What is a Haskell way to implement flags?
In C usual way is to set some bit in integer variable by shifting or oring,and than check flag integer variable by anding with particular flag value.What is Haskell way? Thanks.
The same as C way. You can import Data.Bits and can use the functions.
Prelude> import Data.Bits
Prelude Data.Bits> Data.Bits.
Data.Bits..&. Data.Bits.bitDefault Data.Bits.complementBit
Data.Bits.rotate Data.Bits.shift
Data.Bits.testBitDefault
Data.Bits..|. Data.Bits.bitSize Data.Bits.isSigned
Data.Bits.rotateL Data.Bits.shiftL Data.Bits.unsafeShiftL
Data.Bits.Bits Data.Bits.clearBit Data.Bits.popCount
Data.Bits.rotateR Data.Bits.shiftR Data.Bits.unsafeShiftR
Data.Bits.bit Data.Bits.complement
Data.Bits.popCountDefault Data.Bits.setBit
Data.Bits.testBit Data.Bits.xor
Prelude Data.Bits> (.&.) 1 2
0
Prelude Data.Bits> (.&.) 2 2
2
I wrote a minimum spanning tree code and rather than maintaining the list
of visited nodes, I took a Integer because of arbitrary precision and set
the bits accordingly.
visited :: Int -> Integer -> ( Bool , Integer )
visited x vis = ( t == 0 , vis' ) where
t = ( B..&. ) ( B.shiftL ( 1 :: Integer ) x ) vis
vis' = ( B..|. ) ( B.shiftL ( 1 :: Integer ) x ) vis
Mukesh
On Tue, Feb 19, 2013 at 8:41 PM, Branimir Maksimovic
In C usual way is to set some bit in integer variable by shifting or oring, and than check flag integer variable by anding with particular flag value. What is Haskell way?
Thanks.
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Quoth Branimir Maksimovic
In C usual way is to set some bit in integer variable by shifting or oring, and than check flag integer variable by anding with particular flag value. What is Haskell way?
Of course you may do the very same thing, if you like. I think if there's only one of these for the entire program, most Haskell programmers would use a Boolean value, as the space you save with a single bit is of too little consequence to bother with bit twiddling in Haskell. The various flags that belong together might be housed in a `record'. Or, in some contexts a list of an algebraic type might be useful - like, "if elem FlagDebug flags", or "if any (flip elem flags) [FlagDebug, FlagVerbose]" Donn
On Tue, Feb 19, 2013 at 10:11 AM, Branimir Maksimovic
In C usual way is to set some bit in integer variable by shifting or oring, and than check flag integer variable by anding with particular flag value. What is Haskell way?
You can do that, but a somewhat more idiomatic way would be a list (or, slightly less conveniently but more accurately, a Data.Set) of constructors from a flags ADT. -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
Brandon Allbery
In C usual way is to set some bit in integer variable by shifting or oring, and than check flag integer variable by anding with particular flag value. What is Haskell way?
You can do that, but a somewhat more idiomatic way would be a list (or, slightly less conveniently but more accurately, a Data.Set) of constructors from a flags ADT.
The Set way is the one I would prefer. In fact together with lenses you even get the boolean interface and a nice interface in general. Define your option types: data Flag = Debug | Verbose deriving (Ord) data Options = Options { _optFiles :: Set FilePath, _optFlags :: Set Flag } makeLenses ''Options The fun starts when you have a state monad around Options, because then you can use lenses very easily. Let's add a file: optFiles . contains "blah.txt" .= True Let's set the Verbose flag: optFlags . contains Verbose .= True Let's flip the Verbose flag: optFlags . contains Verbose %= not Are we verbose? verbose <- use (optFlags . contains Verbose) Have fun. =) Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad.
participants (5)
-
Brandon Allbery -
Branimir Maksimovic -
Donn Cave -
Ertugrul Söylemez -
mukesh tiwari