How does one specify a fixed length string?

[danwdart]

I was starting with:

data Variable = Variable {
    length :: !Word32,
    var :: !ByteString,
    tree :: !Word8
} deriving (Generic, Binary, Show)

but I need the var to be a static length of course. What's the recommended way to do this? Arrays? DataKinds types? Smart constructors on custom types?

Ta

[konsumlamm]

I think the best way is to write the Binary instance yourself. You can use putByteString to serialise a ByteString without its length and getByteString to deserialise it, given the length. The instance would look like this:

instance Binary Variable where
    put (Variable l v t) = do
        put l
        putByteString v
        put t
    get = do
        l <- get
        v <- getByteString (fromIntegral l)
        t <- get
        pure (Variable l v t)

Alternatively, if length is always the length of var, you could just not have a length field in your type and use ByteStrings Binary instance for (de)serialisation and Data.ByteString.length to get the length.