A Tale of Two Cats

每学一门语言首先想做的是用它写一个类UNIX下cat命令的东西，简单但至少涉及命令行处理和文件I/O。

Scheme版本：

(define (cat . arg)
  (let ((port (if (null? arg)
                  (current-input-port)
                  (car arg))))
    (let loop ((c (read-char port)))
      (if (not (eof-object? c))
          (begin
            (display c)
            (loop (read-char port)))))))

(define (main args)
  (if (null? (cdr args))
      (cat)
      (for-each (lambda (port) (cat port) (close-input-port port))
                (map open-input-file (cdr args)))))

Haskell版本：

import System.Environment

cat :: String -> IO ()
cat fn = do
  contents <- readFile fn
  putStr contents

main = do args <- getArgs
          if null args then interact id else mapM_ cat args

学了Haskell之后才知道Python里面的缩进、List Comprehension等似乎是从Haskell学来的。上面两个cat功能一样，区别是Haskell cat比较懒一点，那是因为Haskell是惰性求值的。勤快一点的版本只需要bytestring重新实现cat函数：

import qualified Data.ByteString.Lazy as B

cat :: String -> IO ()
cat fn = do
  bs <- B.readFile fn
  B.putStr bs