haskell - Real World Haskell 示例的编译错误

Question

我正在写一段来自“Real World Haskell”的代码:

 ghc --make ch04/InteractWith.hs 
[1 of 1] Compiling Main             ( ch04/InteractWith.hs, ch04/InteractWith.o )

ch04/InteractWith.hs:9:5: parse error on input `args'

dan@dbmint ~/testHaskell $ cat ch04/InteractWith.hs

import System.Environment (getArgs)

interactWith function inputFile outputFile = do
  input <- readFile inputFile
  writeFile outputFile (function input)

main = mainWith myFunction
  where mainWith function = do
    args <- getArgs
    case args of 
      [input, output] -> interactWith function input output
      _ -> putStrLn "error: exactly two arguments needed"

myFunction = id

Answer 1

错误的缩进。 do block (args <- getArgs ... 部分)与 mainWith 的开头处于同一级别定义。

编译没有错误:

import System.Environment (getArgs)

interactWith function inputFile outputFile = do
  input <- readFile inputFile
  writeFile outputFile (function input)

main = mainWith myFunction
  where
    mainWith function = do
      args <- getArgs
      case args of
        [input, output] -> interactWith function input output
        _ -> putStrLn "error: exactly two arguments needed"

myFunction = id