August 2001 - Haskell-Cafe

"foreign export dynamic" in GHC
by Ashley Yakeley 03 Aug '01

03 Aug '01

When I make a function generator with "foreign export dynamic", GHC spits out stub files which it apparently then needs for the link. What are these for? For instance: -- foreign export dynamic mkCallback :: (Int -> IO Int) -> IO Addr foreign import registerCallback :: Addr -> IO () exportCallback :: (Int -> IO Int) -> IO () exportCallback f = do fx <- mkCallback f registerCallback fx -- And then I expect the C side of things to look like this: -- typedef HsInt (*mkCallback_FunPtr) (HsInt arg1); void registerCallback (HsAddr f) { mkCallback_FunPtr callBack = reinterpret_cast<mkCallback_FunPtr>(f); ... // actually call the callback HsInt arg = 3; HsInt result = (*callBack)(arg); ... } -- Is this correct? When I compile the Haskell Test.hs I get these strange stub files, "Test_stub.h", "Test_stub.c" and "Test_stub.o". The _stub.h one looks like this: -- #include "HsFFI.h" extern HsInt Test_dvD(HsStablePtr a0, HsAddr a_, HsInt a1); -- ...while the _stub.c looks like this: -- #include "Stg.h" #include "HsStd.h" #include "HsLang.h" #include "RtsAPI.h" extern HaskellObj Test_zdfmkCallback_closure; HsInt Test_dvD(HsStablePtr a0, HsAddr a_, HsInt a1) { SchedulerStatus rc; HaskellObj ret; rc=rts_evalIO(rts_apply(rts_apply((StgClosure*)&Test_zdfmkCallback_closure, rts_mkStablePtr(a0)),rts_mkInt(a1)),&ret); rts_checkSchedStatus("Test_dvD",rc); return(rts_getInt(ret)); } -- What are the stubs for? The GHC manual, sec. 8.4.1 says this: # An FFI implementation is encouraged to generate the C # typedef corresponding to a foreign export dynamic # declaration, but isn't required to do so. It doesn't say anything about funny little stub functions. And curiously, it doesn't say exactly how GHC's FFI implementation responds to that non-requirement encouragement. There were no typedefs in the stub files. -- Ashley Yakeley, Seattle WA

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Another query (-:
by Mark Carroll 01 Aug '01

01 Aug '01

I've made a bizarre little program that I don't understand the behaviour of. It's: data BoolList = End | Node Bool BoolList instance Show BoolList where show (Node False rest) = "F " ++ show rest show (Node True rest) = "T " ++ show rest show End = "<>" grow list = (Node False rest) where (Node _ rest) = add list add End = (Node True End) add (Node truth rest) = (Node truth (add rest)) The show is fine; it's the add that's confusing me. Where we have, grow list = (Node False rest) where (Node _ rest) = add list I don't really understand what's going on with this binding and matching using "where", so I'm not seeing how the True and False values are managing to propagate up to being results of grow as they do, e.g.: > grow End F <> > grow (grow End) F T <> I have Hudak, Peterson, Fasel's gentle introduction to Haskell 98, and Thompson's "Craft of Functional Programming" book - maybe I just missed something in them? Is it easy to explain what's going on with this "where" behaviour so that I could figure out what "grow"'s results are without evaluating examples? -- Mark

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Re: Novice Haskell query
by Ashley Yakeley 01 Aug '01

01 Aug '01

At 2001-07-31 14:17, Mark Carroll wrote: >Now, "show x" will work, and "show y" won't. How can I write things so >that I can show both x and y? I don't seem to be able to write, say, > > instance Show a => Show (Tree a) where > show (Branch datum left right) = > show left ++ " <- " ++ show datum ++ " -> " ++ show right > > instance Show (Tree a) where > show Leaf = "*" These two instances overlap. You want one instance, with both Branch and Leaf alternatives in the 'show' function. >or, perhaps, > > instance Show (Tree a) where > Show a => show (Branch datum left right) = > show left ++ " <- " ++ show datum ++ " -> " ++ show right > show Leaf = "*" Close, but you're making a promise (that 'Tree a' is Show for _any_ 'a') that you can't fulfill. Attempting to restrict the type of 'a' while defining the function is bogus, the 'a' symbol simply isn't in scope at that point. Clearly the type 'Tree a' is only Show if 'a' is. It's irrelevant that the 'Leaf' part of the definition doesn't need 'a' to be Show: the 'Branch' part does, so the type has to be restricted to 'Show a => Tree a' if it is going to be Show. So you want this: instance Show a => Show (Tree a) where ... -- Ashley Yakeley, Seattle WA

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Re: newbie conceptual question [from haskell list]
by Steinitz, Dominic J 01 Aug '01

01 Aug '01

Alex, The code isn't ready to go public but you can try out a web interface at http://r341-02.king.ac.uk/cgi-bin/ldap.cgi. If you use the search filter objectClass=top you will get the whole tree on our test server. You shouldn't be able to add, modify or delete (let me know if you can!). The web code isn't very resilient. If you specifiy a non-existent attribute or use the wild card character (*), you will get an uninformative error message. Or you can try searching any public ldap server. Dominic. alex(a)shop.com on 01/08/2001 13:30:00 To: Dominic Steinitz cc: hdaume franka haskell-cafe bcc: Subject: Re: newbie conceptual question [from haskell list] Is the LDAP client available somewhere? -Alex- On 1 Aug 2001, Steinitz, Dominic J wrote: > I don't know about functional dependencies but using an existential type turned out to be very useful in writing an LDAP protocol handler. The protocol is specified at an abstract level using ASN.1 and could, in theory, be encoded using any set of encoding rules. It happens to use the Basic Encoding Rules. We used an existential type to "encode" the protocol at an abstract level and the encoding rules take this type and produce a concrete representation ready to send over a transport mechanism. Thus we get a good separation between the abstract protocol and the concrete encoding. So the next time we implement a protocol handler we can re-use the encoding code or we could encode LDAP with a different set of encoding rules without having to touch the LDAP code itself. > > We are presenting a paper which includes this at the Scottish Functional Programming workshop. > > Dominic. > > > > > hdaume(a)ISI.EDU on 31/07/2001 22:29:00 > To: franka > cc: haskell-cafe > bcc: Dominic Steinitz > Subject: Re: newbie conceptual question [from haskell list] > > Hi, > > Frank Atanassow wrote: > > D. Tweed wrote: > > > I've never written a Haskell program using functional dependencies, or > > > existential classes, ... > > > > I find them indispensible, and I know for a fact that I am not the only one > > around our office who feels that way. Though, the people around here > > (Utrecht's software technology group) are not exactly typical programmers... > > :) > > I've been recently experimenting quite a bit with existential classes > and somewhat less with functional dependencies, primarily to help my > understanding of the concepts. However, I've yet to be able to think of > an appropriate place to use them in the "real world". That is, in > something more than a toy thought-experiment. Could you give some > examples of what you are using them for? > > -- > Hal Daume III > > "Computer science is no more about computers | hdaume(a)isi.edu > than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > ------------------------------------------------------------------------------- ------------------ > 21st century air travel http://www.britishairways.com > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > ___________________________________________________________________ S. Alexander Jacobson Shop.Com 1-646-638-2300 voice The Easiest Way To Shop (sm) ------------------------------------------------------------------------------------------------- 21st century air travel http://www.britishairways.com

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Re: newbie conceptual question [from haskell list]
by S. Alexander Jacobson 01 Aug '01

01 Aug '01

Is the LDAP client available somewhere? -Alex- On 1 Aug 2001, Steinitz, Dominic J wrote: > I don't know about functional dependencies but using an existential type turned out to be very useful in writing an LDAP protocol handler. The protocol is specified at an abstract level using ASN.1 and could, in theory, be encoded using any set of encoding rules. It happens to use the Basic Encoding Rules. We used an existential type to "encode" the protocol at an abstract level and the encoding rules take this type and produce a concrete representation ready to send over a transport mechanism. Thus we get a good separation between the abstract protocol and the concrete encoding. So the next time we implement a protocol handler we can re-use the encoding code or we could encode LDAP with a different set of encoding rules without having to touch the LDAP code itself. > > We are presenting a paper which includes this at the Scottish Functional Programming workshop. > > Dominic. > > > > > hdaume(a)ISI.EDU on 31/07/2001 22:29:00 > To: franka > cc: haskell-cafe > bcc: Dominic Steinitz > Subject: Re: newbie conceptual question [from haskell list] > > Hi, > > Frank Atanassow wrote: > > D. Tweed wrote: > > > I've never written a Haskell program using functional dependencies, or > > > existential classes, ... > > > > I find them indispensible, and I know for a fact that I am not the only one > > around our office who feels that way. Though, the people around here > > (Utrecht's software technology group) are not exactly typical programmers... > > :) > > I've been recently experimenting quite a bit with existential classes > and somewhat less with functional dependencies, primarily to help my > understanding of the concepts. However, I've yet to be able to think of > an appropriate place to use them in the "real world". That is, in > something more than a toy thought-experiment. Could you give some > examples of what you are using them for? > > -- > Hal Daume III > > "Computer science is no more about computers | hdaume(a)isi.edu > than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > ------------------------------------------------------------------------------------------------- > 21st century air travel http://www.britishairways.com > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > ___________________________________________________________________ S. Alexander Jacobson Shop.Com 1-646-638-2300 voice The Easiest Way To Shop (sm)

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Re: newbie conceptual question [from haskell list]
by Steinitz, Dominic J 01 Aug '01

01 Aug '01

I don't know about functional dependencies but using an existential type turned out to be very useful in writing an LDAP protocol handler. The protocol is specified at an abstract level using ASN.1 and could, in theory, be encoded using any set of encoding rules. It happens to use the Basic Encoding Rules. We used an existential type to "encode" the protocol at an abstract level and the encoding rules take this type and produce a concrete representation ready to send over a transport mechanism. Thus we get a good separation between the abstract protocol and the concrete encoding. So the next time we implement a protocol handler we can re-use the encoding code or we could encode LDAP with a different set of encoding rules without having to touch the LDAP code itself. We are presenting a paper which includes this at the Scottish Functional Programming workshop. Dominic. hdaume(a)ISI.EDU on 31/07/2001 22:29:00 To: franka cc: haskell-cafe bcc: Dominic Steinitz Subject: Re: newbie conceptual question [from haskell list] Hi, Frank Atanassow wrote: > D. Tweed wrote: > > I've never written a Haskell program using functional dependencies, or > > existential classes, ... > > I find them indispensible, and I know for a fact that I am not the only one > around our office who feels that way. Though, the people around here > (Utrecht's software technology group) are not exactly typical programmers... > :) I've been recently experimenting quite a bit with existential classes and somewhat less with functional dependencies, primarily to help my understanding of the concepts. However, I've yet to be able to think of an appropriate place to use them in the "real world". That is, in something more than a toy thought-experiment. Could you give some examples of what you are using them for? -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ------------------------------------------------------------------------------------------------- 21st century air travel http://www.britishairways.com

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Re: Singular Type Constructor
by Ashley Yakeley 31 Jul '01

31 Jul '01

At 2001-07-30 19:08, I wrote: >Consider: > >-- >data T a = T >-- > >Is there anything in the Prelude or a standard library that looks like >this? Should there be? What should it be called? 'Singleton'? 'Reified'? >'T'? In the end I called it 'Type'. Perhaps something like it should be in the Prelude... See <http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/jvm-bridge/source/Haskell/T ype.hs?rev=HEAD&content-type=text/vnd.viewcvs-markup>. -- Ashley Yakeley, Seattle WA

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