| Copyright | (C) 2018 Google Inc. |
|---|---|
| License | BSD2 (see the file LICENSE) |
| Maintainer | Christiaan Baaij <christiaan.baaij@gmail.com> |
| Safe Haskell | None |
| Language | Haskell2010 |
Clash.Annotations.BitRepresentation
Description
Using ANN pragma's you can tell the Clash compiler to use a custom
bit representation for a data type. See DataReprAnn for documentation.
Synopsis
- data DataReprAnn = DataReprAnn Type Size [ConstrRepr]
- data ConstrRepr = ConstrRepr Name BitMask Value [FieldAnn]
- type BitMask = Integer
- type Value = Integer
- type Size = Int
- type FieldAnn = BitMask
- liftQ :: Lift a => Q a -> Q Exp
Data structures to express a custom bit representation
data DataReprAnn Source #
Annotation for custom bit representations of data types
Using ANN pragma's you can tell the Clash compiler to use a custom bit-representation for a data type.
For example:
data Color = R | G | B
{-# ANN module (DataReprAnn
$(liftQ [t|Color|])
2
[ ConstrRepr 'R 0b11 0b00 []
, ConstrRepr 'G 0b11 0b01 []
, ConstrRepr 'B 0b11 0b10 []
]) #-}
This specifies that R should be encoded as 0b00, G as 0b01, and
B as 0b10. The first binary value in every ConstrRepr in this example
is a mask, indicating which bits in the data type are relevant. In this case
all of the bits are.
Or if we want to annotate Maybe Color:
{-# ANN module ( DataReprAnn
$(liftQ [t|Maybe Color|])
2
[ ConstrRepr 'Nothing 0b11 0b11 []
, ConstrRepr 'Just 0b00 0b00 [0b11]
] ) #-}
By default, Maybe Color is a data type which consumes 3 bits. A single bit
to indicate the constructor (either Just or Nothing), and two bits to encode
the first field of Just. Notice that we saved a single bit by exploiting
the fact that Color only uses three values (0, 1, 2), but takes two bits
to encode it. We can therefore use the last - unused - value (3), to encode
one of the constructors of Maybe. We indicate which bits encode the
underlying Color field of Just by passing [0b11] to ConstrRepr. This
indicates that the first field is encoded in the first and second bit of the
whole datatype (0b11).
NB: BitPack for a custom encoding can be derived using
deriveBitPack.
Constructors
| DataReprAnn Type Size [ConstrRepr] |
Instances
data ConstrRepr Source #
Annotation for constructors. Indicates how to match this constructor based off of the whole datatype.
Constructors
| ConstrRepr Name BitMask Value [FieldAnn] |