Timer API for R7RS Scheme

This library is proposed to the SRFI. See SRFI 120 Timer APIs

Timer is one of the important features to implement a practical program. It is trivial to implement if implementations support thread. However not all Scheme implementations support SRFI-18 nor POSIX thread model. Some of the implementations even don't expose mutex. So it is nice to have the interface to handle timer to hide underlying implementation.

Requirements

This library requires the following SRFIs:

• SRFI 18
• SRFI 19
• SRFI 69
• SRFI 114

To run the test script SRFI 64 is also required.

The only implementation I have tested is Sagittarius (0.6.3) and Gauche (pre0.9.5). I believe it is trivial to support other implementations.

APIs

(make-timer [error-handler])

Creates and starts a timer object. The optional argument error-handler must be a procedure which accepts one argument. If it is given and when a timer task raises an error, then the handler will be invoked and timer will continue if the error-handler wouldn't raise an error. Otherwise whenever an error is raised, timer stops and preserves the error. The error is raised when timer-cancel! procedure is called.

(timer? obj)

Returns #t if given obj is a timer object, otherwise #f.

(timer-cancel! timer)

Stops the given timer. The procedure raises the propagated error if there is. Once a timer is stopped, it will never be able to start again.

(timer-schedule! timer thunk when [period])

Schedules the given thunk as the given timer's task. The when argument specifies when the task will be started. It can be either timer delta object or non negative integer. The task is scheduled on the time when the given when passed from the procedure is called.

If the optional argument period is given, which must be either timer delta object or an integer, then the given task is scheduled as periodical task. The next task is scheduled by adding when and period.

If the period or when is an integer, then it is interpreted as milliseconds.

The executing order of the same timing tasks are not defined.

The procedure returns timer id which is a readable datum such as an integer.

The task is executed on the dynamic environment where the timer is created.

(timer-reschedule! timer id when [period])

Reschedules the task associated to the given id on the given timer. The when and period arguments are the same as timer-schedule!.

Thus to cancel the periodical task, you can specify 0 as period argument.

The procedure returns given id.

(timer-task-remove! timer id)

Removes the task associated to the given id on the given timer. It returns #t if a task is removed, otherwise #f.

(timer-task-exists? timer id)

Returns #t if a task associated to the given id exists, otherwise #f.

(make-timer-delta n unit)

Creates a timer delta object. n must be an integer and unit must be a symbol which represents the time unit. Implementations must support the following units:

• h : hour
• m : minute
• s : second
• ms : millisecond
• us : microsecond
• ns : nanosecond

And may support other unit.

(timer-delta? obj)

Returns #t if given obj is a timer delta object, otherwise #f.

Implementation notes

A task should be able to cancel or reschedule other tasks. But it should not be able to cancel or reschedule itself.

If a task is rescheduled whenever it's executed, the timer doesn't stop its execution. It is rescheduled but the current execution will be continued.