Re: How to Check whether a file exists and IO[String] vs [String]

Thanks it solved my problem. The second one I solved by just opening the file on main function and passing it as an argument using function "lines" straight on the argument but I got the idea. Thanks a lot. Best Regards Alex ----- Original Message ----- From: "Jon Cast" To: "Alexandre Weffort Thenorio" Cc: Sent: Friday, May 02, 2003 11:56 PM Subject: Re: How to Check whether a file exists and IO[String] vs [String]

Alexandre Weffort Thenorio wrote:

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Hello. I am having two problems. I have a program that reads two texts file and writes a new text with its info. So I have done so that if main file doesn't exist I just output an error string but my problem is when it comes to the second file. This file is not exactly needed so if it is not there, the program should just ignore the file. I tried doing that but if the second file is there I also get an error saying that it couldn;t find the file. So is it possible to check whether a file exists

Yes. See Directory.doesFileExist.

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or not outputting and empty string if not or the actual data from it if so without giving an error and making the program not run??

For example

main = do mainfile <- readFile "xxxxx.txt" secondfile <- readMyFile "yyyy.txt" writeFile "test.txt" mainfile++secondfile

readMyFile file = do list <- readFile file if list == "" then return "" else return list

Apparently this doesn't work and if file doesn't exist it still gives me an error.

Correct. readFile always assumes the file exists (not to mention, if list == "", then list = "", so we can calculate:

readMyFile file = do list <- readFile file if list == "" then return "" else return list = do list <- readFile file if list == "" then return list else return list = do list <- readFile file return list = readFile file

so your readMyFile function as written is unnecessary.)

If you want to take a different action if the file doesn't exists you have to use doesFileExist as above.

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Another problem I ran into is with IO[String] and [String].

I have a function like

foo = do nf <- readFile "xxx.txt" return (lines nf)

Then when I try using foo like a [String] in other programs I run into type error.

True. IO a /= a; they're not even isomorphic.

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How can I turn this IO[String] into a [String] so I don't actually have to open this file in the main and then use it as an argument of another function running "lines' on main method????

If you want to pass the result into f, you can say:

do x <- foo return (f x)

or fmap f foo.

Jon Cast