specifies that a variable is special, that is it does not have lexical scope. This covers two cases: if the variable is bound in the dynamic environment (for example by
), then the value of that binding is used; otherwise the value in the global environment is used, if any. An error is signaled in safe code if there is no value in either environment. When
is used with a variable, the value in the dynamic environment is modified if the variable is bound in the dynamic environment, otherwise the value in the global environment is modified. Dynamic variables can have a different value in each thread because each thread has is own dynamic environment. The global environment is shared between all threads.
In SMP LispWorks access to special variables (excluding constants) is a little slower than in non-SMP LispWorks. It can be speeded up by declarations of the symbol, normally by using by proclaim or declaim.
The speedup will be pretty small overall in most cases, because access to specials is usually a small part of a program. However, if the Profiler identifies some piece of code as a bottleneck, you will want to optimize it, and your optimizations may include proclamation of some variable as global or dynamic.
The three declarations described in this section are extensions to Common Lisp. All declare the symbol to be
, along with other information. These three declarations are mutually exclusive between themselves and
. That is, declaring a symbol with any of these declarations eliminates the other declaration:
hcl:special-globaldeclares that the symbol is never bound in the dynamic environment.
is very useful, and because of the checks it is reasonably safe. It is useful not only for speed, but also to guard against unintentionally binding variables that should not be bound.
See also defglobal-parameter.
hcl:special-dynamicdeclares that the symbol is always bound in the dynamic environment when it is accessed.
hcl:special-fast-accessdeclares that a symbol should be "fast access".
The semantics of the declaration is the same as
, except that access to the variable is low safety. In addition, the compiler compiles access to the symbol in a way that speeds up the access, but also introduces a tiny reduction in the speed of the whole system. The balance between these effects is not obvious.
LispWorks User Guide and Reference Manual - 21 Dec 2011