After struggling mentally with this for a few weeks, I would like to have some consultation before I introduce some changes in ECL -- not that I expect many users here, but at least some implementor-fellows and power users of other implementations.<div>

<br></div><div>My concerns right now relate to how declarations should be used by a compiler, and in particular how declarations interact with SAFETY levels. Please correct me if I am wrong, but I have seen more or less the following approaches</div>

<div><br></div><div>[a]- Most implementations blindly believe declarations below a certain safety level. Above it, they seem more or less useless.</div><div><br></div><div>[b]- SBCL takes declarations (and THE) as type assertions. For instance, in (LET ((Y (FOO X))) (DECLARE (FIXNUM Y))) ...) the assignment to Y would be checked to be a FIXNUM. This means the type declaration is actually enforced and believed and only at SAFETY 0 the checks are dropped (*)</div>

<div><br></div><div>In both cases one ends up with a model in which in order to truly believe a declaration and have no extra burden (assertions), one has to drop to SAFETY 0 in all code that is involved with it, which is a mess, because it might inadvertently affect other parts of the code. It is for this reason that I am considering an alternative model for ECL which would grade safety as follows</div>

<div><br></div><div>- Type declarations are always believed</div><div>- SAFETY &gt;= 1 adds type checks to enforce them.</div><div>- SAFETY = 0, no checks.</div><div>- SAFETY = 1, the special form THE or additional proclamations on the functions can be used to deactivate the check. As in (LET ((Y (THE FIXNUM (FOO X))) ...)</div>

<div><br></div><div>This would allow one to keep most code safe, while deactivating some checks when they are really known to be true (**). Do you think this is useful/useless? The problem I see with this approach is that all code around is written for model [a] or [b], but I could not come up with something more sensible so far.</div>

<div><br></div><div>Juanjo</div><div><br></div><div>(*) Actually the checks are also deactivated when SBCL can infer the type of the value that is assigned to Y. This is somewhat contradictory, because (SETF Y (THE FIXNUM (FOO X))) would still generate a check, but proclaiming FOO to return a FIXNUM would completely bypass the check.</div>

<div><br></div><div>(**) Yes, indeed I know that LOCALLY exists for a reason, but it does more than THE. For instance, if I (LOCALLY (DECLARE (SAFETY 0)) (THE FIXNUM (FOO (SLOT-ACCESSOR X)))... this influences the safety of the code that accesses a structure, which is not good.</div>

<div><br></div><div>P.S.: Thanks to Paul Khuong for pointing out that SBCL behaves differently w.r.t. declarations.</div><div><div><br></div>-- <br>Instituto de Física Fundamental, CSIC<br>c/ Serrano, 113b, Madrid 28006 (Spain) <br>

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