codegen.ml

let prologue =
  {|
{-# language DeriveGeneric #-}
module Parser where
import Data.Csv
import qualified Data.Vector as V
import qualified Data.ByteString.Char8 as BS
import qualified Data.ByteString.Lazy.Char8 as BSL
import GHC.Generics
import Control.Monad
import Text.Read(readEither)
import Data.Time.Calendar

instance FromField DayOfWeek where
parseField f = case readEither (BS.unpack f) of
   Left errMsg -> fail errMsg
   Right r -> pure r
|}

let epilogue =
  {|
instance ToRecord InputRecord where
instance FromRecord InputRecord where
instance ToField DayOfWeek where

main = do
  input <- BSL.readFile "input.csv"
  either putStrLn (V.mapM_ (print :: InputRecord -> IO ())) $ decode HasHeader input
|}

let () =
  print_endline prologue ;
  print_endline {|data InputRecord = InputRecord { |} ;
  let inpath = Sys.argv.(1) in
  Csv.Rows.load ~has_header:true inpath
  |> List.map Csv.Row.to_list
  |> List.mapi (fun i -> function
       | [field; typ; _parser] ->
           let i = i + 2 in
           let field = String.lowercase_ascii field in
           Printf.sprintf "{-# LINE %d \"%s\" #-}\n" i inpath ^
           (if i > 2 then "," else "") ^
           Printf.sprintf "%s :: %s" field typ
       | line ->
           failwith ("Unknown line format: " ^ String.concat "," line))
  |> List.iter print_endline ;
  print_endline {|} deriving (Generic, Show)|} ;
  print_endline epilogue

With this dune file:

(executable
 (name codegen)
 (libraries stdio csv)
 )

Generates this output:

dune exec ./codegen.exe ~/code-gen-tutorial/test/input.csv
                     
module Parser where
import Data.Csv
import qualified Data.ByteString.Lazy.Char8 as BS
import GHC.Generics
import Control.Monad

data InputRecord = InputRecord {
{-# LINE "/home/edwin-work/code-gen-tutorial/test/input.csv" 2 #-}
	Name :: String
{-# LINE "/home/edwin-work/code-gen-tutorial/test/input.csv" 3 #-}
	Age :: Int
{-# LINE "/home/edwin-work/code-gen-tutorial/test/input.csv" 4 #-}
	Date :: DayOfWeek
} deriving (Generic, Show)

instance ToRecord InputRecord where
 
main = do
  input <- BS.readFile "input.csv"
  either putStrLn (mapM print) $ decode HasHeader input

Excellent. Now try to compile generated code with GHC, or generate parser in OCaml