[Haskell-cafe] [Pipes] Can pipes solve this problem? How?

[Daniel Hlynskyi]

Hello Cafe.
Consider code, that takes input from handle until special substring matched:

 matchInf a res s | a `isPrefixOf` s = reverse res
 matchInf a res (c:cs)   = matchInf a (c:res) cs
 hTakeWhileNotFound str hdl = hGetContents hdl = return.matchInf str []

It is simple, but the handle is closed after running. That is not good,
because I want to reuse this function.
Code can be rewritten without hGetContent, but it is much less
comprehensible:

hTakeWhileNotFound str hdl = fmap reverse$ findStr str hdl [0] []
 where
   findStr str hdl indeces acc = do
 c - hGetChar hdl
 let newIndeces = [ i+1 | i - indeces, i  length str, str!!i == c]
 if length str `elem` newIndeces
   then return (c : acc)
   else findStr str hdl (0 : newIndeces) (c : acc)

So, the question is - can pipes (any package of them) be the Holy Grail in
this situation, to both keep simple code and better deal with handles (do
not close them specifically)? How?
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[Haskell-cafe] Cannot System,Timeout.timeout Network.accept

[Daniel Hlynskyi]

Hello Cafe.

I cannot accept socket connection with timeout (Windows, GHC 7.4,1).
Consider following code:

 import Network
 import System.Timeout
 main = withSocketsDo $ do
 print :: Starting server ...
 sock - listenOn (PortNumber 4010)
 res - timeout 1 (accept sock)
 print Timed out -- Never output!
 sClose sock

Documentation on
http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-Timeout.html
says:... Standard
I/O functions like ... Network.Socket.accept, ... appear to be blocking,
but they really don't because the runtime system uses scheduling mechanisms
like select(2) to perform asynchronous I/O, so it is possible to interrupt
standard socket I/O or file I/O using this combinator. Neither
Network.accept, nor Network.Socket.accept can be timed out is example above.

Is it bug?
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Re: [Haskell-cafe] How helpful is h-99 (and should we complete the missing ones)?

[Daniel Hlynskyi]

 So here's my question: how useful is h-99 (are they overrated as learning
 tools)? I find myself solve most of them in a from the scratch fashion
 (e.g., no Monad, no Applicative, no Functor aside from List and a few
 Maybe). Some of them are paper-worthy, for example the prime problems. I
 hope some guru-level Haskeller could do away the missing few, and maybe
 dive deeper into the surface to produce more insights (like the knights
 travel page or the sieve paper, which are both beautiful).


As another new-haskeller I say: Yes  No.

Yes for  necessity of several described algorithms and gained intuition
for real programming.
No for needed ammount of work to understand pitfalls of Haskell when real
programming.

And of-course, problems and solutions are not annotated with there typical
real world aplications, they are not obvious for average beginners. Why
would I make Binary tree balanced, when I don't know, what I'll gain
(except balanced binary tree)?
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Re: [Haskell-cafe] [C][enums][newbie] What is natural Haskell representation of such enum?

[Daniel Hlynskyi]

Thanks. This and previous email are answers to question I asked. But not
the answer to question I mean.
I'll describe the whole task, as Yves Parès suggested.

I'm trying to convert C++ code to Haskell. I have such hierarchy: class
Object, class Item : Object, class Container : Item. Another one example:
class Unit : Object, class Player : Unit. Each constructor do things like
this:

Object::Object()
{
objectType= TYPEMASK_OBJECT;
// ... lots of code ...
}


Item::Item()
{
objectType |= TYPEMASK_ITEM;
// ...
}

Container::Container(): Item()
{
objectType |= (TYPEMASK_ITEM | TYPEMASK_CONTAINER);
// ...
}


What is objectType? This field is used when a networksend packet is
created. In the packet it is 1 byte of flags, so it is in object hierarchy.

So the question was: what type should objectType field have in Haskell? I
think it must not mimic enum. What the structure have I to use? There is
one more problem - there may be lots of objects, lots of, so memory
efficiency is also suggested.
And side question: what to do with classes? =) Maybe there is simple rule
to convert OO hierarchy to FP.

23 січня 2012 р. 12:15 Malcolm Wallace malcolm.wall...@me.com написав:


  2012/1/22 Данило Глинський abcz2.upr...@gmail.com
  What is natural Haskell representation of such enum?
 
  enum TypeMask
  {
 UNIT,
 GAMEOBJECT,
 
 CREATURE_OR_GAMEOBJECT = UNIT | GAMEOBJECT
  };

 I don't think that definition makes any sense in C, because UNIT is 0, so
 UNIT | GAMEOBJECT == GAMEOBJECT == 1

 Nevertheless, in Haskell something vaguely similar might be:

 data TypeMask = UNIT | GAMEOBJECT | CREATURE_OR_GAMEOBJECT

  // 1-byte flaged enum
  enum TypeMask
  {
 // ...
 UNIT= 0x0004,
 GAMEOBJECT  = 0x0008,
 // ...
 
 CREATURE_OR_GAMEOBJECT = UNIT | GAMEOBJECT
 WORLDOBJECT = UNIT | PLAYER | GAMEOBJECT | DYNAMICOBJECT | CORPSE
 // ... even more enum combos ...
  };

 import Data.Bits
 data TypeMask = UNIT | GAMEOBJECT | CREATURE_OR_GAMEOBJECT | WORLDOBJECT
 instance Enum TypeMask where
fromEnum UNIT = 0x4
fromEnum GAMEOBJECT = 0x8
fromEnum CREATURE_OR_GAMEOBJECT = fromEnum UNIT .|. fromEnum GAMEOBJECT
fromEnum WORLDOBJECT = fromEnum UNIT .|. fromEnum PLAYER .|. fromEnum
 GAMEOBJECT
   .|. fromEnum DYNAMICOBJECT .|. fromEnum CORPSE

toEnum 0x4 = UNIT
toEnum 0x8 = GAMEOBJECT
toEnum _   = error unspecified enumeration value of type TypeMask

 isCreatureOrGameObject :: Int - Bool
 isCreatureOrGameObject x = (x .|. fromEnum CREATURE_OR_GAMEOBJECT) /= 0

 isWorldObject :: Int - Bool
 isWorldObject x = (x .|. fromEnum WORLDOBJECT) /= 0

 -- But fundamentally, this is not an idiomatic Haskell way of doing things.
 -- The other posts in this thread have shown more Haskell-ish translations.



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