[英]haskell write to file it if it does not exists before reading
I am learning haskell and am stumbled upon the parse error on input contents'
compilation error. 我正在学习haskell并且偶然发现了parse error on input contents'
编译错误的parse error on input contents'
错误。
What i want to do : 我想做的事 :
I store my previous session state state in a file. 我将以前的会话状态存储在一个文件中。 I read this file before starting the program. 我在启动程序之前读了这个文件。 However, during the first run of program, file may not exist. 但是,在第一次运行程序期间,文件可能不存在。 In this case, i want to first create file first with default value and then proceed ahead. 在这种情况下,我想首先使用默认值创建文件,然后继续。
main :: IO()
main = do
-- Take input
let fileName = "ashish-temp.txt"
let dummyBoard = take 5 $ repeat "-----"
fileExist <- doesFileExist fileName
if False == fileExist
then writeFile fileName $ unlines dummyBoard
-- getting an error on this line
contents <- readFile fileName
-- do processing () :)
-- i want the value in contents
putStrLn "Done"
Also, i think that rather than writing dummyBoard
to the file i can just initialize the contents with dummyBoard
. 另外,我认为不是将dummyBoard
写入文件,而是可以使用dummyBoard
初始化内容。 But i also failed in doing it. 但我也做不到。 And i guess the way should be the same for both. 我猜两种方式应该是一样的。
Please help. 请帮忙。 Thanks. 谢谢。
Edit Solution: 编辑方案:
else
is required for every if
in haskell. 在haskell中每个if
else
需要else
。
Also another problem that you would face after this problem is : *** Exception: ashish-temp.txt: openFile: resource busy (file is locked)
此问题后您将面临的另一个问题是: *** Exception: ashish-temp.txt: openFile: resource busy (file is locked)
use import qualified System.IO.Strict as S
and S.redFile
for reading file. 使用import qualified System.IO.Strict as S
和S.redFile
来读取文件。
There are some problem with your codebase: 您的代码库存在一些问题:
else
part of your if
expression. 你缺少if
表达式的else
部分。 In Haskell, since if
is an expression , it would need the else
part as opposed to other languages where the if-else
are statements and the else part is not mandatory. 在Haskell中,因为if
是一个表达式,所以它需要else
部分而不是其他语言,其中if-else
是语句而else部分不是必需的。 dim
? 究竟是什么dim
? You have to define it. 你必须定义它。 A working program which shows similar in concept of what you want to do will look like this: 一个工作程序显示类似于你想要做的概念将如下所示:
main :: IO()
main = do
let fileName = "somefile.txt"
fileExist <- doesFileExist fileName
if not fileExist
then writeFile fileName "something"
else return ()
contents <- readFile fileName
-- do stuff with contents here
putStrLn "Done"
You had a second part of the question which hasn't been answered yet. 你有问题的第二部分尚未得到答复。
Also, i think that rather than writing
dummyBoard
to the file i can just initialize the contents withdummyBoard
. 另外,我认为不是将dummyBoard
写入文件,而是可以使用dummyBoard
初始化内容。 But i also failed in doing it. 但我也做不到。 And i guess the way should be the same for both. 我猜两种方式应该是一样的。
Indeed you can, as follows: 的确,你可以如下:
contents <- if fileExist
then readFile fileName
else return $ unlines dummyBoard
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