The Compiler's Handling of Types

Implementation Limitations

Ideally, the compiler would consider all type declarations to be assertions, so that adding type declarations to a program, no matter how incorrect they might be, would never cause undefined behavior. As of SBCL version 0.8.1, the compiler is known to fall short of this goal in two areas:

• Proclaimed constraints on argument and result types of a function are supposed to be checked by the function. If the function type is proclaimed before function definition, type checks are inserted by the compiler, but the standard allows the reversed order, in which case the compiler will trust the declaration.

• The compiler cannot check types of an unknown number of values; if the number of generated values is unknown, but the number of consumed is known, only consumed values are checked.

• There are a few poorly characterized but apparently very uncommon situations where a type declaration in an unexpected location will be trusted and never checked by the compiler.

These are important bugs, but are not necessarily easy to fix, so they may, alas, remain in the system for a while.

Type Errors at Compile Time

If the compiler can prove at compile time that some portion of the program cannot be executed without a type error, then it will give a warning at compile time. It is possible that the offending code would never actually be executed at run-time due to some higher level consistency constraint unknown to the compiler, so a type warning doesn't always indicate an incorrect program. For example, consider this code fragment:

(defun raz (foo)
  (let ((x (case foo
             (:this 13)
             (:that 9)
             (:the-other 42))))
    (declare (fixnum x))
    (foo x)))

in: DEFUN RAZ
  (CASE FOO (:THIS 13) (:THAT 9) (:THE-OTHER 42))
--> LET COND IF COND IF COND IF
==>
  (COND)
caught WARNING: This is not a FIXNUM:
  NIL

This sort of spurious type warning happens moderately often in the expansion of complex macros and in inline functions. In such cases, there may be dead code that is impossible to correctly execute. The compiler can't always prove this code is dead (could never be executed), so it compiles the erroneous code (which will always signal an error if it is executed) and gives a warning.

Type warnings are inhibited when the sb-ext:inhibit-warnings optimization quality is 3. (See the section on compiler policy.) This can be used in a local declaration to inhibit type warnings in a code fragment that has spurious warnings.

Precise Type Checking

With the default compilation policy, all type declarations are precisely checked, except in a few situations where they are simply ignored instead. Precise checking means that the check is done as though typep had been called with the exact type specifier that appeared in the declaration. In SBCL, adding type declarations makes code safer. (Except that as noted elsewhere, remaining bugs in the compiler's handling of types unfortunately provide some exceptions to this rule.)

If a variable is declared to be (integer 3 17) then its value must always be an integer between 3 and 17. If multiple type declarations apply to a single variable, then all the declarations must be correct; it is as though all the types were intersected producing a single and type specifier.

Argument and result type declarations are automatically enforced. If you declare the type of a function argument, a type check will be done when that function is called. In a function call, the called function does the argument type checking.

The types of structure slots are also checked. The value of a structure slot must always be of the type indicated in any :type slot option.

In traditional Common Lisp compilers, not all type assertions are checked, and type checks are not precise. Traditional compilers blindly trust explicit type declarations, but may check the argument type assertions for built-in functions. Type checking is not precise, since the argument type checks will be for the most general type legal for that argument. In many systems, type declarations suppress what little type checking is being done, so adding type declarations makes code unsafe. This is a problem since it discourages writing type declarations during initial coding. In addition to being more error prone, adding type declarations during tuning also loses all the benefits of debugging with checked type assertions.

To gain maximum benefit from the compiler's type checking, you should always declare the types of function arguments and structure slots as precisely as possible. This often involves the use of or, member, and other list-style type specifiers.

Weakened Type Checking

At one time, CMU CL supported another level of type checking, "weakened type checking", when the value for the speed optimization quality is greater than safety, and safety is not 0. The CMU CL manual still has a description of it, but even the CMU CL code no longer corresponds to the manual. Some of this partial safety checking lingers on in SBCL, but it's not a supported feature, and should not be relied on. If you ask the compiler to optimize speed to a higher level than safety, your program is performing without a safety net, because SBCL may at its option believe any or all type declarations with either partial or nonexistent runtime checking.

Getting Existing Programs to Run

Since SBCL's compiler, like CMU CL's compiler, does much more comprehensive type checking than most Lisp compilers, SBCL may detect type errors in programs that have been debugged using other compilers. These errors are mostly incorrect declarations, although compile-time type errors can find actual bugs if parts of the program have never been tested.

Some incorrect declarations can only be detected by run-time type checking. It is very important to initially compile a program with full type checks (high safety optimization) and then test this safe version. After the checking version has been tested, then you can consider weakening or eliminating type checks. This applies even to previously debugged programs, because the SBCL compiler does much more type inference than other Common Lisp compilers, so an incorrect declaration can do more damage.

The most common problem is with variables whose constant initial value doesn't match the type declaration. Incorrect constant initial values will always be flagged by a compile-time type error, and they are simple to fix once located. Consider this code fragment:

(prog (foo)
  (declare (fixnum foo))
  (setq foo ...)
  ...)

(prog (foo)
  (declare (type (or fixnum null) foo))
  (setq foo ...)
  ...)

(prog ((foo 0))
  (declare (fixnum foo))
  (setq foo ...)
  ...)

Another declaration problem occasionally encountered is incorrect declarations on defmacro arguments. This can happen when a function is converted into a macro. Consider this macro:

(defmacro my-1+ (x)
  (declare (fixnum x))
  `(the fixnum (1+ ,x)))

(my-1+ (+ 4 5))

(defmacro my-1+ (x)
  `(the fixnum (1+ (the fixnum ,x))))

Some more subtle problems are caused by incorrect declarations that can't be detected at compile time. Consider this code:

(do ((pos 0 (position #\a string :start (1+ pos))))
    ((null pos))
  (declare (fixnum pos))
  ...)

In this case, the fix is to weaken the type declaration to (or fixnum null). [1] Note that there is usually little performance penalty for weakening a declaration in this way. Any numeric operations in the body can still assume that the variable is a fixnum, since nil is not a legal numeric argument. Another possible fix would be to say:

(do ((pos 0 (position #\a string :start (1+ pos))))
    ((null pos))
  (let ((pos pos))
    (declare (fixnum pos))
    ...))