[Haskell-cafe] Fw: [darcs-devel] Inferred type is less polymorphic than expected and type witnesses
Forwarding to Haskell Cafe per Eric's suggestion. Begin forwarded message: Date: Sun, 11 Jan 2009 23:01:31 -0600 From: Rob Hoelz r...@hoelzro.net To: darcs-de...@darcs.net Subject: [darcs-devel] Inferred type is less polymorphic than expected and type witnesses Hello again, Darcs users and developers, As I mentioned in my last e-mail, I'm working on http://bugs.darcs.net/issue291. It's actually gone pretty well, and I feel I'm just about finished (I've done all but sorting out the changes after leaving the editor), only I've encountered the compiler error you see in the subject of this message. This error only appears when compiling with witnesses. Here's the source for the function that it's complaining about: compare_changes_with_old (x :: xs) (y :: ys) = case compare_changes_with_old xs ys of nx : ny - if unsafeCompare x y then ((x :: nx) : ny) else (NilFL : (y :: ys)) compare_changes_with_old NilFL NilFL = (NilFL : NilFL) compare_changes_with_old NilFL ys@(_ :: _) = (NilFL : ys) compare_changes_with_old x@(_ :: _) NilFL = (NilFL : NilFL) Now, I have two questions: 1) What exactly does this error mean, and how do I get around it? 2) What are witness types, and what are they used for? I will gladly accept links to fine manuals as answers to either question, but simple explanations would be nice. =D I thought I understood Haskell pretty well, but existentially qualified types have thrown me for a loop. Thanks much, Rob Hoelz ___ darcs-devel mailing list (AUTOMATIC POSTINGS ONLY PLEASE!) darcs-de...@darcs.net http://lists.osuosl.org/mailman/listinfo/darcs-devel ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fw: [darcs-devel] Inferred type is less polymorphic than expected and type witnesses
I should've included these when I forwarded it, but that was pre-coffee today. =P class MyEq p where unsafeCompare :: p C(a b) - p C(c d) - Bool -- more stuff data FL a C(x z) where (::) :: a C(x y) - FL a C(y z) - FL a C(x z) NilFL :: FL a C(x x) data (a1 : a2) C(x y) = FORALL(z) (a1 C(x z)) : (a2 C(z y)) infixr 1 : -- I'm not entirely sure on this one, because type witnesses confuse me. compare_changes_with_old :: (Patchy p) = FL p C(x y) - FL p C(x y) - (FL p : FL p) C(x y) C(args...) is a preprocessor macro that expands to args if Darcs is building with GADT type witnesses. FORALL(args...) expands to forall args. under the same condition. All of the definitions are available at http://darcs.net/api-doc/doc-index.html as well. Thanks, Rob Ryan Ingram ryani.s...@gmail.com wrote: Some questions first: What's the type of this function supposed to be? What's the type of unsafeCompare? How is the data type with NilFL and :: defined? -- ryan On Mon, Jan 12, 2009 at 5:43 AM, Rob Hoelz r...@hoelzro.net wrote: Forwarding to Haskell Cafe per Eric's suggestion. Begin forwarded message: Date: Sun, 11 Jan 2009 23:01:31 -0600 From: Rob Hoelz r...@hoelzro.net To: darcs-de...@darcs.net Subject: [darcs-devel] Inferred type is less polymorphic than expected and type witnesses Hello again, Darcs users and developers, As I mentioned in my last e-mail, I'm working on http://bugs.darcs.net/issue291. It's actually gone pretty well, and I feel I'm just about finished (I've done all but sorting out the changes after leaving the editor), only I've encountered the compiler error you see in the subject of this message. This error only appears when compiling with witnesses. Here's the source for the function that it's complaining about: compare_changes_with_old (x :: xs) (y :: ys) = case compare_changes_with_old xs ys of nx : ny - if unsafeCompare x y then ((x :: nx) : ny) else (NilFL : (y :: ys)) compare_changes_with_old NilFL NilFL = (NilFL : NilFL) compare_changes_with_old NilFL ys@(_ :: _) = (NilFL : ys) compare_changes_with_old x@(_ :: _) NilFL = (NilFL : NilFL) Now, I have two questions: 1) What exactly does this error mean, and how do I get around it? 2) What are witness types, and what are they used for? I will gladly accept links to fine manuals as answers to either question, but simple explanations would be nice. =D I thought I understood Haskell pretty well, but existentially qualified types have thrown me for a loop. Thanks much, Rob Hoelz ___ darcs-devel mailing list (AUTOMATIC POSTINGS ONLY PLEASE!) darcs-de...@darcs.net http://lists.osuosl.org/mailman/listinfo/darcs-devel ___ 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] Damnit, we need a CPAN.
Neil Mitchell [EMAIL PROTECTED] wrote: Hi Rationale: We need a CPAN We choose to spell CPAN as Hackage , a cabal that is smart enough to know what to to, even if building depends on make Why should building depend on make? Shouldn't cabal build stuff for us? We need a cabal that is clever enough that we don't need a make. , grapefruit authors that commit to hackage Or someone to help show the grapefruit authors the light. I helped put smallcheck on hackage, others have done other packages. Perhaps you could do grapefruit? , a cabal-inst that can install from darcs (or at least from a local directory) Yes, that would be lovely! , or maybe just a make-replacement for haskell, like java has with ant (which is really cool if you successfully do not notice the xml-syntax). We have replaced a lot of make with just --make in GHC. Cabal replaces more. What more bits of make do you need? If you can say why make is still necessary, people may be able to eliminate it. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Hackage and Cabal are nice, but a command line tool for automatically searching Hackage and installing Hackage packages (like the cpan program, or easy_install) would be nice. Unless I haven't done my homework and this tool exists... Thanks, Rob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Haskell interface file (.hi) format?
Hello fellow Haskellers, Does anyone know if/where I can find a specification for the .hi files generated by GHC? I ask because I want to write an omni-completion plugin for Vim to make Haskell hacking a bit nicer. Thanks, Rob Hoelz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell interface file (.hi) format?
Claus Reinke [EMAIL PROTECTED] wrote: Does anyone know if/where I can find a specification for the .hi files generated by GHC? I ask because I want to write an omni-completion plugin for Vim to make Haskell hacking a bit nicer. you might find it easier to use GHCi's :browse command $ ghc -e ':browse Control.Concurrent.MVar' modifyMVar :: MVar a - (a - IO (a, b)) - IO b modifyMVar_ :: MVar a - (a - IO a) - IO () readMVar :: MVar a - IO a swapMVar :: MVar a - a - IO a withMVar :: MVar a - (a - IO b) - IO b data MVar a = GHC.IOBase.MVar (GHC.Prim.MVar# GHC.Prim.RealWorld a) addMVarFinalizer :: MVar a - IO () - IO () isEmptyMVar :: MVar a - IO Bool newEmptyMVar :: IO (MVar a) newMVar :: a - IO (MVar a) putMVar :: MVar a - a - IO () takeMVar :: MVar a - IO a tryPutMVar :: MVar a - a - IO Bool tryTakeMVar :: MVar a - IO (Maybe a) that is what the haskell mode plugins for vim use, for one of their completion modes, anyway;-) http://www.cs.kent.ac.uk/~cr3/toolbox/haskell/Vim/ (actually, that completion is wrt to imported identifiers, so we simply do a ':browse *current_module'; another completion mode is based on haddock indices, and then there are the standard occurs-in-imported-source-files and occurs-in-tags-file completions) claus Wow! Thanks for saving me a ton of work, everyone! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] IO Monad operation madness
Hello again, I'm still working on that binding, and I've come to a problem that I just can't figure out. I'm sure it's staring me right in the face, so I think another pair of eyes to take a look. So if you look at my previous message, I defined a linked list data type in C. I'm trying to convert it to a Haskell list of strings, so here's my function: linkedListToHaskellStringList :: LinkedList - IO [String] linkedListToHaskellStringList listPtr = let convertList' ptr = if listIsNull ptr then [] else let str = peekCString = (linked_list_getdata ptr) next - linked_list_next ptr str : (convertList' next) in sequence $ convertList' listPtr listIsNull :: LinkedList - Bool listIsNull (LinkedList ptr) = ptr == nullPtr So here's the compile error: Option.hsc:63:14: Couldn't match expected type `[t]' against inferred type `IO LinkedList' In a 'do' expression: next - linked_list_next ptr In the expression: if listIsNull ptr then [] else do let str = peekCString = linked_list_getdata ptr next - linked_list_next ptr str : (convertList' next) In the definition of `convertList'': convertList' ptr = if listIsNull ptr then [] else do let str = ... next - linked_list_next ptr str : (convertList' next) Could anyone tell me what I'm doing wrong? Thanks a lot! -Rob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO Monad operation madness
Isaac Dupree [EMAIL PROTECTED] wrote: linkedListToHaskellStringList :: LinkedList - IO [String] linkedListToHaskellStringList listPtr = let convertList' ptr = convertList' :: LinkedList - [IO String], I infer? Correct. if listIsNull ptr then [] else are you doing IO here or not? You need a 'do' if you are (else this is a syntax error), but IO /= [], so what is the function returning? My mistake; I left off the 'do.' My original code does have it, though. I figure since the null case returns [] (which should be a valid [IO String], and str is an IO String, it should work. In fact, if I take out the next - ... line and substitute ptr for next, it compiles. But that's not exactly desirable behavior...=P let str = peekCString = (linked_list_getdata ptr) next - linked_list_next ptr str : (convertList' next) in sequence $ convertList' listPtr listIsNull :: LinkedList - Bool listIsNull (LinkedList ptr) = ptr == nullPtr I'd recommend recursion without the 'sequence', I think, so you can do IO along the way - traversing the LinkedList - and then 'return' the list made from (:). (and throw in unsafeInterleaveIO if you're feeling sadistical and want parts of the traversal to be performed at unspecified later dates) The problem I have with that is that I'd have to do something like this: str - peekCString = (linked_list_getdata ptr) next - linked_list_next ptr rest - linkedListToHaskellStringList next return (str : rest) I don't like this because it's not really tail recursive. I've been told tail recursion is overrated, but since I don't really know how long the C linked lists are going to be (could be 10 elements, could be 100), I don't think avoiding a stack overflow is overrated. Thanks, Rob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO Monad operation madness
Isaac Dupree [EMAIL PROTECTED] wrote: Isaac Dupree wrote: liftM2 (:) (peekCString = (linked_list_getdata ptr)) (linkedListToHaskellStringList = linked_list_next ptr) formerly missing parenthesis fixed in the above before it bites you :( Isaac Thanks for the help, Isaac. I didn't know how good of a job GHC did when optimizing code like that. I don't have a problem with that code I wrote as a rule (it doesn't look ugly to me), but I just wanted to make sure my binding didn't cause crashes. I like the liftM2 thing, though; I think I'll go with that. Thanks, Rob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Safe/Unsafe calls with the GHC Foreign Function Interface
More questions from Rob regarding his mysterious bindings... So I've been reading the docs for the FFI, and it's my understanding that foreign functions imported as unsafe are faster, but they've got a problem with callbacks. From what I read, I believe that I should just make a foreign function safe if it sets up a callback; is this correct? Normally I wouldn't bother people with questions like this, but I'd rather be absolutely certain and avoid tracking down a strange bug later. Many thanks as always, Rob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Safe/Unsafe calls with the GHC Foreign Function Interface
Stefan O'Rear [EMAIL PROTECTED] wrote: On Thu, May 17, 2007 at 10:32:11PM -0500, Rob Hoelz wrote: More questions from Rob regarding his mysterious bindings... So I've been reading the docs for the FFI, and it's my understanding that foreign functions imported as unsafe are faster, but they've got a problem with callbacks. From what I read, I believe that I should just make a foreign function safe if it sets up a callback; is this correct? Normally I wouldn't bother people with questions like this, but I'd rather be absolutely certain and avoid tracking down a strange bug later. Many thanks as always, Rob If it *calls* a callback into haskell. Also, for unfathomable reasons, safety has been overloaded to include forking OS threads. If it could block (like getchar), you must make it safe. Stefan Ouch...how bad is the overhead for a safe function? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Tail Recursion within the IO Monad
Hello everyone,
You may have seen my message about how I'm writing a binding to a C
library. This is another question related to that.
So, let's say I have a linked list implemented in C. Here's what its
definition looks like:
struct __linked_list {
void *data;
struct __linked_list *next;
};
typedef struct __linked_list linked_list_t;
void *linked_list_getdata(linked_list_t *);
linked_list_t *linked_list_next(linked_list_t *);
Keep in mind, this is just a segment.
So using the Haskell FFI, I import these into my .hsc file:
data LinkedList = LL (Ptr linked_list_t)
foreign import ccall unsafe linked_list.h
linked_list_getdata :: Ptr LinkedList - IO Ptr a
foreign import ccall unsafe linked_list.h
linked_list_next :: Ptr LinkedList - IO Ptr LinkedList
So now that that's done, I attempt to write a Ptr LinkedList -
[String] function (assuming the given LinkedList is holding c strings):
linkedListToStringList :: Ptr LinkedList - IO [String]
linkedListToStringList listPtr =
if listPtr == nullPtr
then
return []
else do
item - linked_list_getdata listPtr
next - linked_list_next listPtr
cStr - peek item
hStr - peekCString cStr
t - linkedListToStringList next
return (hStr : t)
This is just ugly...making the recursive call first, THEN consing the
value on? However, this is the only way I could think of doing it. I
figure there are three possibilities from here:
1) Leave this code alone, as GHC will optimize it because it's smart.
2) There's a way to more effectively write this code! Change it!
3) Roll my own optimization.
I know how to do 3, but I'd rather avoid it. I guess I'm looking for
an answer to 2, but if 1 is true, that'd be ok too. Could anyone give
me a hand?
And as long as I'm asking, is there some kind of monadic function
composition operator? I'd like to clean up the above with something
like peekCString . peek . linked_list_getdata...
Many thanks,
Rob Hoelz
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Re: [Haskell-cafe] Tail Recursion within the IO Monad
Brandon S. Allbery KF8NH [EMAIL PROTECTED] wrote: On May 16, 2007, at 12:23 , Rob Hoelz wrote: And as long as I'm asking, is there some kind of monadic function composition operator? I'd like to clean up the above with something like peekCString . peek . linked_list_getdata... (=)? Thanks for the reply; I can't believe I missed that one! But while looking over the documentation, completely humbled, I discovered sequence, which allows me to write my code cleanly! Thanks for the help! -Rob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Converting CTime - Int
Hello everyone, I'm writing an interface to C code, and one of the functions I'm wrapping returns time_t. I see that this maps to CTime in Foreign.C.Types, but I can't figure out how to convert it to an Int (or any other useful Haskell type, for that matter) for the life of me. I've poured over the standard library docs, but to no avail. Could someone give me a hint? Thanks, Rob Hoelz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Any Haskellers anywhere? (was Re: Any Haskellers in St Louis, MO?)
Sounds like a good idea to me. I'd like to see if any Haskellers are in Madison. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Any Haskellers anywhere?
Gabor Greif [EMAIL PROTECTED] wrote: Am 06.05.2007 um 03:52 schrieb Rob Hoelz: Sounds like a good idea to me. I'd like to see if any Haskellers are in Madison. Doesn't Google have a service for visualizing locations on a map? The wiki could point there, for example... Gabor I bet Google does have one, but first thing that came to my mind is Frappr: http://www.frappr.com/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
