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(cl)Blocks and Exits

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Blocks and Exits
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Common Lisp "blocks" provide a non-local exit mechanism very similar to
`catch' and `throw', but lexically rather than dynamically scoped.
This package actually implements `block' in terms of `catch'; however,
the lexical scoping allows the optimizing byte-compiler to omit the
costly `catch' step if the body of the block does not actually
`return-from' the block.

 - Special Form: block name forms...
     The FORMS are evaluated as if by a `progn'.  However, if any of
     the FORMS execute `(return-from NAME)', they will jump out and
     return directly from the `block' form.  The `block' returns the
     result of the last FORM unless a `return-from' occurs.

     The `block'/`return-from' mechanism is quite similar to the
     `catch'/`throw' mechanism.  The main differences are that block
     NAMEs are unevaluated symbols, rather than forms (such as quoted
     symbols) which evaluate to a tag at run-time; and also that blocks
     are lexically scoped whereas `catch'/`throw' are dynamically
     scoped.  This means that functions called from the body of a
     `catch' can also `throw' to the `catch', but the `return-from'
     referring to a block name must appear physically within the FORMS
     that make up the body of the block.  They may not appear within
     other called functions, although they may appear within macro
     expansions or `lambda's in the body.  Block names and `catch'
     names form independent name-spaces.

     In true Common Lisp, `defun' and `defmacro' surround the function
     or expander bodies with implicit blocks with the same name as the
     function or macro.  This does not occur in Emacs Lisp, but this
     package provides `defun*' and `defmacro*' forms which do create
     the implicit block.

     The Common Lisp looping constructs defined by this package, such
     as `loop' and `dolist', also create implicit blocks just as in
     Common Lisp.

     Because they are implemented in terms of Emacs Lisp `catch' and
     `throw', blocks have the same overhead as actual `catch'
     constructs (roughly two function calls).  However, the optimizing
     byte compiler will optimize away the `catch' if the block does not
     in fact contain any `return' or `return-from' calls that jump to
     it.  This means that `do' loops and `defun*' functions which don't
     use `return' don't pay the overhead to support it.

 - Special Form: return-from name [result]
     This macro returns from the block named NAME, which must be an
     (unevaluated) symbol.  If a RESULT form is specified, it is
     evaluated to produce the result returned from the `block'.
     Otherwise, `nil' is returned.

 - Special Form: return [result]
     This macro is exactly like `(return-from nil RESULT)'.  Common
     Lisp loops like `do' and `dolist' implicitly enclose themselves in
     `nil' blocks.