# defer(3tcl) 1 defer "Defered execution ala Go"

## Name

defer - Defered execution

## Synopsis

- package require Tcl 8.6
- package require defer ?1?

- ::defer::defer ?command? ?arg1? ?arg2? ?argN...?
- ::defer::with variableList script
- ::defer::autowith script
- ::defer::cancel ?id...?

## Description

The defer commands allow a developer to schedule actions to happen as part of the current variable scope terminating. This is most useful for dealing with cleanup activities. Since the defered actions always execute, and always execute in the reverse order from which the defer statements themselves execute, the programmer can schedule the cleanup of a resource (for example, a channel) as soon as that resource is acquired. Then, later if the procedure or lambda ends, either due to an error, or an explicit return, the cleanup of that resource will always occur.

## COMMANDS

::defer::defer ?command? ?arg1? ?arg2? ?argN...?

  Defers execution of some code until the current variable scope ends. Each argument is concatencated together to form the script to execute at deferal time. Multiple defer statements may be used, they are executed in the order of last-in, first-out. The return value is an identifier which can be used later with defer::cancel

::defer::with variableList script

  Defers execution of a script while copying the current value of some variables, whose names specified in variableList, into the script. The script acts like a lambda but executes at the same level as the defer::with call. The return value is the same as ::defer::defer

::defer::autowith script

  The same as ::defer::with but uses all local variables in the variable list.

::defer::cancel ?id...?

  Cancels the execution of a defered action. The id argument is the identifier returned by ::defer::defer, ::defer::with, or ::defer::autowith. Any number of arguments may be supplied, and all of the IDs supplied will be cancelled.

## EXAMPLES

    	package require defer 1
    	apply {{} {
    		set fd [open /dev/null]
    		defer::defer close $fd
    	}}

## REFERENCES

## AUTHORS

Roy Keene

## Bugs, Ideas, Feedback

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category defer of the Tcllib Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

When proposing code changes, please provide unified diffs, i.e the output of diff -u.

Note further that attachments are strongly preferred over inlined patches. Attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar.

## Keywords

cleanup, golang

## Category

Utility

## Copyright

Copyright © 2017, Roy Keene