e = l ++ xs | otherwise = x : subst e l xs and that your solution substitutes only first match in a list: subst 1 [1,1] [0] = [0,1] -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Problems interpreting
On Mon, Sep 18, 2006 at 04:52:33PM +0400, Bulat Ziganshin wrote: but the goal is not keystrokes itself but easy of understanding. for me, first solution looks rather idiomatic and intuitively understandable. second solution requires more time to got it, but seems easier for novices that are not yet captured higher-level Haskell idioms. I was obviously kidding, as the ;-) should have made clear. ;-) Apart for the bug (I did not understand that all the occurrences should be replaced) I wrote something that was as close as possible to Albert's first attempt. For the rest, I completely agree with you and find the second one a lot easier... Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Problems interpreting
Andrea Rossato [EMAIL PROTECTED] writes: On Mon, Sep 18, 2006 at 12:42:59PM +0100, Jón Fairbairn wrote: And if you do that, you can write it like this: subst e l' = concat . map subst_elem where subst_elem x | x == e = l' | otherwise = [x] Pretty. Just to many keystrokes. Keystrokes? Learn to touchtype! This should take two keystrokes less, probably: subst e l [] = [] subst e l (x:xs) = if x == e then l ++ xs else x : subst e l xs but if you want short, do this: subst e l' = concat.map (\x-if x==e then l' else [x]) which beats yours by twenty seven characters and one bug ;-P -- Jón Fairbairn [EMAIL PROTECTED] http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2006-09-13) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Problems interpreting
Hi subst e l' = concat.map (\x-if x==e then l' else [x]) subst e l' = concatMap (\x-if x==e then l' else [x]) Let's save an extra character :) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Problems interpreting
Hi, Am Montag, den 18.09.2006, 16:00 +0100 schrieb Neil Mitchell: subst e l' = concat.map (\x-if x==e then l' else [x]) subst e l' = concatMap (\x-if x==e then l' else [x]) Let's save an extra character :) We are talking keystrokes here, so count the shift key! Greetings, Joachim -- Joachim Breitner e-Mail: [EMAIL PROTECTED] Homepage: http://www.joachim-breitner.de ICQ#: 74513189 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Problems interpreting
Hi, On 9/18/06, Joachim Breitner [EMAIL PROTECTED] wrote: Hi, Am Montag, den 18.09.2006, 16:00 +0100 schrieb Neil Mitchell: subst e l' = concat.map (\x-if x==e then l' else [x]) subst e l' = concatMap (\x-if x==e then l' else [x]) Let's save an extra character :) We are talking keystrokes here, so count the shift key! Greetings, Joachim Sorry, couldn't resist... If we *really* talking keystrokes, it much depends on auto-completion features of your editor! :) V.Rudenko -- λ is the ultimate ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Problems interpreting
Or even shorter: subst e l = concatMap $ \x-if x==e then l else [x] I kinda like the list comprehension version too subst e l1 l2 = [ r | x - l2, r - if x==e then l1 else [x] ] On Sep 18, 2006, at 10:54 , Jón Fairbairn wrote: Andrea Rossato [EMAIL PROTECTED] writes: On Mon, Sep 18, 2006 at 12:42:59PM +0100, Jón Fairbairn wrote: And if you do that, you can write it like this: subst e l' = concat . map subst_elem where subst_elem x | x == e = l' | otherwise = [x] Pretty. Just to many keystrokes. Keystrokes? Learn to touchtype! This should take two keystrokes less, probably: subst e l [] = [] subst e l (x:xs) = if x == e then l ++ xs else x : subst e l xs but if you want short, do this: subst e l' = concat.map (\x-if x==e then l' else [x]) which beats yours by twenty seven characters and one bug ;-P -- Jón Fairbairn [EMAIL PROTECTED] http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2006-09-13) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Problems interpreting
Hi, Out of curiosity, I've been developing a tool called Dr Haskell, for a sample run: module Test where substitute1 :: Eq a = a - [a] - [a] - [a] substitute1 e l1 l2= [c | c - check_elem l1] where check_elem [] = l1 check_elem (x:xs) = if x == e then (l2 ++ xs) else [x] ++ check_elem xs substitute2 e l l' = concat (map subst_elem l) where subst_elem x | x == e = l' | otherwise = [x] subst3 e l [] = [] subst3 e l (x:xs) = if x == e then l ++ xs else x : subst3 e l xs subst4 e l' = concat.map (\x-if x==e then l' else [x]) drhaskell Test.hs I can apply Hints.concat_map in Test.subst4 I can apply Hints.concat_map in Test.substitute2 I can apply Hints.box_append in Test.Test.Prelude.200.check_elem For the curious, see the darcs repo: http://www.cs.york.ac.uk/fp/darcs/drhaskell/ (Requires Yhc) Thanks Neil PS. dons also contributed some of the earlier discussion to this tool, so deserves some credit. On 9/18/06, wld [EMAIL PROTECTED] wrote: Hi, On 9/18/06, Joachim Breitner [EMAIL PROTECTED] wrote: Hi, Am Montag, den 18.09.2006, 16:00 +0100 schrieb Neil Mitchell: subst e l' = concat.map (\x-if x==e then l' else [x]) subst e l' = concatMap (\x-if x==e then l' else [x]) Let's save an extra character :) We are talking keystrokes here, so count the shift key! Greetings, Joachim Sorry, couldn't resist... If we *really* talking keystrokes, it much depends on auto-completion features of your editor! :) V.Rudenko -- λ is the ultimate ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Problems interpreting
On 2006-09-18, Jón Fairbairn [EMAIL PROTECTED] wrote: Andrea Rossato [EMAIL PROTECTED] writes: On Mon, Sep 18, 2006 at 12:42:59PM +0100, Jón Fairbairn wrote: And if you do that, you can write it like this: subst e l' = concat . map subst_elem where subst_elem x | x == e = l' | otherwise = [x] Pretty. Just to many keystrokes. Keystrokes? Learn to touchtype! One has only a finite number of keystrokes before one's hands give out. Use them wisely. -- Aaron Denney -- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Problems interpreting
On 18/09/06, Aaron Denney [EMAIL PROTECTED] wrote: One has only a finite number of keystrokes before one's hands give out. Use them wisely. i agr rdndncy = bd shd b stmpd out wsts bndwth 2 Slightly more seriously, a few extra keystrokes can -really- improve clarity. For example, you can write English and still be understood (reasonably) well without most of the vowels. But how on Earth do you interpret ld mn gd t shp? Or any Perl/Ruby/whatever golf entry? I'd rather waste keystrokes writing clearer code than brain cycles understanding obfuscated code, personally. --Sam ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Problems interpreting
On Mon, Sep 18, 2006 at 11:04:27AM -0400, Lennart Augustsson wrote:
Or even shorter:
subst e l = concatMap $ \x-if x==e then l else [x]
I kinda like the list comprehension version too
subst e l1 l2 = [ r | x - l2, r - if x==e then l1 else [x] ]
This is the version I first wanted to (try to) implement (improvements
thanks to the thread, obviously :-):
newtype SF a b = SF { runSF :: [a] - [b] }
instance Arrow SF where
arr f = SF (map f)
SF f SF g = SF (f g)
first (SF f) = SF (unzip first f uncurry zip)
substitute e l = arr (\x-if x==e then l else [x]) SF concat
I was studying Hughes when I read the first mail of this thread. But
you can see it yourself...
Andrea
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[Haskell-cafe] darcs get-together tuesday evening (2006-09-19 18:00)
Dear darcs users, hackers and observers (*), We're getting together around 18:00 this Tuesday to discuss and/or hack on darcs. This will be after the ICFP sessions for that day, in Portland, Oregon, specifically the Marriott downtown waterfront LL1. David Roundy will be there, Ian Lynagh and me too. Hope to see you there. (*) Sorry for the spam, Haskellers. I figured we might as well try to catch ICFP bystanders. -- Eric Kow http://www.loria.fr/~kow PGP Key ID: 08AC04F9 Merci de corriger mon français. pgpMDJnheBANk.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: foreach
Brandon Moore wrote: Couldn't '\' delimit a subexpression, as parentheses do? Would there be any ambiguity in accepting code like State \s - (s, s) instead of requiring State $ \s - (s, s), or taking main = do args - getArgs foreach args \arg - do foreach [1..3] \n - do putStrLn ((show n) ++ ) ++ arg It would be a bit odd to have a kind of grouping the always starts explicitly and ends implicitly, but other than that it seems pretty handy, harmless, and natural (I know I've tried to write this sort of thing often enough) Sounds like an extremely good idea to me. Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foreach
Couldn't '\' delimit a subexpression, as parentheses do? Would there be
any ambiguity in accepting code like State \s - (s, s) instead of
requiring State $ \s - (s, s), or taking
Looking at the Haskell 98 language definition it seems that a whole
class of these expressions are disallowed inside function applications:
exp10 - \ apat1 ... apatn - exp
| let decls in exp
| if exp then exp else exp
| case exp of { alts }
| do { stmts }
| fexp
This means that none of the following are legal Haskell declarations,
even though they are unambiguous:
a = State \s - (s, s)
b = map let f x = x + 1 in f
c = return if a then b else c
d = catch do x - getLine
return x
It can be argued that this is mostly obfuscation, and that it can
sometimes be confusing, especially with let and do, but it saves on the
amount of parentheses or $s. What was the original reasoning for
disallowing these more complex expressions as the rightmost argument in
an fexp? Or was this simply not considered?
Twan
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Re: [Haskell-cafe] Re: Optimization problem
On Sun, Sep 17, 2006 at 01:08:04PM +0200, [EMAIL PROTECTED] wrote: Ross Paterson wrote: It's interesting that these composed transformations don't seem to cost too much. That the composed transformations are indeed cheap is not necessarily disturbing. I meant the composition of the transformations of the tree (update or reverse insert) that Bertram does for each list element. They're cheap because they're bounded by the depth of the tree, and even cheaper if you're probing some other part of the tree. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Optimization problem
On Mon, Sep 18, 2006 at 12:23:11AM +0100, Ross Paterson wrote: To prove that (even for partial and infinite lists ps) splitSeq ps = [(a, seconds a ps) | a - nub ps] [...] we can establish, by induction on the input list, (1) fst (splitSeq' s ps) = [(a, seconds a ps) | a - nub ps, not (member a s)] (2) member x s = get x s (snd (splitSeq' s ps)) = seconds x ps Oops, nub ps should be nub (map fst ps) in each case. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Either e Monad
Dear all, Where is the Monad instance declaration of Either e? From the description of Control.Monad.Error, I deduce that Either e is an instance of Monad. http://haskell.org/ghc/docs/latest/html/libraries/mtl/Control-Monad-Error.html class Monad m = MonadError e m | m - e where ... Error e = MonadError e (Either e) But, I cannot find the Monad instance declaration of Either anywhere. Thanks. -- Deokhwan Kim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
