For encoding and decoding newtype quantities as scientific with a fixed number of decimal places and with units.

With these unit definitons;

[u| m |]
[u| km = 1000 m |]

Let's say we have distances in kilometres we'd like encoded with 3 decimal places.

>>> :{
newtype Distance a = Distance a deriving (Eq, Ord, Show)
instance (q ~ Quantity Double [u| km |]) => DefaultDecimalPlaces (Distance q) where
    defdp _ = DecimalPlaces 3
instance (q ~ Quantity Double [u| km |]) => Newtype (Distance q) q where
    pack = Distance
    unpack (Distance a) = a
:}

Encoding and decoding JSON.

>>> :{
instance (q ~ Quantity Double [u| km |]) => ToJSON (Distance q) where
    toJSON x = toJSON $ ViaQ x
instance (q ~ Quantity Double [u| km |]) => FromJSON (Distance q) where
    parseJSON o = do ViaQ x <- parseJSON o; return x
:}

>>> [u| 112233.445566 km |]
[u| 112233.445566 km |]
>>> encode (Distance [u| 112233.445566 km |])
"\"112233.446 km\""
>>> let Just x :: Maybe (Distance (Quantity Double [u| km |])) = decode (encode (Distance [u| 112233.445566 km |])) in x
Distance [u| 112233.446 km |]

Similarly for CSV.

>>> :{
instance (q ~ Quantity Double [u| km |]) => ToField (Distance q) where
    toField x = toField $ ViaQ x
instance (q ~ Quantity Double [u| km |]) => FromField (Distance q) where
    parseField c = do ViaQ x <- parseField c; return x
:}

>>> let d = Distance [u| 112233.445566 km |]
>>> Csv.encode [("A", d)]
"A,112233.446 km\r\n"
>>> Csv.decode NoHeader (Csv.encode [("B", d)]) == Right (fromList [("B", d)])
False
>>> Csv.decode NoHeader (Csv.encode [("C", d)]) == Right (fromList [("C", Distance [u| 112233.446 km |])])
True