[EMAIL PROTECTED] writes:
>I am trying to emulate bash in haskell, and i got a function called shell that waits
>fot the command and then executes it. The problem is that if the file does not exist,
>the program returns to Main (it gets out of the shell). The only thing i want to do
>is to return to the shell after an IOError.
>
>How can i perform such a thing?
You could test if the file exists, like you said before, but that test
would fail once in a while, when another process deletes the file after
your test but before the file is opened. This is called a "race condition"
and should be avoided when possible.
The best thing is to use catch which has the type IO a -> (IOError -> IO
a) -> IO a. An example (untested code) is
catch
(do
hdl<-openFile "/home/bracaman/foo" ReadMode
char<-hGetChar hdl
string<-hGetLine hdl )
--we caught an exception:
(\errorCode->
if isDoesNotExistError errorCode then "return to shell" else
if isPermissionError errorCode then "return to shell" else
--something strange happened.
--re-raise the exception, which will probably return control to Main.
ioError errorCode )
P.S.
Instead of
(do
hdl<-openFile "/home/bracaman/foo" ReadMode
char<-hGetChar hdl
string<-hGetLine hdl
... )
you could have
(do
fileContentsAsABigString<-readFile "/home/bracaman/foo"
... )
but I try to stay away from readFile and getContents because they're
not referentially transparent.
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