Question

This seems so simple, but for some reason I’m confusing myself. The “thing” line is giving me the error.
The parsing functions are correct (stolen from RWH). I just have a type error.

Thanks!

import Text.ParserCombinators.Parsec
import System.IO

main = do 
   csv_cont <- openFile "aCSV.txt" ReadMode
   csv_cont1 <- hGetContents csv_cont
   thing <- parseCSV csv_cont1
   return () 


csvFile = endBy line eol
line = sepBy cell (char ',')
cell = many (noneOf ",\n")
eol = char '\n'

parseCSV :: String -> Either ParseError [[String]]
parseCSV input = parse csvFile "(unknown)" input

Answer 1

parseCSV is a pure function (note, no IO in the type). So you don’t use “do notation” to bind its result. Instead, just regular let is appropriate:

import Text.ParserCombinators.Parsec
import System.IO

main = do
   h <- openFile "aCSV.txt" ReadMode
   s <- hGetContents h
   let thing = parseCSV s
   print thing


csvFile = endBy line eol
line    = sepBy cell (char ',')
cell    = many (noneOf ",\n")
eol     = char '\n'

parseCSV :: String -> Either ParseError [[String]]
parseCSV s = parse csvFile "(unknown)" s

Here, with more idiomatic naming and identation.