[Haskell-cafe] Type families in export lists
Hi, How do I include type families (used as associated types) in a module export list? E.g.: class MyClass a where type T a :: * coolFunction :: Ta - a (...) If I just include MyClass and its functions in the list, instances in other modules complain they don't know T, but I wasn't able to find how (where) to include T in the list. Thanks, Maurício ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Error message reform
wren ng thornton wrote: (Though it doesn't necessarily generalize to cover similar messages like: Prelude :t (\x - x) :: a - b interactive:1:7: Couldn't match expected type `b' against inferred type `a' `b' is a rigid type variable bound by the polymorphic type `forall a b. a - b' at interactive:1:0 `a' is a rigid type variable bound by the polymorphic type `forall a b. a - b' at interactive:1:0 In the expression: x ) I find this slightly more complicated case quite confusing with the current wording: Prelude :t (\x - x) :: (a - b) - (a - a) interactive:1:7: Couldn't match expected type `a' against inferred type `b' `a' is a rigid type variable bound by an expression type signature at interactive:1:14 `b' is a rigid type variable bound by an expression type signature at interactive:1:19 In the expression: x In the expression: (\ x - x) :: (a - b) - (a - a) This message suggests that ghc has inferred type b for x. Tillmann ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Error message reform
I find this slightly more complicated case quite confusing with the current wording: Prelude :t (\x - x) :: (a - b) - (a - a) interactive:1:7: Couldn't match expected type `a' against inferred type `b' `a' is a rigid type variable bound by an expression type signature at interactive:1:14 `b' is a rigid type variable bound by an expression type signature at interactive:1:19 In the expression: x In the expression: (\ x - x) :: (a - b) - (a - a) This message suggests that ghc has inferred type b for x. Not really; but this looks like a nice simple example of what I was referring to: GHC is so careful not to bother the user with scary type stuff that it mentions types as little as possible. In particular, it never mentions the type of 'x'! It only mentions that it has run into 'a' and 'b' somewhere *while looking into 'x's type*, so those are conflicting types for some parts of 'x's type, not 'x's type itself. In more complex code, this peephole view can be really unhelpful, which is why I suggested [1] that type errors should give types for all expressions mentioned in the type error context (here 'x' and '(\x-x)', the latter's type is there only because it was part of the input). Claus [1] http://hackage.haskell.org/trac/ghc/ticket/1928#comment:2 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec float
On Saturday 30 May 2009 03:10:11 Bryan O'Sullivan wrote:
On Fri, May 29, 2009 at 5:04 PM, Bartosz Wójcik bar...@sudety.it wrote:
I don't undersdand what is being missed.
Brevity.
liftM f m1 = do { x1 - m1; return (f x1) }
so
liftM fromIntegral integer
will result the same.
Yes, and there's less code to read if you use liftM or $, hence fewer
moving parts to understand.
OK, thats clear. BTW: reading RWH I could not memorize what those liftM
funtions meant.
Best,
Bartek
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] can someone point me to more help about Database.HDBC.ODBC?
Try http://sites.google.com/site/haskell/notes/connecting-to-mysql-with-haskell that I wrote up. An important thing that I don't think was documented anywhere is that the trailing ';' is required. On Fri, May 29, 2009 at 11:01 PM, Michael P Mossey m...@alumni.caltech.edu wrote: I'm trying to use Database.HDBC.ODBC to connect to a MySQL server, and I cannot figure out the docs. I want to establish a connection, and I know the server url, username, and password. According to these docs... http://software.complete.org/static/hdbc-odbc/doc//HDBC-odbc/Database-HDBC-ODBC.html ...I am supposed to pass a string to connectODBC. These docs do not explain the meaning of the passed string, but refer me to a microsoft document: http://msdn2.microsoft.com/en-us/library/ms715433(VS.85).aspx I cannot make any sense of this microsoft document. I guess I don't know enough about database interfaces in general. Can someone explain how to make a connect to a specific URL, or point me to some clearer docs? Thanks, Mike ___ 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] Type families in export lists
On Sat, May 30, 2009 at 7:35 PM, Maurício briqueabra...@yahoo.com wrote: Hi, How do I include type families (used as associated types) in a module export list? E.g.: class MyClass a where type T a :: * coolFunction :: Ta - a (...) If I just include MyClass and its functions in the list, instances in other modules complain they don't know T, but I wasn't able to find how (where) to include T in the list. In export list, you can treat 'type T a' as normal type declaration, ie, write T(..) in export list. lee ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec float
2009/05/30 Bartosz Wójcik bar...@sudety.it: ...reading RWH I could not memorize what those liftM funtions meant. The basic one, `liftM`, means `fmap`, though specialized for functors that are monads. Prelude Control.Monad :t liftM liftM :: forall a b (m :: * - *). (Monad m) = (a - b) - m a - m b Prelude Control.Monad :t fmap fmap :: forall a b (f :: * - *). (Functor f) = (a - b) - f a - f b I think we have `liftM` either to help the inferencer or due to the absence of a `(Functor m)` constraint in the definition of the `Monad` typeclass. -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec float
On Sat, May 30, 2009 at 1:12 PM, Jason Dusek jason.du...@gmail.com wrote: 2009/05/30 Bartosz Wójcik bar...@sudety.it: ...reading RWH I could not memorize what those liftM funtions meant. The basic one, `liftM`, means `fmap`, though specialized for functors that are monads. Prelude Control.Monad :t liftM liftM :: forall a b (m :: * - *). (Monad m) = (a - b) - m a - m b Prelude Control.Monad :t fmap fmap :: forall a b (f :: * - *). (Functor f) = (a - b) - f a - f b I think we have `liftM` either to help the inferencer or due to the absence of a `(Functor m)` constraint in the definition of the `Monad` typeclass. It's the latter effectively. liftM doesn't make anything easier for the type checker. liftM simply has a different type than fmap, not a more specialized one, but even if Monad did have a Functor constraint, liftM would still never lead to any ambiguity being resolved. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Concurrent Haskell Actions with Timeout
Hi how could one implement a function in concurrent haskell that either returns 'a' successfully or due timeout 'b'? timed :: Int → IO a → b → IO (Either a b) timed max act def = do Best Regards, Cetin Sert ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Missing a Deriving?
The following code is from Section 8.4.2, pgs. 111-112 (PDF paging) of YAHT. It compiles fine, but upon trying it I get the following error message. It seems to be trying to 'Show' the Computation class but I'm not sure where to put the 'Deriving'. Michael Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( graph4.hs, interpreted ) Ok, modules loaded: Main. *Main let g = Graph [(1,'a'),(2,'b'),(3,'c'),(4,'d')] [(1,2,'p'),(2,3,'q'),(1,4,'r'),(4,3,'s')] *Main searchAll g 1 3 interactive:1:0: No instance for (Show (c [Int])) arising from a use of `print' at interactive:1:0-14 Possible fix: add an instance declaration for (Show (c [Int])) In a stmt of a 'do' expression: print it data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] class Computation c where success :: a - c a failure :: String - c a augment :: c a - (a - c b) - c b combine :: c a - c a - c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure no path search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Concurrent Haskell Actions with Timeout
2009/5/30 Cetin Sert cetin.s...@gmail.com Hi how could one implement a function in concurrent haskell that either returns 'a' successfully or due timeout 'b'? timed :: Int → IO a → b → IO (Either a b) timed max act def = do Something like (warning, untested code - no compiler atm). timed timeout act fallback = do res - newEmptyMVar tid - forkIO $ act = writeMVar res threadDelay timeout stillRunning - isEmptyMVar res if stillRunning then killThread tid return fallback else takeMVar res -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Concurrent Haskell Actions with Timeout
Thank you for your reply, I'd come up with the following: timed :: Int → IO a → b → IO (Either b a) timed max act def = do r ← new t ← forkIO $ do a ← act r ≔ Right a s ← forkIO $ do wait max e ← em r case e of True → do kill t r ≔ Left def False → return () takeMVar r - *Network.Port.Scan timed 500 (wait 5 return 0) 'x' Left 'x' *Network.Port.Scan timed 50 (wait 5 return 0) 'x' Right 0 - before reading your reply: timed timeout act fallback = do res - newEmptyMVar tid - forkIO $ act = writeMVar res threadDelay timeout stillRunning - isEmptyMVar res if stillRunning then killThread tid return fallback else takeMVar res - *Network.Port.Scan timed2 500 (wait 5 return 0) 'x' interactive:1:33: No instance for (Num Char) arising from the literal `0' at interactive:1:33 Possible fix: add an instance declaration for (Num Char) In the first argument of `return', namely `0' In the second argument of `()', namely `return 0' In the second argument of `timed2', namely `(wait 5 return 0)' Regards, Cetin Sert 2009/5/30 Sebastian Sylvan sebastian.syl...@gmail.com 2009/5/30 Cetin Sert cetin.s...@gmail.com Hi how could one implement a function in concurrent haskell that either returns 'a' successfully or due timeout 'b'? timed :: Int → IO a → b → IO (Either a b) timed max act def = do Something like (warning, untested code - no compiler atm). timed timeout act fallback = do res - newEmptyMVar tid - forkIO $ act = writeMVar res threadDelay timeout stillRunning - isEmptyMVar res if stillRunning then killThread tid return fallback else takeMVar res -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Concurrent Haskell Actions with Timeout
On Sat, May 30, 2009 at 10:32 PM, Cetin Sert cetin.s...@gmail.com wrote: Thank you for your reply, I'd come up with the following: timed :: Int → IO a → b → IO (Either b a) timed max act def = do r ← new t ← forkIO $ do a ← act r ≔ Right a s ← forkIO $ do wait max e ← em r case e of True → do kill t r ≔ Left def False → return () takeMVar r - *Network.Port.Scan timed 500 (wait 5 return 0) 'x' Left 'x' *Network.Port.Scan timed 50 (wait 5 return 0) 'x' Right 0 - before reading your reply: timed timeout act fallback = do res - newEmptyMVar tid - forkIO $ act = writeMVar res threadDelay timeout stillRunning - isEmptyMVar res if stillRunning then killThread tid return fallback else takeMVar res - *Network.Port.Scan timed2 500 (wait 5 return 0) 'x' interactive:1:33: No instance for (Num Char) arising from the literal `0' at interactive:1:33 Possible fix: add an instance declaration for (Num Char) In the first argument of `return', namely `0' In the second argument of `()', namely `return 0' In the second argument of `timed2', namely `(wait 5 return 0)' Right, I forgot about the Either bit so you'd have to make sure the action's result and the default has the same type (or modify it to return an Either). -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing a Deriving?
It's trying to 'Show' the 'c [Int]' type, but doesn't know which 'c' to use; so it's trying to find a generic instance, which doesn't exist. You can't fix this with 'deriving' or anything like this; instead, provide the type annotation like this: *Main searchAll g 1 3 :: Maybe [Int] On 31 May 2009, at 00:50, michael rice wrote: The following code is from Section 8.4.2, pgs. 111-112 (PDF paging) of YAHT. It compiles fine, but upon trying it I get the following error message. It seems to be trying to 'Show' the Computation class but I'm not sure where to put the 'Deriving'. Michael Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( graph4.hs, interpreted ) Ok, modules loaded: Main. *Main let g = Graph [(1,'a'),(2,'b'),(3,'c'),(4,'d')] [(1,2,'p'), (2,3,'q'),(1,4,'r'),(4,3,'s')] *Main searchAll g 1 3 interactive:1:0: No instance for (Show (c [Int])) arising from a use of `print' at interactive:1:0-14 Possible fix: add an instance declaration for (Show (c [Int])) In a stmt of a 'do' expression: print it data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] class Computation c where success :: a - c a failure :: String - c a augment :: c a - (a - c b) - c b combine :: c a - c a - c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure no path search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es ___ 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] Missing a Deriving?
michael rice nowg...@yahoo.com writes: The following code is from Section 8.4.2, pgs. 111-112 (PDF paging) of YAHT. It compiles fine, but upon trying it I get the following error message. It seems to be trying to 'Show' the Computation class but I'm not sure where to put the 'Deriving'. My guess is that your expression is polymorphic, returning Computation c = c [Int] Since Haskell knows how to show this for both of the instances defined for Computation (i.e. Maybe [Int] and [[Int]]), perhaps you could give one of those as an explicit type signature? -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] Concurrent Haskell Actions with Timeout
The proper way is just to wrap System.Timeout, which does some rather clever things with regards to exception semantics. The code for it is a joy to read, by the way. --S. On May 30, 2009, at 5:36 PM, Sebastian Sylvan wrote: On Sat, May 30, 2009 at 10:32 PM, Cetin Sert cetin.s...@gmail.com wrote: Thank you for your reply, I'd come up with the following: timed :: Int → IO a → b → IO (Either b a) timed max act def = do r ← new t ← forkIO $ do a ← act r ≔ Right a s ← forkIO $ do wait max e ← em r case e of True → do kill t r ≔ Left def False → return () takeMVar r - *Network.Port.Scan timed 500 (wait 5 return 0) 'x' Left 'x' *Network.Port.Scan timed 50 (wait 5 return 0) 'x' Right 0 - before reading your reply: timed timeout act fallback = do res - newEmptyMVar tid - forkIO $ act = writeMVar res threadDelay timeout stillRunning - isEmptyMVar res if stillRunning then killThread tid return fallback else takeMVar res - *Network.Port.Scan timed2 500 (wait 5 return 0) 'x' interactive:1:33: No instance for (Num Char) arising from the literal `0' at interactive:1:33 Possible fix: add an instance declaration for (Num Char) In the first argument of `return', namely `0' In the second argument of `()', namely `return 0' In the second argument of `timed2', namely `(wait 5 return 0)' Right, I forgot about the Either bit so you'd have to make sure the action's result and the default has the same type (or modify it to return an Either). -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862 ___ 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] get Scale of window in HOpenGL?
Hi, I want to move one object to the border of window, then go back to the start point. Does anyone one have an idea to implement it ? Thank you! ___ 好玩贺卡等你发，邮箱贺卡全新上线！ http://card.mail.cn.yahoo.com/___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Lazy Parsing
Dear Doaitse, It is my pleasure to announce that after 5 days of experimenting with uu-parsinglib I have absolutely no clue, whatsoever, on how to use it. Period. I do not even manage to write a parser for even a mere digit or a simple character. I have read the tutorial from a to a to z and from z to a and there were a few words I recognized. I mean I'd like to be able to turn 12.05.2009 into something like (12, 5, 2009) and got no clue what the code would have to look like. I do know almost every variation what the code must not look like :). I am guessing here that when one does define a parsing function, since all the parser combinators aren't function but methods, one *must* also provide a type signature so that the compiler knows the actual *instance* method? Günther ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Concurrent Haskell Actions with Timeout
-__- hehe why did I not let Hayoo or Hoogle help me there *sigh* Thanks!! 2009/5/31 Sterling Clover s.clo...@gmail.com The proper way is just to wrap System.Timeout, which does some rather clever things with regards to exception semantics. The code for it is a joy to read, by the way. --S. On May 30, 2009, at 5:36 PM, Sebastian Sylvan wrote: On Sat, May 30, 2009 at 10:32 PM, Cetin Sert cetin.s...@gmail.com wrote: Thank you for your reply, I'd come up with the following: timed :: Int → IO a → b → IO (Either b a) timed max act def = do r ← new t ← forkIO $ do a ← act r ≔ Right a s ← forkIO $ do wait max e ← em r case e of True → do kill t r ≔ Left def False → return () takeMVar r - *Network.Port.Scan timed 500 (wait 5 return 0) 'x' Left 'x' *Network.Port.Scan timed 50 (wait 5 return 0) 'x' Right 0 - before reading your reply: timed timeout act fallback = do res - newEmptyMVar tid - forkIO $ act = writeMVar res threadDelay timeout stillRunning - isEmptyMVar res if stillRunning then killThread tid return fallback else takeMVar res - *Network.Port.Scan timed2 500 (wait 5 return 0) 'x' interactive:1:33: No instance for (Num Char) arising from the literal `0' at interactive:1:33 Possible fix: add an instance declaration for (Num Char) In the first argument of `return', namely `0' In the second argument of `()', namely `return 0' In the second argument of `timed2', namely `(wait 5 return 0)' Right, I forgot about the Either bit so you'd have to make sure the action's result and the default has the same type (or modify it to return an Either). -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862 ___ 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
Re: [Haskell-cafe] [] == []
On Fri, May 29, 2009 at 5:36 AM, Max Rabkin max.rab...@gmail.com wrote: On Fri, May 29, 2009 at 12:29 PM, Paul Keir pk...@dcs.gla.ac.uk wrote: f''' = ([]::[()]) == ([]::[()]) (Very pretty.) So why doesn't ghc have 'default' instances? It does. I believe Num defaults to Integer and then to Double. Generally, though, defaults are convenient in exploratory usage but confusing in compiled code. In compiled code, you don't want arbitrary choices of defaults to affect performance and correctness. I've had programs run much slower than expected because the types defaulted to Integer rather than Int. http://www.haskell.org/ghc/docs/latest/html/users_guide/interactive-evaluation.html#extended-default-rules ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] HPC and derived instances
Hello! Why isn't there an option to control whether HPC, the Haskell Program Coverage, will consider derived instances coverable. I'm using it and my top level coverage is 52% while my expression coverage is at 92%. Looking carefully we see that most non-tested top level definitions are derived Show, Eq and Ord instances that I always derive for most data types (as this is a library). I think it is nice that it recognizes class instances, but there should be a flag to ignore at least the derived ones. Am I missing something? Thanks! :) -- Felipe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing a Deriving?
Hi Miguel, That works. but it gives just a single solution [1,2,3] when there are supposed to be two [[1,2,3],[1,4,3]]. Of course the code in YAHT may be in error. Also, how the heck does Haskell decide which success, failure, augment, and combine to use in function searchAll, since there are five possibilities. MIchael --- On Sat, 5/30/09, Miguel Mitrofanov miguelim...@yandex.ru wrote: From: Miguel Mitrofanov miguelim...@yandex.ru Subject: Re: [Haskell-cafe] Missing a Deriving? To: michael rice nowg...@yahoo.com Cc: haskell-cafe@haskell.org Date: Saturday, May 30, 2009, 5:36 PM It's trying to 'Show' the 'c [Int]' type, but doesn't know which 'c' to use; so it's trying to find a generic instance, which doesn't exist. You can't fix this with 'deriving' or anything like this; instead, provide the type annotation like this: *Main searchAll g 1 3 :: Maybe [Int] On 31 May 2009, at 00:50, michael rice wrote: The following code is from Section 8.4.2, pgs. 111-112 (PDF paging) of YAHT. It compiles fine, but upon trying it I get the following error message. It seems to be trying to 'Show' the Computation class but I'm not sure where to put the 'Deriving'. Michael Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( graph4.hs, interpreted ) Ok, modules loaded: Main. *Main let g = Graph [(1,'a'),(2,'b'),(3,'c'),(4,'d')] [(1,2,'p'),(2,3,'q'),(1,4,'r'),(4,3,'s')] *Main searchAll g 1 3 interactive:1:0: No instance for (Show (c [Int])) arising from a use of `print' at interactive:1:0-14 Possible fix: add an instance declaration for (Show (c [Int])) In a stmt of a 'do' expression: print it data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] class Computation c where success :: a - c a failure :: String - c a augment :: c a - (a - c b) - c b combine :: c a - c a - c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure no path search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es ___ 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] Missing a Deriving?
On Sat, May 30, 2009 at 9:00 PM, michael rice nowg...@yahoo.com wrote: That works. but it gives just a single solution [1,2,3] when there are supposed to be two [[1,2,3],[1,4,3]]. Of course the code in YAHT may be in error. Works for me. *Main searchAll g 1 3 :: [[Int]] [[1,2,3],[1,4,3]] *Main searchAll g 1 3 :: Maybe [Int] Just [1,2,3] *Main searchAll g 1 3 :: Failable [Int] Success [1,2,3] Also, how the heck does Haskell decide which success, failure, augment, and combine to use in function searchAll, since there are five possibilities. *Main :t searchAll searchAll :: (Computation c) = Graph t t1 - Int - Int - c [Int] The way searchAll is written, the choice of which functions to use depends on the type variable c. That's determined by the calling context of searchAll, which is why you need to provide a type signature when using it at the GHCi command line. -- Dave Menendez d...@zednenem.com http://www.eyrie.org/~zednenem/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing a Deriving?
On Sat, May 30, 2009 at 6:33 PM, David Menendez d...@zednenem.com wrote: *Main :t searchAll searchAll :: (Computation c) = Graph t t1 - Int - Int - c [Int] The way searchAll is written, the choice of which functions to use depends on the type variable c. That's determined by the calling context of searchAll, which is why you need to provide a type signature when using it at the GHCi command line. This is actually one of the most interesting and important things to get about typeclasses; it's not *just* like an interface, because the instance type can appear in the result of a function and *not* in the arguments at all. In contrast, in Java/C++, the method to use is always chosen by the object being called; one could argue that all of COM is an attempt to get around this problem. Some examples: fromInteger :: Num a = Integer - a fromDynamic :: Typeable a = Dynamic - Maybe a In both of these cases, the choice of which instance to use is made by the caller, and often automatically by type inference: test x = case fromDynamic x of Just s - s == hello Nothing - False Now (test $ toDynamic hello) = True, but (test $ toDynamic 'a') = False. Notice that I never directly specified what type fromDynamic should return; but the case statement forces it to return Maybe String, since I compare (s == hello) -- ryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing a Deriving?
I figured out the [[Int]] case for myself, but hadn't considered the Failure case. Thanks. In function searchAll, given a calling context Failable [Int], for the line where search' [] = failure no path failure would be Fail, a constructor that takes a String. Right? But using either of the other two contexts, where failure equals either const Nothing or const [] it would seem like that same string argument no path would be passed to either Nothing or [], which doesn't make any sense. Explanation? Michael --- On Sat, 5/30/09, David Menendez d...@zednenem.com wrote: From: David Menendez d...@zednenem.com Subject: Re: [Haskell-cafe] Missing a Deriving? To: michael rice nowg...@yahoo.com Cc: Miguel Mitrofanov miguelim...@yandex.ru, haskell-cafe@haskell.org Date: Saturday, May 30, 2009, 9:33 PM On Sat, May 30, 2009 at 9:00 PM, michael rice nowg...@yahoo.com wrote: That works. but it gives just a single solution [1,2,3] when there are supposed to be two [[1,2,3],[1,4,3]]. Of course the code in YAHT may be in error. Works for me. *Main searchAll g 1 3 :: [[Int]] [[1,2,3],[1,4,3]] *Main searchAll g 1 3 :: Maybe [Int] Just [1,2,3] *Main searchAll g 1 3 :: Failable [Int] Success [1,2,3] Also, how the heck does Haskell decide which success, failure, augment, and combine to use in function searchAll, since there are five possibilities. *Main :t searchAll searchAll :: (Computation c) = Graph t t1 - Int - Int - c [Int] The way searchAll is written, the choice of which functions to use depends on the type variable c. That's determined by the calling context of searchAll, which is why you need to provide a type signature when using it at the GHCi command line. -- Dave Menendez d...@zednenem.com http://www.eyrie.org/~zednenem/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing a Deriving?
Belay that last question. I just realized that its the const function being used rather than a constant declaration in const Nothing and const []. MIchael --- On Sat, 5/30/09, David Menendez d...@zednenem.com wrote: From: David Menendez d...@zednenem.com Subject: Re: [Haskell-cafe] Missing a Deriving? To: michael rice nowg...@yahoo.com Cc: Miguel Mitrofanov miguelim...@yandex.ru, haskell-cafe@haskell.org Date: Saturday, May 30, 2009, 9:33 PM On Sat, May 30, 2009 at 9:00 PM, michael rice nowg...@yahoo.com wrote: That works. but it gives just a single solution [1,2,3] when there are supposed to be two [[1,2,3],[1,4,3]]. Of course the code in YAHT may be in error. Works for me. *Main searchAll g 1 3 :: [[Int]] [[1,2,3],[1,4,3]] *Main searchAll g 1 3 :: Maybe [Int] Just [1,2,3] *Main searchAll g 1 3 :: Failable [Int] Success [1,2,3] Also, how the heck does Haskell decide which success, failure, augment, and combine to use in function searchAll, since there are five possibilities. *Main :t searchAll searchAll :: (Computation c) = Graph t t1 - Int - Int - c [Int] The way searchAll is written, the choice of which functions to use depends on the type variable c. That's determined by the calling context of searchAll, which is why you need to provide a type signature when using it at the GHCi command line. -- Dave Menendez d...@zednenem.com http://www.eyrie.org/~zednenem/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing a Deriving?
Hi Ryan, Is there something missing or mislabeled in your post, because I don't see any definition of toDynamic. Michael --- On Sun, 5/31/09, Ryan Ingram ryani.s...@gmail.com wrote: From: Ryan Ingram ryani.s...@gmail.com Subject: Re: [Haskell-cafe] Missing a Deriving? To: David Menendez d...@zednenem.com Cc: michael rice nowg...@yahoo.com, haskell-cafe@haskell.org, Miguel Mitrofanov miguelim...@yandex.ru Date: Sunday, May 31, 2009, 12:45 AM On Sat, May 30, 2009 at 6:33 PM, David Menendez d...@zednenem.com wrote: *Main :t searchAll searchAll :: (Computation c) = Graph t t1 - Int - Int - c [Int] The way searchAll is written, the choice of which functions to use depends on the type variable c. That's determined by the calling context of searchAll, which is why you need to provide a type signature when using it at the GHCi command line. This is actually one of the most interesting and important things to get about typeclasses; it's not *just* like an interface, because the instance type can appear in the result of a function and *not* in the arguments at all. In contrast, in Java/C++, the method to use is always chosen by the object being called; one could argue that all of COM is an attempt to get around this problem. Some examples: fromInteger :: Num a = Integer - a fromDynamic :: Typeable a = Dynamic - Maybe a In both of these cases, the choice of which instance to use is made by the caller, and often automatically by type inference: test x = case fromDynamic x of Just s - s == hello Nothing - False Now (test $ toDynamic hello) = True, but (test $ toDynamic 'a') = False. Notice that I never directly specified what type fromDynamic should return; but the case statement forces it to return Maybe String, since I compare (s == hello) -- ryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing a Deriving?
Oops, it's called toDyn; from Data.Dynamic [1] toDyn :: Typeable a = a - Dynamic -- ryan [1] http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Dynamic.html On Sat, May 30, 2009 at 10:18 PM, nowg...@yahoo.com wrote: Hi Ryan, Is there something missing or mislabeled in your post, because I don't see any definition of toDynamic. Michael --- On Sun, 5/31/09, Ryan Ingram ryani.s...@gmail.com wrote: From: Ryan Ingram ryani.s...@gmail.com Subject: Re: [Haskell-cafe] Missing a Deriving? To: David Menendez d...@zednenem.com Cc: michael rice nowg...@yahoo.com, haskell-cafe@haskell.org, Miguel Mitrofanov miguelim...@yandex.ru Date: Sunday, May 31, 2009, 12:45 AM On Sat, May 30, 2009 at 6:33 PM, David Menendez d...@zednenem.com wrote: *Main :t searchAll searchAll :: (Computation c) = Graph t t1 - Int - Int - c [Int] The way searchAll is written, the choice of which functions to use depends on the type variable c. That's determined by the calling context of searchAll, which is why you need to provide a type signature when using it at the GHCi command line. This is actually one of the most interesting and important things to get about typeclasses; it's not *just* like an interface, because the instance type can appear in the result of a function and *not* in the arguments at all. In contrast, in Java/C++, the method to use is always chosen by the object being called; one could argue that all of COM is an attempt to get around this problem. Some examples: fromInteger :: Num a = Integer - a fromDynamic :: Typeable a = Dynamic - Maybe a In both of these cases, the choice of which instance to use is made by the caller, and often automatically by type inference: test x = case fromDynamic x of Just s - s == hello Nothing - False Now (test $ toDynamic hello) = True, but (test $ toDynamic 'a') = False. Notice that I never directly specified what type fromDynamic should return; but the case statement forces it to return Maybe String, since I compare (s == hello) -- ryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
