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45. Expect

The macros in this section are made available with:

(use-modules (ice-9 expect))

expect is a macro for selecting actions based on the output from a port. The name comes from a tool of similar functionality by Don Libes. Actions can be taken when a particular string is matched, when a timeout occurs, or when end-of-file is seen on the port. The expect macro is described below; expect-strings is a front-end to expect based on regexec (see the regular expression documentation).

Macro: expect-strings clause ...
By default, expect-strings will read from the current input port. The first term in each clause consists of an expression evaluating to a string pattern (regular expression). As characters are read one-by-one from the port, they are accumulated in a buffer string which is matched against each of the patterns. When a pattern matches, the remaining expression(s) in the clause are evaluated and the value of the last is returned. For example:

(with-input-from-file "/etc/passwd"
  (lambda ()
      ("^nobody" (display "Got a nobody user.\n")
                 (display "That's no problem.\n"))
      ("^daemon" (display "Got a daemon user.\n")))))

The regular expression is compiled with the REG_NEWLINE flag, so that the ^ and $ anchors will match at any newline, not just at the start and end of the string.

There are two other ways to write a clause:

The expression(s) to evaluate can be omitted, in which case the result of the regular expression match (converted to strings, as obtained from regexec with match-pick set to "") will be returned if the pattern matches.

The symbol => can be used to indicate that the expression is a procedure which will accept the result of a successful regular expression match. E.g.,

("^daemon" => write)
("^d\\(aemon\\)" => (lambda args (for-each write args)))
("^da\\(em\\)on" => (lambda (all sub)
                         (write all) (newline)
                         (write sub) (newline)))

The order of the substrings corresponds to the order in which the opening brackets occur.

A number of variables can be used to control the behaviour of expect (and expect-strings). Most have default top-level bindings to the value #f, which produces the default behaviour. They can be redefined at the top level or locally bound in a form enclosing the expect expression.

A port to read characters from, instead of the current input port.
expect will terminate after this number of seconds, returning #f or the value returned by expect-timeout-proc.
A procedure called if timeout occurs. The procedure takes a single argument: the accumulated string.
A procedure called if end-of-file is detected on the input port. The procedure takes a single argument: the accumulated string.
A procedure to be called every time a character is read from the port. The procedure takes a single argument: the character which was read.
Flags to be used when compiling a regular expression, which are passed to make-regexp See section 21.5.1 Regexp Functions. The default value is regexp/newline.
Flags to be used when executing a regular expression, which are passed to regexp-exec See section 21.5.1 Regexp Functions. The default value is regexp/noteol, which prevents $ from matching the end of the string while it is still accumulating, but still allows it to match after a line break or at the end of file.

Here's an example using all of the variables:

(let ((expect-port (open-input-file "/etc/passwd"))
      (expect-timeout 1)
        (lambda (s) (display "Times up!\n")))
        (lambda (s) (display "Reached the end of the file!\n")))
      (expect-char-proc display)
      (expect-strings-compile-flags (logior regexp/newline regexp/icase))
      (expect-strings-exec-flags 0))
     ("^nobody"  (display "Got a nobody user\n"))))

Macro: expect clause ...
expect is used in the same way as expect-strings, but tests are specified not as patterns, but as procedures. The procedures are called in turn after each character is read from the port, with two arguments: the value of the accumulated string and a flag to indicate whether end-of-file has been reached. The flag will usually be #f, but if end-of-file is reached, the procedures are called an additional time with the final accumulated string and #t.

The test is successful if the procedure returns a non-false value.

If the => syntax is used, then if the test succeeds it must return a list containing the arguments to be provided to the corresponding expression.

In the following example, a string will only be matched at the beginning of the file:

(let ((expect-port (open-input-file "/etc/passwd")))
     ((lambda (s eof?) (string=? s "fnord!"))
        (display "Got a nobody user!\n"))))

The control variables described for expect-strings also influence the behaviour of expect, with the exception of variables whose names begin with expect-strings-.

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