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Common Lisp blocks provide a non-local exit mechanism very
similar to catch and throw, with lexical scoping.
This package actually implements cl-block
in terms of catch; however, the lexical scoping allows the
byte-compiler to omit the costly catch step if the
body of the block does not actually cl-return-from the block.
The forms are evaluated as if by a progn. However,
if any of the forms execute (cl-return-from name),
they will jump out and return directly from the cl-block form.
The cl-block returns the result of the last form unless
a cl-return-from occurs.
The cl-block/cl-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) that evaluate to a tag at run-time; and
also that blocks are always lexically scoped.
In a dynamically scoped catch, functions called from the
catch body can also throw to the catch. This
is not an option for cl-block, where
the cl-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 lambdas 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 cl-defun and cl-defmacro
forms, which do create the implicit block.
The Common Lisp looping constructs defined by this package,
such as cl-loop and cl-dolist, also create implicit blocks
just as in Common Lisp.
Because they are implemented in terms of Emacs Lisp’s catch
and throw, blocks have the same overhead as actual
catch constructs (roughly two function calls). However,
the byte compiler will optimize away the catch
if the block does
not in fact contain any cl-return or cl-return-from calls
that jump to it. This means that cl-do loops and cl-defun
functions that don’t use cl-return don’t pay the overhead to
support it.
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.
This macro is exactly like (cl-return-from nil result).
Common Lisp loops like cl-do and cl-dolist implicitly enclose
themselves in nil blocks.
This macro executes statements while allowing for control transfer to
user-defined labels. Each element of labels-or-statements can
be either a label (an integer or a symbol), or a cons-cell
(a statement). This distinction is made before macroexpansion.
Statements are executed in sequence, discarding any return value.
Any statement can transfer control at any time to the statements that follow
one of the labels with the special form (go label).
Labels have lexical scope and dynamic extent.
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