GADTs in the wild

[Scott Michel]

Probably useful to include a "mkFixed" function example as well, to show
how a Fixed can be constructed using the "optimal" data representation:

-- | Make a fixed field.
--
-- Note that this type constructor function chooses the minimal type
-- representation for the fixed value stored. Unsigned representations
-- are preferred over signed.
mkFixed :: String -> Int -> Integer -> Member
mkFixed name len val
  | len <= 0  = error $ "mkFixed " ++ name ++ ": length < 0"
  | len < 8  && validUnsigned len val = Fixed name len False (fromIntegral
val :: Word8)
  | len < 8  && validSigned   len val = Fixed name len True  (fromIntegral
val :: Int8)
  | len < 16 && validUnsigned len val = Fixed name len False (fromIntegral
val :: Word16)
  | len < 16 && validSigned   len val = Fixed name len True  (fromIntegral
val :: Int16)
  | len < 32 && validUnsigned len val = Fixed name len False (fromIntegral
val :: Word32)
  | len < 32 && validSigned   len val = Fixed name len True  (fromIntegral
val :: Int32)
  | len < 64 && validUnsigned len val = Fixed name len False (fromIntegral
val :: Word64)
  | len < 64 && validSigned   len val = Fixed name len True  (fromIntegral
val :: Int64)
  | otherwise = error $ "mkFixed " ++ name ++ ": cannot represent this bit
field"


On Thu, Aug 23, 2012 at 11:47 AM, Scott Michel <scooter.phd at gmail.com>wrote:

> Here's an example (not a complete module) I was using to represent
> bit-oriented structures as they occur in certain space applications,
> notably GPS frames. "Fixed" allows for fixed-sized fields and lets the end
> user choose the integral type that's best for the structure.
>
> At least it's not a parser or language example. :-)
>
>
> -scooter
>
> -- | Member fields, etc., that comprise a 'BitStruct'
> data Member where
>   Field    :: String                    -- Field name
>               -> Int                    -- Field length
>               -> Bool                   -- Signed (True) or unsigned
> (False)
>               -> Member
>   ZeroPad  :: String                    -- Field name
>               -> Int                    -- Field length
>               -> Member
>   OnesPad  :: String                    -- Field name
>               -> Int                    -- Field length
>               -> Member
>   ArbPad   :: String                    -- Field name
>               -> Int                    -- Field length
>               -> Member
>   Reserved :: String                    -- Field name
>               -> Int                    -- Field length
>               -> Member
>   Fixed    :: (Integral x, Show x) =>
>               String                    -- Field name
>               -> Int                    -- Field length
>               -> Bool                   -- Signed (True) or unsigned
> (False)
>               -> x                      -- Type of the fixed field's value
>               -> Member
>   Variant  :: (Integral x, Show x) =>
>               String                    -- Variant prefix name
>               -> Maybe BitStruct        -- Header before the tag
>               -> TagElement             -- The tag element itself
>               -> Maybe BitStruct        -- Common elements after the tag
>               -> Seq (x, BitStruct)     -- Variant element tuples (value,
> structure)
>               -> Member
>   -- Mult-value variant: Use this when multiple variant tag values have the
>   -- same structure:
>   MultiValueVariant :: (Integral x, Show x) =>
>               String                            -- Variant prefix name
>               -> Maybe BitStruct                -- Header before the tag
>               -> TagElement                     -- The tag element itself
>               -> Maybe BitStruct                -- Common elements after
> the tag
>               -> Seq ([x], BitStruct)           -- Variant element tuples
> ([values], structure)
>               -> Member
>
>
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