Cassava Skip or Ignore Columns - csv

Question

So I have a decent grasp on Cassava for simple use cases but I have this one .csv that has 25 columns, and I only care about the 2nd and 5th columns. Is there any way to do a partial parse of each line or do I need to make 20 _ :: Text parameter declarations in the labmda declaration like below?
Right v -> V.forM_ v $ \ (_ :: Text, thingA :: Text, _ :: Text, _ :: Text, _ :: Text, thingB :: Text,, _ :: Text, _ :: Text, _ :: Text ..... etc
Edit: Also just discovered there's no instance for a 25 column CSV anyhow so even my ridiculous 336 character signature can't work.
Edit': Seems that one solution might be named records (suggested here as a fix for dealing with super wide documents] )

Answer 1

You can write your own FromRecord instance for this. You just need to write a parseRecord method that takes a Record (which is type Vector Field), extracts the desired columns at indexes 1 and 4, and loads them into your data type.
Something like the following will work:
{-# LANGUAGE OverloadedStrings #-}
import Data.Csv
import qualified Data.Vector as V
import Data.Text (Text)
data SomeFields = SomeFields Text Text deriving (Show)
instance FromRecord SomeFields where
parseRecord r = SomeFields <$> parseField (r V.! 1) <*> parseField (r V.! 4)
main = do
print $ (decode NoHeader "1,2,3,4,5,6,7\na,b,c,d,e,f,g\n"
:: Either String (V.Vector SomeFields))

Answer 2

Ended up solving it this way.
data ThingAtoThinBMapping = ThingAtoThinBMapping {
thingA :: Text
, thingB :: Text
} deriving (Eq, Show, Read)
instance FromNamedRecord ThingAtoThinBMapping where
parseNamedRecord r = ThingAtoThinBMapping
<$> r .: "thing_a"
<*> r .: "thing_b"
printABMap = do
csvData <- BL.readFile "reallywide.csv"
case decodeByNameWith decodeOpts csvData of
Left err -> putStrLn err
Right (h, v) -> do
putStrLn $ show h
V.forM_ v $ \ m ->
putStrLn $ show (thingA m, thingB m)