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guarded-commands Notes

What it Is

guarded-commands is a Common Lisp port of my favourite Perl module, Commands::Guarded. As its inspiration says, “Better scripts through guarded commands.”.

Usage

(let ((a 0))
  (with-task
    (with-step "something"
      (ensure (= a 1))
      (using (setf a 1)))))

Tasks

A task is a collection of steps working towards a specific goal.

DEFINE-TASK name args &body
Creates a function which performs a task.
WITH-TASK &body
Creates an unnamed task. Basically a progn, but with appropriate wrappers for rolling back steps and so forth.

Steps

A step accomplishes something. Generally, it accomplishes one thing.

WITH-STEP name (step-part &body)+
A step. May be nested.

Parts of a Step

Each step is broken up into several declarative pieces:

ENSURE
Defines a condition which checks that a step has completed successfully, or does not need to be run. If ENSURE is false, the USING block will be run; if USING has already been run, an error will be signalled.
USING
Defines code to affect the ENSURE condition. Will not be run if ENSURE is already true. USING supresses the interactive debugger (like ignore-errors, except condition handlers outside of the USING block may still be run) unless guarded-commands::*debug* is true.
SANITY
Defines an invariant condition, which must be true both before and after this step has run. It is checked before ENSURE, and after USING.
ROLLBACK
Defines code to revert this step. If this step fails—or any step after it within the task, for that matter—the rollback will be run. Rollbacks are run in LIFO order. Warning: A non-local exit from within a rollback will cause the remaining rollbacks to be aborted.

ENSURE is required, all others are optional.

Differences from Commands::Guarded

  • steps should be grouped within a define-task or with-task. The -task forms are what provide the rollback functionality.
  • USING blocks prevent errors from hitting the debugger. C::G makes no attempt to shield you from an error in USING aborting your program, which seems to me to be somewhat contrary to its goals. Debugger avoidance is coded specifically to allow HANDLER-BIND and friends work fine. See also guarded-commands::*debug*, which is a macroexpansion-time option to allow hitting the interactive debugger.
  • STEPs are not objects, and can not be collected and called later.
  • STEP is called WITH-STEP in guarded-commands to avoid conflicting with CL:STEP.

Commands::Guarded Notes

Stuff in C::G:

  • step <name>
  • ensure <condition> The condition to determine whether the step needs to be performed, and if it successfully completed.
  • using <body> The code that performs the step.
  • sanity <condition> An invariant that should be true both before the step is executed (before ensure, actually), and after it completes.
  • rollback <body> Code to reset things if this step, or any subsequent step, fails.
  • clear-rollbacks ? Clears the list of rollbacks to be performed. May not be neccessary, if we can group things with a macro or whatever.

Useful utilities provided by C::G:

  • fgrep <regexp> <file> Returns true if <file> has a line matching <regexp>.
  • readf / writef / appendf

Examples

(step “create directory” (ensure (probe-file dir)) (using (create-directory dir))) => something like => (flet ((#:ensure-132 () (probe-file dir))) (unless (#:ensure-132) (ignore-errors (create-directory dir))) (unless (#:ensure-132) (error “step ~a failed!” “create directory”)))

(step “retarget symlink” (ensure (string= dest (read-symlink file)) (using (create-symlink file dest)) (rollback (create-symlink file orig-dest))))