Re: [Haskell-cafe] Missing a "Deriving"?

[Ryan Ingram]

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,   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  wrote:
>
> From: Ryan Ingram 
> Subject: Re: [Haskell-cafe] Missing a "Deriving"?
> To: "David Menendez" 
> Cc: "michael rice" , haskell-cafe@haskell.org, "Miguel
> Mitrofanov" 
> Date: Sunday, May 31, 2009, 12:45 AM
>
> On Sat, May 30, 2009 at 6:33 PM, David Menendez  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"?

[nowgate]

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  wrote:

From: Ryan Ingram 
Subject: Re: [Haskell-cafe] Missing a "Deriving"?
To: "David Menendez" 
Cc: "michael rice" , haskell-cafe@haskell.org, "Miguel 
Mitrofanov" 
Date: Sunday, May 31, 2009, 12:45 AM

On Sat, May 30, 2009 at 6:33 PM, David Menendez 
 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"?

[michael rice]

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  wrote:

From: David Menendez 
Subject: Re: [Haskell-cafe] Missing a "Deriving"?
To: "michael rice" 
Cc: "Miguel Mitrofanov" , haskell-cafe@haskell.org
Date: Saturday, May 30, 2009, 9:33 PM

On Sat, May 30, 2009 at 9:00 PM, michael rice  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 




  ___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

Re: [Haskell-cafe] Missing a "Deriving"?

[michael rice]

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  wrote:

From: David Menendez 
Subject: Re: [Haskell-cafe] Missing a "Deriving"?
To: "michael rice" 
Cc: "Miguel Mitrofanov" , haskell-cafe@haskell.org
Date: Saturday, May 30, 2009, 9:33 PM

On Sat, May 30, 2009 at 9:00 PM, michael rice  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 




  ___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

Re: [Haskell-cafe] Missing a "Deriving"?

[Ryan Ingram]

On Sat, May 30, 2009 at 6:33 PM, David Menendez  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"?

[David Menendez]

On Sat, May 30, 2009 at 9:00 PM, michael rice  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 

___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

Re: [Haskell-cafe] Missing a "Deriving"?

[michael rice]

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  wrote:

From: Miguel Mitrofanov 
Subject: Re: [Haskell-cafe] Missing a "Deriving"?
To: "michael rice" 
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
> 
> :1:0:
>     No instance for (Show (c [Int]))
>       arising from a use of `print' at :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

[Haskell-cafe] HPC and derived instances

[Felipe Lessa]

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] [] == []

[Derek Elkins]

On Fri, May 29, 2009 at 5:36 AM, Max Rabkin  wrote:
> On Fri, May 29, 2009 at 12:29 PM, Paul Keir  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

Re: [Haskell-cafe] Concurrent Haskell Actions with Timeout

[Cetin Sert]

-__-" hehe why did I not let Hayoo or Hoogle help me there *sigh*

Thanks!!

2009/5/31 Sterling Clover 

> 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 
>> 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'
>>
>> :1:33:
>>No instance for (Num Char)
>>  arising from the literal `0' at :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

[Haskell-cafe] Re: Lazy Parsing

[GüŸnther Schmidt]

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

[Haskell-cafe] get Scale of window in HOpenGL?

[yu yang]

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

Re: [Haskell-cafe] Concurrent Haskell Actions with Timeout

[Sterling Clover]

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   
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'

:1:33:
No instance for (Num Char)
  arising from the literal `0' at :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

Re: [Haskell-cafe] Missing a "Deriving"?

[Ketil Malde]

michael rice  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] Missing a "Deriving"?

[Miguel Mitrofanov]

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

:1:0:
No instance for (Show (c [Int]))
  arising from a use of `print' at :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] Concurrent Haskell Actions with Timeout

[Sebastian Sylvan]

On Sat, May 30, 2009 at 10:32 PM, Cetin Sert  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'
>
> :1:33:
> No instance for (Num Char)
>   arising from the literal `0' at :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] Concurrent Haskell Actions with Timeout

[Cetin Sert]

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'

:1:33:
No instance for (Num Char)
  arising from the literal `0' at :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 

>
>
> 2009/5/30 Cetin Sert 
>
>> 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

[Sebastian Sylvan]

2009/5/30 Cetin Sert 

> 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

[Haskell-cafe] Missing a "Deriving"?

[michael rice]

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

:1:0:
    No instance for (Show (c [Int]))
  arising from a use of `print' at :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] Concurrent Haskell Actions with Timeout

[Cetin Sert]

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

Re: [Haskell-cafe] Parsec float

[Derek Elkins]

On Sat, May 30, 2009 at 1:12 PM, Jason Dusek  wrote:
> 2009/05/30 Bartosz Wójcik :
>> ...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

Re: [Haskell-cafe] Parsec float

[Jason Dusek]

2009/05/30 Bartosz Wójcik :
> ...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] Type families in export lists

[Lee Duhem]

On Sat, May 30, 2009 at 7:35 PM, Maurí­cio  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] can someone point me to more help about Database.HDBC.ODBC?

[Wei Hu]

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
 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] Parsec float

[Bartosz Wójcik]

On Saturday 30 May 2009 03:10:11 Bryan O'Sullivan wrote:
> On Fri, May 29, 2009 at 5:04 PM, Bartosz Wójcik  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] Re: Error message reform

[Claus Reinke]

I find this slightly more complicated case quite confusing with the 
current wording:


  Prelude> :t (\x -> x) :: (a -> b) -> (a -> a)
  :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 :1:14
`b' is a rigid type variable bound by
an expression type signature at :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] Re: Error message reform

[Tillmann Rendel]

wren ng thornton wrote:

(Though it doesn't necessarily generalize to cover similar messages like:

Prelude> :t (\x -> x) :: a -> b
: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 :1:0
  `a' is a rigid type variable bound by
  the polymorphic type `forall a b. a -> b' at :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)
  :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 :1:14
`b' is a rigid type variable bound by
an expression type signature at :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

[Haskell-cafe] Type families in export lists

[Maurí­cio]

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