[Peter Lewerin] 2014-01-11: ''with open file'' is an old programming pattern or idiom that abstracts the following:

   1. open a channel to a file
   1. do something with the channel
   1. close the channel

(When I started this page, I first searched this wiki for similar pages, but still missed a couple of pages (keyword search will only get you so far): [Playing with with] and [using], both of which address the same subject.)

In Tcl code, e.g. on this wiki, this procedure is usually written step-by-step, usually without error handling (and sometimes without actually closing the file):

======
set f [open $myfile]
# ... $f ...
close $f
======

Abstracting away such sequences will make the code less cluttered. A couple of examples:

======
proc slurp {filename} {
    set f [open $filename]
    set res [chan read $f]
    chan close $f
    set res
}

proc fileputs {filename str} {
    set f [open $filename a]
    puts $f $str
    chan close $f
}
======

Doing it this way, one command is needed for every file action. Instead, a generic command can be used for all cases where some kind of operation is to be done on a temporarily open file:

======
proc withOpenFile args {
    if {[llength $args] < 3} {
        error {wrong # args: should be "with-open-file channelIdName filename ?access? ?permissions? script"}
    }

    upvar 1 [lindex $args 0] channelid

    try {
        open {*}[lrange $args 1 end-1]
    } on ok channelid {
        uplevel 1 [lindex $args end]
    } finally {
        catch {chan close $channelid}
    }
}
======

The point of having a `[try]` structure without error handling is that it ensures that the channel will be closed even if the script terminates with an exception.

So, instead of

======
set filename foo.bar
set mytext {foo bar}
# ...
set f [open $filename a]
puts $f $str
chan close $f
# ...
set f [open $filename]
set result [chan read $f]
chan close $f
======

you could write 

======
set filename foo.bar
set mytext {foo bar}
# ...
fileputs $filename $mytext
# ...
set result [slurp $filename]
======

or

======
set filename foo.bar
set mytext {foo bar}
# ...
withOpenFile f $filename a {chan puts $f $mytext}
# ...
set result [withOpenFile f $filename {chan read $f}]
======

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