GNU Guile fails test suite

[christoff-buerger]

GNU Guile fails some of the rewrite and Petri net tests (rewrite-basics, rewrite-lists, rewrite-refine-abstract, purchase-processing, runtime-structure-example-slide).

Further investigation for the reasons and fixes, if it is not an implementation error of GNU Guile, are required.

Note: Other R6RS Scheme systems pass these tests; most likely GNU Guile is bugged.

[christoff-buerger]

The equal? implementation of GNU Guile does not satisfy the R6RS standard. It has problems with cyclic entities. RACR ASTs are cyclic (each node knows its parent, the parent knows its children etc). This problem is particularly tricky, because nodes can be cached or used as keys for caching; since RACR uses equal-hashtables this may break Guile.

[christoff-buerger]

To fix the Petri net composition tests (purchase-processing and runtime-structure-example-slide) is more complicated (cf. ac0c706). It is postponed because the reason seems to be the erroneous equal? implementation of Guile. It looks like a Guile fix requires to switch to eq? hashtables instead of equal?-based hashtables for attribute caching.

Regarding the failed Petri net test cases, the circularity cache used for the evaluation of the fused-places attribute seems to fail because of the usage of equal?-based hashtables. A hint for this explanation is, that the Petri net test cases non-deterministically (i.e., sometimes) fail with a stack overflow, which indicates missing cache hits. The reason is not 100% clear though.

[christoff-buerger]

The issues of equal? in Guile are documented in Guile's reference manual, Section 7.6.1 Incompatibilities with the R6RS (http://www.gnu.org/software/guile/manual/html_node/R6RS-Incompatibilities.html#R6RS-Incompatibilities).

[christoff-buerger]

Since commit 8cb1b57 the composed Petri net examples are excluded from RACR's test suite for GNU Guile.

[christoff-buerger]

Regarding to issue #55, the problems of GNU Guile may also be due to its equal-hash implementation not working properly if cyclic entities are used as keys in hash-tables. Such keys are used to cache parameterised attributes whose arguments are AST nodes, which are by definition cyclic data structures (each node knows its parent, which in turn knows its children etc).