How does one specify a fixed length string?

[danwdart]

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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