5.7.1 Defining Guile Actions

A Guile action is defined as follows:

(define (function-name header body . rest)
 ...)

Its arguments are:

header

List of message headers. Each list element is a cons

(name . value)

where name is the name of the header field, and value is its value with final CRLF stripped off. Both name and value are strings.

body

A string containing the message body.

rest

Any additional arguments passed to the function from the configuration file (see section Invoking Guile Actions). This argument may be absent if the function is not expected to take optional arguments.

The function must return a cons whose car contains the new message headers, and cdr contains the new message body. If the car is #t, it means that no headers are changed. If the cdr is #t, it means that the body has not changed. If the cdr is #f, Anubis will delete the entire message body.

As the first example, let’s consider a no-operation action, i.e. an action that does not alter the message in any way. It can be written in two ways:

(define (noop-1 header body)
  (cons header body))
  
(define (noop-2 header body)
  (cons #t #t))

The following example is a function that deletes the message body and adds an additional header:

(define (proc header body)
  (cons (append header
                (cons "X-Body-Deleted" "yes"))
        #f))

Let’s consider a more constructive example. The following function checks if the Subject header starts with string ‘ODP:’ (a Polish equivalent to ‘Re:’), and if it does, replaces it with ‘Re:’. It also adds the header

X-Processed-By: GNU Anubis

Additionally, an optional argument can be used. If it is given, it will be appended to the body of the message.

(define (fix-subject hdr body . rest)
  "If the Subject: field starts with characters \"ODP:\", replace
them with \"Re:\".
If REST is not empty, append its car to BODY"
  (cons (append
         (map (lambda (x)
                (if (and (string-ci=? (car x) "subject")
                         (string-ci=? (substring (cdr x) 0 4) "ODP:"))
                    (cons (car x)
                          (string-append "Re:"
                                         (substring (cdr x) 4)))
                    x))
              hdr)
         (list (cons "X-Processed-By" "GNU Anubis")))
        (if (null? rest)
            #t
            (string-append body "\n" (car rest)))))

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