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 ()
    (expect-strings
      ("^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). By default they are all bound at the top level 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.

expect-port

A port to read characters from, instead of the current input port.

expect-timeout

expect will terminate after this number of seconds, returning #f or the value returned by expect-timeout-proc.

expect-timeout-proc

A procedure called if timeout occurs. The procedure takes a single argument: the accumulated string.

expect-eof-proc

A procedure called if end-of-file is detected on the input port. The procedure takes a single argument: the accumulated string.

expect-char-proc

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.

Here's an example using all of the variables:

(let ((expect-port (open-input-file "/etc/passwd"))
      (expect-timeout 1)
      (expect-timeout-proc
        (lambda (s) (display "Times up!\n")))
      (expect-eof-proc
        (lambda (s) (display "Reached the end of the file!\n")))
      (expect-char-proc display))
   (expect-strings
     ("^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 the value of the accumulated string as the argument. 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")))
  (expect
     ((lambda (s) (string=? s "fnord!"))
        (display "Got a nobody user!\n"))))

The control variables described for expect-strings can also be used with expect.

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