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• egg

hopefully

Simple composable transactional memory.

The API is similar to Clojure and STMX. However (currently) limited.

Overview

This egg provides two modules: hopefully and hopefully-current.

Module hopefully contains bindings recommended to use. These transactions are roughly as fast as normal locking (ca. 10% faster than locking).

Module hopefully-current adds support for the concept of an implicit current transaction, which is nice but and order of magnitude more expensive at runtime.

Modules

hopefully

[syntax] define-a-record

Like define-record, defines procedures according to the following pattern.

[procedure] (make-TYPE FIELDS ...) -> AREC
[procedure] (TYPE-FIELD-ref AREC TNX) => REF

Return a reference for use with CELL-REF (alias "@") and ALTER!.

Note: Try to use this accessor just once per object+field. Multiple calls (within a lightweight transaction) will produce independent references. (If these become inconsistent, the commit will fail nevertheless.) Only those references used to (set! (@ REF) val) will return the in-transaction value.

[procedure] (TYPE-FIELD AREC)

Return the FIELD value visible without transactions in effect.

[procedure] (TYPE-tag AREC)

Purely for debugging. Mayb e removed. Return the internal version tag of the slot.

[procedure] (cell-ref REF)

Retrieve the in-transaction value from the REFerence (and add it to the transactions dependencies).

[procedure] (@ REF)

Alias to cell-ref. With generalized setter.

[procedure] (alter! REF val)

Alter a REFerence (produced by the type-field-ref accessors to hold the new value val. This also adds the cell to the dependencies of the transaction associated with the REF.

[procedure] (call-with-transaction PROC)

Call PROC with one argument, a fresh (lightweight) transaction. PROC may be called multiple times in case of a conflict. (See hopefully-current for the difference to heavy transactions).

Returns whatever PROC returns.

Note: One should not pass the transaction argument around among threads or capture it. Most (if not all) resulting conditions should be handled. But it is no good idea.

hopefully-current

This module introduces the concept of a default current-transaction and heavy transactions.

When a reference is added to a heavy transaction, the transactions dependencies are searched and if a reference to the same object+slot is found, it is returned.

[syntax] define-ac-record

Like define-a-record.

define-ac-record is provided for maximum compatibility with define-record. Just changing the record definition should make code aware of the current transaction.

[procedure] (make-TYPE FIELDS..) -> ACREC

Accessors:

[procedure] (TYPE-FIELD-ref ACREC TNX) => REF

Return a reference to the in-transaction value of field in ACREC for use with CELL-REF (alias "@") and ALTER!. See sibling definition in hopefully.

[procedure] (TYPE-FIELD ACREC)

Return the value of field in ACREC. Returns the in-transaction value with respect to the current-transaction or the outside value if there is no current-transaction in the current-thread.

Note: this is roughly an order of magnitude slower than the corresponding accessor from define-a-record

[procedure] (TYPE-FIELD-set! ACREC val) -> undefined

Set the value of field to val. Changes the in-transaction value respect to the current-transaction or the outside value if there is no current-transaction in the current-thread.

Note: this is roughly an order of magnitude slower than the corresponding accessor from define-a-record

[procedure] (with-current-transaction THUNK)

Establish a new current-transaction and call THUNK. After thunk completed, commit the current transaction. If that failes, THUNK is called again.

Returns whatever THUNK returns.

Examples

(define-a-record gbox v)
(define b1 (make-gbox 0))
(define b2 (make-gbox 1))

(call-with-transaction
 (lambda (tnx)
   (let ((x (gbox-v-ref b1 tnx))
         (i (gbox-v-ref b2 tnx)))
     (let ((ni (@ i)))
       (alter! x (+ (@ x) ni))
       (alter! i (add1 ni))))))

See also tests/run.scm.

About this egg

Source

Authors

Jörg F. Wittenberger