CODEWITHSUNDEEP
haskell
J Fritsch
J Fritsch

Reputation: 3358

Creating a bitfield with Data.Bits

I want to use a bitfield with 32 bits that store the result of the last 32 computations in sequence. E.g. 0110 ... would be fail pass pass fail ... . Ideally the latest result should just be pushed on the bitfield whilst then the oldest result "falls away" on the other side. For example: 0110 where 1 is the newest result should become 1101.

I am struggling with the use of Data.Bits and don't know how to initialise / create a bitfield. The errors I get are all somewhat similar to 

 No instance for (Bits a0) arising from a use of `bitDefault'
    The type variable `a0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there are several potential instances:
      instance Bits Int -- Defined in `Data.Bits'
      instance Bits Integer -- Defined in `Data.Bits'
      instance Bits GHC.Types.Word -- Defined in `Data.Bits'

Once I add type signatures GHC tells me back about 

 Illegal literal in type (use -XDataKinds to enable): 32

or similar errors. Sometimes it requests the Kind * etc.. I think this is some very basic what I am missing here. How can I create a BitField?

Upvotes: 1

Views: 1247

Answers (1)

Dmytro Sirenko
Dmytro Sirenko

Reputation: 5083

Just use Word32 from Data.Word as a bitmap. It is a type that has Bits instance and thus you can use methods of typeclass Bits on it:

import Data.Bits
import Data.Word

type Bitmap32 = Word32
bitmap :: Word32
bitmap = 0   -- initialize with zeros

...
testBit bitmap 10 -- test 10th bit

...
pushNewBit :: Bool -> Bitmap32 -> Bitmap32
pushNewBit b bitmap = (bitmap `shiftL` 1) .|. (if b then 1 else 0)

Upvotes: 3

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