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

2 May 2003

      Alexandre Weffort Thenorio  wrote:
...
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.
...
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.
...
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.
...
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

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

Jon Cast

Alexandre Weffort Thenorio

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