2 ms·

> To be pedantic, those aren't type declarations. They are values of optimization qualities and guide the compiler strategy. > More importantly, the (un)safet

by lispm 9y ago

> To be pedantic, those aren't type declarations. They are values of optimization qualities and guide the compiler strategy. > More importantly, the (un)safety of type declaration isn't predicated on any specific safety/speed settings. The mere use of a type declaration implies that the programmer certifies their accuracy inside its scope; With an optimize quality 3 for SAFETY a compiler will not reduce safety, even though there are type declarations. The Common Lisp standard REQUIRES that safety at 3 causes 'safe code': http://www.lispworks.com/documentation/HyperSpec/Body/03_eaa.htm http://www.lispworks.com/documentation/HyperSpec/Body/03_eaa... Which usually means full runtime checks in interpreted and compiled code, calls, system calls, ... > an additional optimize declaration is not required. Sure you need to have the right compiler qualities set to see those effects of your type declarations. LispWorks: CL-USER 4 > (proclaim '(optimize (safety 3))) NIL CL-USER 5 > (defun foo (x) (declare (string x)) (+ x 2)) FOO CL-USER 6 > (compile 'foo) FOO NIL NIL CL-USER 7 > (foo 'bar) Error: In + of (BAR 2) arguments should be of type NUMBER. 1 (continue) Return a value to use. 2 Supply a new first argument. 3 (abort) Return to top loop level 0. Type :b for backtrace or :c <option number> to proceed. Type :bug-form "<subject>" for a bug report template or :? for other options. CL-USER 8 : 1 > Oh, the LispWorks compiler has ignored the type declaration at safety 3. The runtime checks are still there. The code has the same safety as without a type declaration.

YouAreGreat 9y ago
Violation of a type declaration is UB in safe code. SBCL obviously makes use of that fact when inserting assertions.
- kazinator 9y ago
  Undefined behavior gives the implementation the freedom to insert any behavior whatsoever (like the proverbial demons flying out of the nose). An assertion isn't any behavior, it is a form of run-time error checking. Run-time error checking is what the "safety" optimization option means! From CLHS: Name Meaning compilation-speed speed of the compilation process debug ease of debugging safety run-time error checking space both code size and run-time space speed speed of the object code (optimize (safety 3)) means "greatest possible run-time error checking". Why wouldn't the implementation insert error checking based on the user's type declaration, when asked to provide the greatest possible run-time error checking?
  - YouAreGreat 9y ago
    > An assertion isn't any behavior It is. For example, a failed assertion inhibits effects that should have occurred before the "actual" type error happens. The only reason SBCL is able to insert assertions into safe code without giving up conforming implementation status is that, as you say, UB allows anything, and violating a declaration is UB even in safe code.
    - kazinator 9y ago
      There isn't necessarily an actual type error! Remember, the declaration provided by the user can be an inaccurate estimate of what the real type constraint is at that program node. The program might in fact require a string there, but the programmer's declaration says integer. So the assertion goes off when a string value occurs, when that would in fact correct in the absence of the declaration. This alteration of behavior by the assertion is not an arbitrary choice of behavior; it is a diagnostic behavior, in line with the diagnostic meaning of safety. ANSI CL does literally say under "Declarations" that "The consequences are undefined if a program violates a declaration or a proclamation". Yet, it's not reasonable to allow any behavior whatsoever if a type declaration is violated by a run-time situation, in safe code. It just makes no sense at all to allow a crash, or random bits being silently flipped, etc. Given that safe code, in the absence of declarations, catches type errors; why would declarations regress from that, in safe code.
      - YouAreGreat 9y ago
        If you re-read everything in this subthread, you'll find a lot of argument from seasoned lispers that I can and should ignore the fact that type declarations are unsafe even at the highest safety settings because of "what implementations do." If this discussion had taken place before SBCL, the logical conclusion from what these seasoned lispers say would have been that I can and should use type declarations in safe code for documentation purposes (say), because implementations ignore them at high safety, e.g. from lispm: "Oh, the LispWorks compiler has ignored the type declaration at safety 3. ... The code has the same safety as without a type declaration." If I had followed such advice from seasoned lispers, liberally putting type declarations into safe code for documentation (say), SBCL's new behavior would have burned me, causing aborts in perfectly working code just because I got my "documentation" wrong. Some observations: (1) SBCL would be allowed to burn me because I invoked UB. Nothing else gives them license to break working code in the presence of a wrong declaration, but they have that license, and I can't sue. (2) I shouldn't have listened to lispm. Risking UB in safe code because most existing implementations wouldn't have burned me (before SBCL) would have been bad advice. Peppering safe code with declarations does nothing to improve safety, risking UB can only ever deduct from safety. (3) As a consequence of (2), encouraging the use of declarations everywhere is wrong. To do so, in the words of PuercoPop, is "missing the point of the Lisp's type system." SBCL is wrong to do it.
- lispm 9y ago
  Exactly. Most implementation will ignore type declarations in safe code. Adding or removing those has no effect. SBCL honors type declarations. Thus it is defined behavior. SBCL defines it. I propose to give up the irrational fear of added services provided by SBCL and actually use it as an additional tool.

by lispm 9y ago

YouAreGreat 9y ago

Violation of a type declaration is UB in safe code. SBCL obviously makes use of that fact when inserting assertions.

kazinator 9y ago
Undefined behavior gives the implementation the freedom to insert any behavior whatsoever (like the proverbial demons flying out of the nose). An assertion isn't any behavior, it is a form of run-time error checking. Run-time error checking is what the "safety" optimization option means! From CLHS: Name Meaning compilation-speed speed of the compilation process debug ease of debugging safety run-time error checking space both code size and run-time space speed speed of the object code (optimize (safety 3)) means "greatest possible run-time error checking". Why wouldn't the implementation insert error checking based on the user's type declaration, when asked to provide the greatest possible run-time error checking?
- YouAreGreat 9y ago
  > An assertion isn't any behavior It is. For example, a failed assertion inhibits effects that should have occurred before the "actual" type error happens. The only reason SBCL is able to insert assertions into safe code without giving up conforming implementation status is that, as you say, UB allows anything, and violating a declaration is UB even in safe code.
  - kazinator 9y ago
    There isn't necessarily an actual type error! Remember, the declaration provided by the user can be an inaccurate estimate of what the real type constraint is at that program node. The program might in fact require a string there, but the programmer's declaration says integer. So the assertion goes off when a string value occurs, when that would in fact correct in the absence of the declaration. This alteration of behavior by the assertion is not an arbitrary choice of behavior; it is a diagnostic behavior, in line with the diagnostic meaning of safety. ANSI CL does literally say under "Declarations" that "The consequences are undefined if a program violates a declaration or a proclamation". Yet, it's not reasonable to allow any behavior whatsoever if a type declaration is violated by a run-time situation, in safe code. It just makes no sense at all to allow a crash, or random bits being silently flipped, etc. Given that safe code, in the absence of declarations, catches type errors; why would declarations regress from that, in safe code.
    - YouAreGreat 9y ago
      If you re-read everything in this subthread, you'll find a lot of argument from seasoned lispers that I can and should ignore the fact that type declarations are unsafe even at the highest safety settings because of "what implementations do." If this discussion had taken place before SBCL, the logical conclusion from what these seasoned lispers say would have been that I can and should use type declarations in safe code for documentation purposes (say), because implementations ignore them at high safety, e.g. from lispm: "Oh, the LispWorks compiler has ignored the type declaration at safety 3. ... The code has the same safety as without a type declaration." If I had followed such advice from seasoned lispers, liberally putting type declarations into safe code for documentation (say), SBCL's new behavior would have burned me, causing aborts in perfectly working code just because I got my "documentation" wrong. Some observations: (1) SBCL would be allowed to burn me because I invoked UB. Nothing else gives them license to break working code in the presence of a wrong declaration, but they have that license, and I can't sue. (2) I shouldn't have listened to lispm. Risking UB in safe code because most existing implementations wouldn't have burned me (before SBCL) would have been bad advice. Peppering safe code with declarations does nothing to improve safety, risking UB can only ever deduct from safety. (3) As a consequence of (2), encouraging the use of declarations everywhere is wrong. To do so, in the words of PuercoPop, is "missing the point of the Lisp's type system." SBCL is wrong to do it.
lispm 9y ago
Exactly. Most implementation will ignore type declarations in safe code. Adding or removing those has no effect. SBCL honors type declarations. Thus it is defined behavior. SBCL defines it. I propose to give up the irrational fear of added services provided by SBCL and actually use it as an additional tool.