📝Common Lisp: special operators

Special operators are forms that are neither functions, nor macros. They provide lower-level building blocks for the rest of the functions and macros.

In Common Lisp, there are 25 special operators.

• Controlling evaluation:

  • quote, if, progn

• Manipulating the Lexical Environment

  • let, let*

  • setq

  • flet, labels: similar to let but define functions. flet names can only be references in the body, but labels names can be references immediately within labels definitions.

  • macrolet, symbol-macrolet

  • function: gets function object (reader macro: #')

• Local Flow of Control

  • block + return-from: return from a block/function immediately

    • block + return-from work correctly across function boundaries unwinding the stack. (The block label is lexically scoped, not dynamically.)

      • important note: the labels have dynamic extent—you cannot return-from a block that is no longer on a stack.

  • tagbody + go: low-level goto construct

    • tagbody + go also work across function boundaries unwinding the stack as needed

• Unwinding the Stack

  • catch and throw: are dynamic counterparts of block and return-from

    • mostly unused because of condition system

  • unwind-protect: ensure some code is always executed if stack is unwinding

    (unwind-protect protected-form
      cleanup-form*)
    

• Multiple Values

  • multiple-value-call (see Common Lisp: multiple values)

• eval-when

  ;; Basic form:
  (eval-when (situation*)
    body-form*)
  

  Possible situations: :compile-toplevel, :load-toplevel, and :execute

• other

  • locally

  • the

  • load-time-value

  • progv