[Haskell-cafe] Simulating OO programming with type classes; writing a factory fu nction
Alistair Bayley wrote: There's a small problem: how to write a factory function that returns values of various subtypes. The makeSubType function below won't compile, obviously because the returns types are different (they're not the same 'm'). Indeed, expressions in both branches of an `if' statement if s == SubBase1 then SubBase1 3 else SubBase2 (SubBase1 4) must be of the same type. If we had intersection types (I'm not complaining!), the compiler would have derived the intersection by itself. As things are now, we have to make the intersection manually: we have to abstract away irrelevant pieces. Expressions `SubBase1 3' and `SubBase2 (SubBase1 4)' have in common the fact that both have types that are instances of a Method class. So, we have to write that common piece of information explicitly. There are two ways of doing this, which can be called direct style and CPS style. In direct style, we do data WM = forall m. Method m = WM m makeSubType1 :: String - WM makeSubType1 s = if s == SubBase1 then WM $ SubBase1 3 else WM $ SubBase2 (SubBase1 4) test1 = let foo x = case x of WM y - method2 y in map (foo . makeSubType1) [SubBase1, SubBase2] The CPS style is just the inverse: -- Higher-ranked type: signature is required! makeSubType2:: (forall m. Method m = m - w) - String - w makeSubType2 consumer s = if s == SubBase1 then consumer $ SubBase1 3 else consumer $ SubBase2 (SubBase1 4) test2 = let foo x = method2 x in map (makeSubType2 foo) [SubBase1, SubBase2] The CPS style involves less tagging (no need to add and remove the tag WM). Also, the CPS style is more general: makeSubType1' s = makeSubType2 WM s test3 = let foo x = case x of WM y - method2 y in map (foo . makeSubType1') [SubBase1, SubBase2] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Space questions about intern and sets
Gracjan Polak [EMAIL PROTECTED] writes: intern :: Ord a = a - a intern x = unsafePerformIO $ internIO x iorefset :: Ord a = IORef(Map.Map a a) iorefset = unsafePerformIO $ do newIORef $ Map.empty It will not work because you can't put values of different types as keys of the same dictionary, as you can't compare them. -- __( Marcin Kowalczyk \__/ [EMAIL PROTECTED] ^^ http://qrnik.knm.org.pl/~qrczak/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Type extensions
Hi, I was wondering if I hat missed something and it was possible to do this within the Haskell type system or not... Essentially I would like some sort of inderritance property for Haskell types, I often find myself wanting to for example extend a tree with black/white colouring, or later extend the tree with some sort of ID, etc. So I would eno up with three data types data MyTree = Branch MyTree MyTree | Leaf type BwTag = Bool data MyBwTree = Branch BwTag MyBwTree MyBwTree | Node BwTag data MyBwTaggedTree = Branch BwTag Int MyBwTaggedTree MyBwTaggedTree | Node BwTag Int and several functions to move from one to another. (Or define the most complex and not always use all the attrdbutes). What I would prefer is to be able to spocify something like: data MyTree = Branch MyTree MyTree | Leaf type BwTag = Bool data MyBwTree extends MyTree with BwTag=True data MyBwTaggedTree extends MyBwTree with Int=0 Specifying that myBwTree is the same as MyTree but with an extra argument on each node, and to generate functions to translate between them that fill in True as a default value for the extra tag. Is this possible? Thanks Tom Davie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type extensions
On Wed, 1 Jun 2005, Thomas Davie wrote: Hi, I was wondering if I hat missed something and it was possible to do this within the Haskell type system or not... Essentially I would like some sort of inderritance property for Haskell types, I often find myself wanting to for example extend a tree with black/white colouring, or later extend the tree with some sort of ID, etc. So I would eno up with three data types data MyTree = Branch MyTree MyTree | Leaf type BwTag = Bool data MyBwTree = Branch BwTag MyBwTree MyBwTree | Node BwTag What about data MyTree a = Branch a (MyTree a) (MyTree a) | Node a and the types MyTree () MyTree Bool MyTree (Bool, Int) ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type extensions
On 1 Jun 2005, at 15:54, Henning Thielemann wrote: On Wed, 1 Jun 2005, Thomas Davie wrote: Hi, I was wondering if I hat missed something and it was possible to do this within the Haskell type system or not... Essentially I would like some sort of inderritance property for Haskell types, I often find myself wanting to for example extend a tree with black/white colouring, or later extend the tree with some sort of ID, etc. So I would eno up with three data types data MyTree = Branch MyTree MyTree | Leaf type BwTag = Bool data MyBwTree = Branch BwTag MyBwTree MyBwTree | Node BwTag What about data MyTree a = Branch a (MyTree a) (MyTree a) | Node a and the types MyTree () MyTree Bool MyTree (Bool, Int) ? That's exactly what I would normally do, but my data type is in a library and is not parameterised. Tom Davie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type extensions
On Wed, 1 Jun 2005, Thomas Davie wrote: On 1 Jun 2005, at 15:54, Henning Thielemann wrote: What about data MyTree a = Branch a (MyTree a) (MyTree a) | Node a and the types MyTree () MyTree Bool MyTree (Bool, Int) ? That's exactly what I would normally do, but my data type is in a library and is not parameterised. Then it is the wrong library. :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Wash broken with GHC 6.4
Hi, I'm trying to use Wash with GHC 6.4. I have applied the patch from http://article.gmane.org/gmane.comp.lang.haskell.libraries/3160, and now it at least compiles. However, despite using -package WASH -package WASH-CGI -package WASHHTML, ghc is not automatically sending the input files through wash2hs. GHC 6.2 did this. Incidentally, is Wash dead upstream? I am surprised that there is no upstream version yet that even tries to work with GHC 6.4. -- John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] CGI module almost useless
My apologies if this sounds like a bit of a rant; I know people put good effort into this, but The Network.CGI module in fptools (and GHC) is not very useful. I think that it should be removed or re-tooled. Here are the main problems with it: 1. It does not permit custom generation of output headers. Thus the CGI script cannot do things like set cookies, manage HTTP auth, etc. 2. It does not permit generation of anything other than text/html documents. Many CGI scripts are used to manage other types of documents. Notably this makes it incompatible with serving up even basic things like stylesheets and JPEGs. 3. It does not permit the use of any custom design to serve up HTML, forcing *everything* to go through Text.Html. This makes it impossible to do things like serving up HTML files from disk. 4. There is documentation in the code, but it is as comments only, and doesn't show up in the Haddock-generated GHC library reference. (Should be an easy fix) 5. It does not appear to support file uploads in any sane fashion Is there a better CGI module out there somewhere that I'm missing, or should I just set about writing my own? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] CGI module almost useless
Hello, I have done all of those things in WASH. But, don't let that stop you from writing something better :) I think some people started a project to write a CGI interface based on a 'Category' -- where a 'Category' is like an 'Arrow' without the 'pure/arr' function... Jeremy Shaw. At Wed, 1 Jun 2005 17:44:38 + (UTC), John Goerzen wrote: My apologies if this sounds like a bit of a rant; I know people put good effort into this, but The Network.CGI module in fptools (and GHC) is not very useful. I think that it should be removed or re-tooled. Here are the main problems with it: 1. It does not permit custom generation of output headers. Thus the CGI script cannot do things like set cookies, manage HTTP auth, etc. 2. It does not permit generation of anything other than text/html documents. Many CGI scripts are used to manage other types of documents. Notably this makes it incompatible with serving up even basic things like stylesheets and JPEGs. 3. It does not permit the use of any custom design to serve up HTML, forcing *everything* to go through Text.Html. This makes it impossible to do things like serving up HTML files from disk. 4. There is documentation in the code, but it is as comments only, and doesn't show up in the Haddock-generated GHC library reference. (Should be an easy fix) 5. It does not appear to support file uploads in any sane fashion Is there a better CGI module out there somewhere that I'm missing, or should I just set about writing my own? ___ 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: Python
Hello Stijn,
Tuesday, May 31, 2005, 2:36:07 PM, you wrote:
SDS Yes. Many people I know state the functional part as a big reason why
SDS they chose python.
are you know Ruby? :) it has all the same stuff, including code
blocks, which can refer to variables from outer context
for example, arr.inject { |sum,a| sum+a } will count sum of all
elements in array arr
--
Best regards,
Bulatmailto:[EMAIL PROTECTED]
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Re: [Haskell-cafe] CGI module almost useless
On Wed, Jun 01, 2005 at 10:54:54AM -0700, Jeremy Shaw wrote: Hello, I have done all of those things in WASH. But, don't let that stop you from writing something better :) I think some people started a project to write a CGI interface based on a 'Category' -- where a 'Category' is like an 'Arrow' without the 'pure/arr' function... From what I can tell, there are two problems with WASH: 1) Everything must be done the WASH way 2) WASH is mostly broken with GHC 6.4 Let me elaborate a bit on #1. Let's say I have a CGI interface pre-defined; I take certain parameters from a GET request and do certain things, generating certain headers no the result. WASH is all centered around generating its own forms, naming its own fields, passing around its own state. It's not at all clear how to handle this myself, as I would with Perl or PHP. If there is a way, please explain it and I'll be happy :-) I think that the GHC 6.4 brokenness is fixable, but it makes me nervous that upstream has said nothing about it to date. -- John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] CGI module almost useless
Shameless plug warning. From what I can tell, there are two problems with WASH: 1) Everything must be done the WASH way 2) WASH is mostly broken with GHC 6.4 Let me elaborate a bit on #1. Let's say I have a CGI interface pre-defined; I take certain parameters from a GET request and do certain things, generating certain headers no the result. WASH is all centered around generating its own forms, naming its own fields, passing around its own state. It's not at all clear how to handle this myself, as I would with Perl or PHP. If there is a way, please explain it and I'll be happy :-) Perhaps you want to have a look at HSP? :-) It doesn't solve all your problems though; it still requires the output to be text/html (or text/xhtml rather), but hopefullly a future incarnation will be more general. Anyway, you can get it from http://www.cs.chalmers.se/~d00nibro/hsp Feedback greatly appreciated. :-) /Niklas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] CGI module almost useless
I'd like to hear more about people using Arrows-minus-arr, as I ran into the same in a project I'm working on for interactive construction of GUI-wrapped functional values code. - Conal -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jeremy Shaw Sent: Wednesday, June 01, 2005 10:55 AM To: John Goerzen Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] CGI module almost useless Hello, I have done all of those things in WASH. But, don't let that stop you from writing something better :) I think some people started a project to write a CGI interface based on a 'Category' -- where a 'Category' is like an 'Arrow' without the 'pure/arr' function... Jeremy Shaw. At Wed, 1 Jun 2005 17:44:38 + (UTC), John Goerzen wrote: My apologies if this sounds like a bit of a rant; I know people put good effort into this, but The Network.CGI module in fptools (and GHC) is not very useful. I think that it should be removed or re-tooled. Here are the main problems with it: 1. It does not permit custom generation of output headers. Thus the CGI script cannot do things like set cookies, manage HTTP auth, etc. 2. It does not permit generation of anything other than text/html documents. Many CGI scripts are used to manage other types of documents. Notably this makes it incompatible with serving up even basic things like stylesheets and JPEGs. 3. It does not permit the use of any custom design to serve up HTML, forcing *everything* to go through Text.Html. This makes it impossible to do things like serving up HTML files from disk. 4. There is documentation in the code, but it is as comments only, and doesn't show up in the Haddock-generated GHC library reference. (Should be an easy fix) 5. It does not appear to support file uploads in any sane fashion Is there a better CGI module out there somewhere that I'm missing, or should I just set about writing my own? ___ 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 mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] G machine in FORTH
Hi - Brace yourself... I work in an environment where FORTH is still used. I've been thinking about writing a G-machine interpreter in FORTH so that one could write Haskell like programs that would compile down and run graph-reduction style on the FORTH machine. Many developers think FORTH is nice, but the language is so, shall we say, terse. I'm curious about what people think about this; having the expressiveness of a Haskell-like language that compiles to this environment might provide the best of both worlds, simple hardware architecture and an advanced programming language. let me know what you think, -andrew ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] G machine in FORTH
Andrew Harris writes: Brace yourself... I work in an environment where FORTH is still used. I've been thinking about writing a G-machine interpreter in FORTH so that one could write Haskell like programs that would compile down and run graph-reduction style on the FORTH machine. let me know what you think, Nice. Just two questions. Does your FORTH has a decent memory management system, permitting to implement GC? Do you have some idea how to modify the standard threading execution of FORTH in order to have laziness? Jerzy Karczmarczuk ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] G machine in FORTH
Actually I am very impressed from the FORTH simplicity and efficiency. I was developing one FORTH system for DOS a couple of years ago and I think it is very useful for small programs that have to perform low level/hardware tasks. Unfortunatelly it doesn't scale well for larger applications. I don't know how well it can be used as G-machine interpreter but its execution model has a lot of common with STG machine. In STG each closure has a header word which is a pointer to its entry code. In FORTH each procedure has a header word which also points to some entry code. The entry code receives the address of the entered procedure in special (usually EBX on x86) register just like in STG (where the used register is ESI). The execution of any FORTH program is a sequence of jumps from one entry code to another just like in STG. Of course STG is much more complicated because the closures are dynamically allocated and there is a garbage collector too. Thanks to its simple execution model I still think that FORTH is the fastest interpreted language, just like STG is the fastest virtual machine for Haskell. Cheers, Krasimir On 6/1/05, Andrew Harris [EMAIL PROTECTED] wrote: Hi - Brace yourself... I work in an environment where FORTH is still used. I've been thinking about writing a G-machine interpreter in FORTH so that one could write Haskell like programs that would compile down and run graph-reduction style on the FORTH machine. Many developers think FORTH is nice, but the language is so, shall we say, terse. I'm curious about what people think about this; having the expressiveness of a Haskell-like language that compiles to this environment might provide the best of both worlds, simple hardware architecture and an advanced programming language. let me know what you think, -andrew ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] G machine in FORTH
I always thought Forth was way cool, but I've never managed to get anything significant written in it. I think that Forth has echoes of the point-free style in Haskell, but Haskell is a lot friendlier. Is the Forth environment part of the hardware? If your Forth is just a threaded interpreter written in software then it seems wasteful to compile Haskell down to an interpreted environment. If it's part of the hardware then I think it would be (at the very least) an interesting exercise. Mike Date: Wed, 1 Jun 2005 17:25:24 -0400 From: Andrew Harris [EMAIL PROTECTED] Reply-To: Andrew Harris [EMAIL PROTECTED] Hi - Brace yourself... I work in an environment where FORTH is still used. I've been thinking about writing a G-machine interpreter in FORTH so that one could write Haskell like programs that would compile down and run graph-reduction style on the FORTH machine. Many developers think FORTH is nice, but the language is so, shall we say, terse. I'm curious about what people think about this; having the expressiveness of a Haskell-like language that compiles to this environment might provide the best of both worlds, simple hardware architecture and an advanced programming language. let me know what you think, -andrew ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type extensions
G'day all. Quoting Thomas Davie [EMAIL PROTECTED]: Essentially I would like some sort of inderritance property for Haskell types, I often find myself wanting to for example extend a tree with black/white colouring, or later extend the tree with some sort of ID, etc. Have you had a look at these? http://haskell.org/hawiki?IndirectComposite http://haskell.org/hawiki?DecoratingStructures Cheers, Andrew Bromage ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] G machine in FORTH
Andrew Harris wrote: I've been thinking about writing a G-machine interpreter in FORTH so that one could write Haskell like programs that would compile down and run graph-reduction style on the FORTH machine. Many developers think FORTH is nice, but the language is so, shall we say, terse. I'm curious about what people think about this; having the expressiveness of a Haskell-like language that compiles to this environment might provide the best of both worlds, simple hardware architecture and an advanced programming language. IMHO Koopman's TIGRE (http://www.ece.cmu.edu/~koopman/tigre/) might be a close approximation of what you want. I had some (rather non-serious) experience with Forth, and I had great pleasure reading Koopman's paper on TIGRE. Unfortunately I couldn't find any further development on this topic (although I saw some later Koopman's publications on Forth itself). I guess, only someone thinking Forth (as in the Brodie's book) might develop such a thing. Dimitry Golubovsky Middletown, CT ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
