Re: [Haskell-cafe] Category theory monad <----> Haskell monad
The explanation given below might be a bit heavy for someone who didn't know much about category theory. For those individuals I'd recommend Phil Wadler's papers: http://homepages.inf.ed.ac.uk/wadler/topics/monads.html I especially recommend "Monads for Functional Programming", "The Essence of Functional Programming" and "Comprehending Monads". Basically, though, the Haskell implementation _is_ the category theoretic definition of monad, with bind/return used instead of (f)map/join/return as described below. Mike > Date: Thu, 18 Aug 2005 20:39:37 -0400 > From: Cale Gibbard <[EMAIL PROTECTED]> > Cc: haskell-cafe@haskell.org > > On 14/08/05, Carl Marks <[EMAIL PROTECTED]> wrote: > > Is there any text/article which makes precise/rigorous/explicit the > > connection > > between the category theoretic definition of monad with the haskell > > implementation? > > Well, a monad over a category C is an endofunctor T on C, together > with a pair of natural transformations eta: 1 -> T, and mu: T^2 -> T > such that > 1) mu . (mu . T) = mu . (T . mu) > 2) mu . (T . eta) = mu . (eta . T) = id_C > > In Haskell, a monad is an endofunctor on the category of all Haskell > types and Haskell functions between them. Application of the > endofunctor to an object is given by applying a type constructor (the > one which is made an instance of the Monad class). Application of the > endofunctor to a function is carried out by fmap or liftM. The natural > transformation eta is called return, and mu is called join (found in > the Monad library). > > Haskell uses a somewhat different (but equivalent) basis for a monad, > in that it is not map, return, and join which need defining to make a > type an instance of the Monad class, but return and (>>=), called > "bind" or "extend". > > One can define bind in terms of fmap, and join as > x >>= f = join (fmap f x) > > and one can get back join and fmap from return and bind: > join x = x >>= id > fmap f x = x >>= (return . f) > > hope this helps, > - Cale > ___ > 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] static typing and interactivity
On Thu, 2005-08-18 at 15:17 +0200, Ketil Malde wrote: > Hi, > > One slight annoyance using Haskell is the inability to load modules > with type problems in the interactive environment (i.e. GHCi). When I > have a type error, it would be nice to have an interactive way to > explore what the compiler thinks about the types involved -- as it is, > I have to resort to adding type signatures more or less at random to > narrow down the problem. > > I'm not sure if it is technically feasible to (partially) load a > module with definitions that fail type checking, but if it were, I > thing it would make developing Haskell programs even nicer :-) You might be interested in Chameleon: http://www.comp.nus.edu.sg/~sulzmann/chameleon/ especially the type debugger. Bernie. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Category theory monad <----> Haskell monad
On 14/08/05, Carl Marks <[EMAIL PROTECTED]> wrote: > Is there any text/article which makes precise/rigorous/explicit the connection > between the category theoretic definition of monad with the haskell > implementation? Well, a monad over a category C is an endofunctor T on C, together with a pair of natural transformations eta: 1 -> T, and mu: T^2 -> T such that 1) mu . (mu . T) = mu . (T . mu) 2) mu . (T . eta) = mu . (eta . T) = id_C In Haskell, a monad is an endofunctor on the category of all Haskell types and Haskell functions between them. Application of the endofunctor to an object is given by applying a type constructor (the one which is made an instance of the Monad class). Application of the endofunctor to a function is carried out by fmap or liftM. The natural transformation eta is called return, and mu is called join (found in the Monad library). Haskell uses a somewhat different (but equivalent) basis for a monad, in that it is not map, return, and join which need defining to make a type an instance of the Monad class, but return and (>>=), called "bind" or "extend". One can define bind in terms of fmap, and join as x >>= f = join (fmap f x) and one can get back join and fmap from return and bind: join x = x >>= id fmap f x = x >>= (return . f) hope this helps, - Cale ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Category theory monad <----> Haskell monad
Is there any text/article which makes precise/rigorous/explicit the connection between the category theoretic definition of monad with the haskell implementation? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Oracle + Haskell advice?
2005/8/18, John Goerzen <[EMAIL PROTECTED]>: > I would suspect that you could find Oracle drivers for either unixODBC, > or some custom ODBC system that is nearly API-compatible. Once you have > that, you have HSQL. I am planing to add support for Oracle in HSQL since now I am using it at office, but I will do that when I have enough time to hack with it. If there is any volunteer to do the job then I will be pretty happy to incorporate its work. Cheers, Krasimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Oracle + Haskell advice?
On 2005-08-18, Brian Strand <[EMAIL PROTECTED]> wrote: > I'm thinking about (re)writing some perl code in Haskell (for performance and > correctness reasons). Has anyone done much with Oracle and Haskell? So far Not Oracle specifically, but other databases, both free and proprietary. > Before I wade in too deep, I'm wondering if anyone has done much "business > app" programming with Haskell (where "business app" is rather fuzzily defined > as database access, web UIs, interfacing with various systems over ip, etc.). > Does anyone have any advice? eg "don't go there" or "works great!" or "try > OCaml" :) . I'm doing a lot of it. There's a good deal of web and database activity going on here. Haskell is also my language of choice for scripting projects, especially data transfer (thanks in part to my FTP library in MissingH ) I use HSQL for everything database-related. HaskellDB did not work out well. It proved rather buggy, cumbersome, inflexible (it appeared to be impossible to do a "SELECT" without "DISTINCT"), and poorly-scalable (try having it scan a database with several thousand tables). HSQL is more low-level, but I'm comfortable with that, having used Perl's DBI and Python's DB-API before. HSQL natively supports the common Free databases (MySQL, PostgreSQL, SQLite, etc.) We use PostgreSQL for everything that we can. It's no less speedy than MySQL when correctly tuned, and far more reliable. It also seems to be a heck of a lot easier to install and work with than Oracle ;-) One of our more important systems is in a Progress database. They provide their own ODBC system for Unix. HSQL has an ODBC interface, and it took only minor tweaking to port it from unixODBC to Progress's ODBC. Configuration was trickier, but that was more because of poor docs from Progress. It is working highly reliably for us. I would suspect that you could find Oracle drivers for either unixODBC, or some custom ODBC system that is nearly API-compatible. Once you have that, you have HSQL. You didn't mention what platform you're on, but if you're on Windows, I'd imagine you'd have no problem making ODBC work. Unix is slightly more tricky, but again, I'd check out the unixODBC route. It's included in Debian by default, and I wouldn't be surprised if other OSs/distros have it too. -- John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] static typing and interactivity
I look at the source code and think about it... Generally I code in vi, then run ghci, or compile and run. I find from experience the type errors are normally easy to fix, you just look at the error, and study the structure of the function. If I still have problems I edit the code to return or output intermediate values. From years of doing this I can generally spot all my mistakes quickly... leaving only those situations where I don't fully understand the algorithm as requiring serious thought. Of course these are precisely those kind of problems for which a debugger is not much good either. Keean. Jake Luck wrote: One slight annoyance using Haskell is the inability to load modules with type problems in the interactive environment (i.e. GHCi). When I have a type error, it would be nice to have an interactive way to explore what the compiler thinks about the types involved -- as it is, I have to resort to adding type signatures more or less at random to narrow down the problem. I'm not sure if it is technically feasible to (partially) load a module with definitions that fail type checking, but if it were, I thing it would make developing Haskell programs even nicer :-) Along similiar lines, it would be quite nice if one can mark their haskell code(working or not) with "breakpoints" and drop the programmer into GHCi so they can poke around, especially inside a do-construct. e.g. something the evalutation engine can check during reduction maybe? I find myself writing a lot of testing frameworks, maybe this is a good thing!, when I program. How do most of the folks here debug their large code base? jake ___ 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] static typing and interactivity
Hi Jake, > program. How do most of the folks here debug their large code base? You might have some success with Hat, http://www.haskell.org/hat/, for debugging. Unfortunately unless you are doing Monadic computations, breakpoints don't really work as well as in strict imperative programs. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] static typing and interactivity
One slight annoyance using Haskell is the inability to load modules with type problems in the interactive environment (i.e. GHCi). When I have a type error, it would be nice to have an interactive way to explore what the compiler thinks about the types involved -- as it is, I have to resort to adding type signatures more or less at random to narrow down the problem. I'm not sure if it is technically feasible to (partially) load a module with definitions that fail type checking, but if it were, I thing it would make developing Haskell programs even nicer :-) Along similiar lines, it would be quite nice if one can mark their haskell code(working or not) with "breakpoints" and drop the programmer into GHCi so they can poke around, especially inside a do-construct. e.g. something the evalutation engine can check during reduction maybe? I find myself writing a lot of testing frameworks, maybe this is a good thing!, when I program. How do most of the folks here debug their large code base? jake ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] static typing and interactivity
Hi, One slight annoyance using Haskell is the inability to load modules with type problems in the interactive environment (i.e. GHCi). When I have a type error, it would be nice to have an interactive way to explore what the compiler thinks about the types involved -- as it is, I have to resort to adding type signatures more or less at random to narrow down the problem. I'm not sure if it is technically feasible to (partially) load a module with definitions that fail type checking, but if it were, I thing it would make developing Haskell programs even nicer :-) -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] embedding prolog in haskell.
Christian Maeder wrote: Keean Schupke wrote: implementation of unify? For example can the algorithm be simplified from my nieve attempt? Most importantly is it correct? It will not be correct without occurs check. You may also get different terms for the same variable in your substitution list. Prolog does not use an occurs check... and this is embedding prolog. However I accept this point, thats why there was the comment about no occurs check in the code. I actually want to cope with recursive definitions as Prolog does, so the solution to: X = f(X) should be: f(f(f(f(... which is an infinite recursion. The simplest form of unification does not take a substitution as input and uses functions to compose two substitutions and to apply a substitution to a term. unify :: Subst -> (Term,Term) -> [Subst] This signature came from the paper... The input subst is an accumulator and it would normally be Id when calling - so there is effectively no input substitution. Do you ever get real lists? The result type "Maybe Subst" is more appropriate. No, I dont think the algorithm gives real lists, Maybe would be better, although I think I will get it working before playing with changing the rest of the code. Is it possible to ever have more than one meaningful answer from unification? unify' s [] [] = [s] unify' s (t0:ts) (u0:us) = case unify s (t0,u0) of s@(_:_) -> unify' (concat s) ts us _ -> [] input lists of different lengths should not cause a runtime error but only a unification failure (indicated by "Nothing" or "[]" in your case.) Aha, a genuine bug... thanks! HTH Christian Here's a part of my version: unify' (t0:ts) (u0:us) = do s1 <- unify (t0,u0) s2 <- unify' (map (applySubst s1) ts) (map (applySubst s1) us) return (composeSubst s1 s2) I am now using: unify' :: Subst -> [Term] -> [Term] -> [Subst] unify' s (t0:ts) (u0:us) = case unify s (t0,u0) of s@(_:_) -> unify' (concat s) ts us _ -> [] unify' s [] [] = [s] unify' _ _ _ = [] Regards, Keean. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] embedding prolog in haskell.
Keean Schupke wrote: > implementation of unify? For example can the algorithm be simplified > from my nieve attempt? Most importantly is it correct? It will not be correct without occurs check. You may also get different terms for the same variable in your substitution list. The simplest form of unification does not take a substitution as input and uses functions to compose two substitutions and to apply a substitution to a term. >unify :: Subst -> (Term,Term) -> [Subst] Do you ever get real lists? The result type "Maybe Subst" is more appropriate. >unify' s [] [] = [s] >unify' s (t0:ts) (u0:us) = case unify s (t0,u0) of >s@(_:_) -> unify' (concat s) ts us >_ -> [] input lists of different lengths should not cause a runtime error but only a unification failure (indicated by "Nothing" or "[]" in your case.) HTH Christian Here's a part of my version: unify' (t0:ts) (u0:us) = do s1 <- unify (t0,u0) s2 <- unify' (map (applySubst s1) ts) (map (applySubst s1) us) return (composeSubst s1 s2) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] embedding prolog in haskell.
Okay, I have found the missing definition (code was split by page break), and found the type error: type Subst = [(Var,Term)] should be: type Subst = [(Vname,Term)] And have written a simple implementation of unify from a description of the algorithm. I was wondering if anyone has any comments on my implementation of unify? For example can the algorithm be simplified from my nieve attempt? Most importantly is it correct? type Subst = [(Vname,Term)] data Term = Func Fname [Term] | Var Vname deriving (Eq,Show) type Fname = String data Vname = Name String | Auto Integer deriving (Eq,Show) unify :: Subst -> (Term,Term) -> [Subst] unify s (t,u) | t == u = [s] unify s (Var x,t) = [(x,t):s] -- no occurs check unify s (t,Var x) = [(x,t):s] -- no occurs check unify s (Func f ts,Func g us) | f == g = unify' s ts us | otherwise = [] unify' :: Subst -> [Term] -> [Term] -> [Subst] unify' s [] [] = [s] unify' s (t0:ts) (u0:us) = case unify s (t0,u0) of s@(_:_) -> unify' (concat s) ts us _ -> [] Keean. Keean Schupke wrote: Does anyone know if the source code for the embedded prolog (by Silvija Seres & Michael Spivey) is available for download from anywhere? I have read the paper and found some of the types are wrong, some critical definitions are missing, and the definition of unify is missing. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Oracle + Haskell advice?
> From: Brian Strand [mailto:[EMAIL PROTECTED] > > I've gotten takusen > (http://cvs.sf.net/viewcvs.py/haskell-libs/libs/takusen/) > to compile and run on my Suse 9.3 x86-64 box against Oracle > 10.1. Cool. I'm keen for feedback. I can also give general advice as far as interfacing to Oracle is concerned. To be fair, Takusen is usable, but far from feature-complete (no support for BLOBs, CLOBs, invoking procedures). You might want to compare it to HaskellDB over ODBC, if you can get HaskellDB running (I don't know how usable ODBC-on-Unix is). HaskellDB is a higher-level approach to database access, and might be better suited to "business apps". > Before I wade in too deep, I'm wondering if anyone has done > much "business > app" programming with Haskell (where "business app" is rather > fuzzily defined > as database access, web UIs, interfacing with various systems > over ip, etc.). As for the other stuff, here are a few projects which might interest you. I believe these represent the state-of-the-art in Haskell as far as this sort of stuff is concerned: http://www.informatik.uni-freiburg.de/~thiemann/haskell/WASH/ : WASH, for building browser-based apps http://www.haskell.org/haxr/ : HaXR, for XML-RPC (the "interfacing with various systems over ip" requirement) http://www.repton-world.org.uk/mediawiki/index.php/HAIFA_Wiki : HAIFA. Looks like it does SOAP. See also: http://haskell.org/~shae/haifa_tmr.pdf There are also a few XML libraries around; I can't comment on how well these work. It should be clear that there's not yet one clear bundle or approach that we can point to and say "here, use this". Like, say, Ruby-on-rails, Zope, Seaside, etc. Things are still a bit more DIY in Haskell land. I recommend starting with WASH and adding what you need from there. 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. * ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
