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

[Alexandre Weffort Thenorio]

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" <jcast@ou.edu>
To: "Alexandre Weffort Thenorio" <alethenorio@home.se>
Cc: <haskell-cafe@haskell.org>
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 <alethenorio@home.se> 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
>