Commit 8b480acb authored by Richard M. Stallman's avatar Richard M. Stallman
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(Printed Representation): Clarify read syntax vs print.

(Floating Point Type): Explain meaning better.
(Symbol Type): Explain uniqueness better.
(Cons Cell Type): Explain empty list sooner.  CAR and CDR later.
List examples sooner.
(Box Diagrams): New subnode broken out.
Some examples moved from old Lists as Boxes node.
(Dotted Pair Notation): Clarify intro.
(Array Type): Clarify.
(Type Predicates): Add hash-table-p.
parent 4182ecfc
......@@ -68,36 +68,37 @@ to use these types can be found in later chapters.
The @dfn{printed representation} of an object is the format of the
output generated by the Lisp printer (the function @code{prin1}) for
that object. The @dfn{read syntax} of an object is the format of the
input accepted by the Lisp reader (the function @code{read}) for that
object. @xref{Read and Print}.
Most objects have more than one possible read syntax. Some types of
object have no read syntax, since it may not make sense to enter objects
of these types directly in a Lisp program. Except for these cases, the
printed representation of an object is also a read syntax for it.
In other languages, an expression is text; it has no other form. In
Lisp, an expression is primarily a Lisp object and only secondarily the
text that is the object's read syntax. Often there is no need to
emphasize this distinction, but you must keep it in the back of your
mind, or you will occasionally be very confused.
that object. Every data type has a unique printed representation.
The @dfn{read syntax} of an object is the format of the input accepted
by the Lisp reader (the function @code{read}) for that object. This
is not necessarily unique; many kinds of object have more than one
syntax. @xref{Read and Print}.
@cindex hash notation
Every type has a printed representation. Some types have no read
syntax---for example, the buffer type has none. Objects of these types
are printed in @dfn{hash notation}: the characters @samp{#<} followed by
a descriptive string (typically the type name followed by the name of
the object), and closed with a matching @samp{>}. Hash notation cannot
be read at all, so the Lisp reader signals the error
@code{invalid-read-syntax} whenever it encounters @samp{#<}.
@kindex invalid-read-syntax
In most cases, an object's printed representation is also a read
syntax for the object. However, some types have no read syntax, since
it does not make sense to enter objects of these types as constants in
a Lisp program. These objects are printed in @dfn{hash notation}: the
characters @samp{#<} followed by a descriptive string (typically the
type name followed by the name of the object), and closed with a
matching @samp{>}. For example:
@example
(current-buffer)
@result{} #<buffer objects.texi>
@end example
@noindent
Hash notation cannot be read at all, so the Lisp reader signals the
error @code{invalid-read-syntax} whenever it encounters @samp{#<}.
@kindex invalid-read-syntax
In other languages, an expression is text; it has no other form. In
Lisp, an expression is primarily a Lisp object and only secondarily the
text that is the object's read syntax. Often there is no need to
emphasize this distinction, but you must keep it in the back of your
mind, or you will occasionally be very confused.
When you evaluate an expression interactively, the Lisp interpreter
first reads the textual representation of it, producing a Lisp object,
and then evaluates that object (@pxref{Evaluation}). However,
......@@ -204,9 +205,11 @@ leading @samp{+} or a final @samp{.}.
@subsection Floating Point Type
Floating point numbers are the computer equivalent of scientific
notation. The precise number of significant figures and the range of
possible exponents is machine-specific; Emacs always uses the C data
type @code{double} to store the value.
notation; you can think of a floating point number as a fraction
together with a power of ten. The precise number of significant
figures and the range of possible exponents is machine-specific; Emacs
uses the C data type @code{double} to store the value, and internally
this records a power of 2 rather than a power of 10.
The printed representation for floating point numbers requires either
a decimal point (with at least one digit following), an exponent, or
......@@ -474,9 +477,10 @@ following text.)
@node Symbol Type
@subsection Symbol Type
A @dfn{symbol} in GNU Emacs Lisp is an object with a name. The symbol
name serves as the printed representation of the symbol. In ordinary
use, the name is unique---no two symbols have the same name.
A @dfn{symbol} in GNU Emacs Lisp is an object with a name. The
symbol name serves as the printed representation of the symbol. In
ordinary Lisp use, with one single obarray (@pxref{Creating Symbols},
a symbol's name is unique---no two symbols have the same name.
A symbol can serve as a variable, as a function name, or to hold a
property list. Or it may serve only to be distinct from all other Lisp
......@@ -606,18 +610,10 @@ Lisp are implicit.
A @dfn{list} is a series of cons cells, linked together so that the
@sc{cdr} slot of each cons cell holds either the next cons cell or the
empty list. @xref{Lists}, for functions that work on lists. Because
most cons cells are used as part of lists, the phrase @dfn{list
structure} has come to refer to any structure made out of cons cells.
The names @sc{car} and @sc{cdr} derive from the history of Lisp. The
original Lisp implementation ran on an @w{IBM 704} computer which
divided words into two parts, called the ``address'' part and the
``decrement''; @sc{car} was an instruction to extract the contents of
the address part of a register, and @sc{cdr} an instruction to extract
the contents of the decrement. By contrast, ``cons cells'' are named
for the function @code{cons} that creates them, which in turn was named
for its purpose, the construction of cells.
empty list. The empty list is actually the symbol @code{nil}.
@xref{Lists}, for functions that work on lists. Because most cons
cells are used as part of lists, the phrase @dfn{list structure} has
come to refer to any structure made out of cons cells.
@cindex atom
Because cons cells are so central to Lisp, we also have a word for
......@@ -627,7 +623,18 @@ for its purpose, the construction of cells.
@cindex parenthesis
The read syntax and printed representation for lists are identical, and
consist of a left parenthesis, an arbitrary number of elements, and a
right parenthesis.
right parenthesis. Here are examples of lists:
@example
(A 2 "A") ; @r{A list of three elements.}
() ; @r{A list of no elements (the empty list).}
nil ; @r{A list of no elements (the empty list).}
("A ()") ; @r{A list of one element: the string @code{"A ()"}.}
(A ()) ; @r{A list of two elements: @code{A} and the empty list.}
(A nil) ; @r{Equivalent to the previous.}
((A B C)) ; @r{A list of one element}
; @r{(which is a list of three elements).}
@end example
Upon reading, each object inside the parentheses becomes an element
of the list. That is, a cons cell is made for each element. The
......@@ -636,8 +643,26 @@ slot refers to the next cons cell of the list, which holds the next
element in the list. The @sc{cdr} slot of the last cons cell is set to
hold @code{nil}.
The names @sc{car} and @sc{cdr} derive from the history of Lisp. The
original Lisp implementation ran on an @w{IBM 704} computer which
divided words into two parts, called the ``address'' part and the
``decrement''; @sc{car} was an instruction to extract the contents of
the address part of a register, and @sc{cdr} an instruction to extract
the contents of the decrement. By contrast, ``cons cells'' are named
for the function @code{cons} that creates them, which in turn was named
for its purpose, the construction of cells.
@menu
* Box Diagrams:: Drawing pictures of lists.
* Dotted Pair Notation:: A general syntax for cons cells.
* Association List Type:: A specially constructed list.
@end menu
@node Box Diagrams
@subsubsection Drawing Lists as Box Diagrams
@cindex box diagrams, for lists
@cindex diagrams, boxed, for lists
A list can be illustrated by a diagram in which the cons cells are
shown as pairs of boxes, like dominoes. (The Lisp reader cannot read
such an illustration; unlike the textual notation, which can be
......@@ -688,19 +713,6 @@ buttercup)}, sketched in a different manner:
to the symbol @code{nil}. In other words, @code{nil} is both a symbol
and a list.
Here are examples of lists written in Lisp syntax:
@example
(A 2 "A") ; @r{A list of three elements.}
() ; @r{A list of no elements (the empty list).}
nil ; @r{A list of no elements (the empty list).}
("A ()") ; @r{A list of one element: the string @code{"A ()"}.}
(A ()) ; @r{A list of two elements: @code{A} and the empty list.}
(A nil) ; @r{Equivalent to the previous.}
((A B C)) ; @r{A list of one element}
; @r{(which is a list of three elements).}
@end example
Here is the list @code{(A ())}, or equivalently @code{(A nil)},
depicted with boxes and arrows:
......@@ -715,27 +727,64 @@ depicted with boxes and arrows:
@end group
@end example
@menu
* Dotted Pair Notation:: An alternative syntax for lists.
* Association List Type:: A specially constructed list.
@end menu
Here is a more complex illustration, showing the three-element list,
@code{((pine needles) oak maple)}, the first element of which is a
two-element list:
@example
@group
--- --- --- --- --- ---
| | |--> | | |--> | | |--> nil
--- --- --- --- --- ---
| | |
| | |
| --> oak --> maple
|
| --- --- --- ---
--> | | |--> | | |--> nil
--- --- --- ---
| |
| |
--> pine --> needles
@end group
@end example
The same list represented in the first box notation looks like this:
@example
@group
-------------- -------------- --------------
| car | cdr | | car | cdr | | car | cdr |
| o | o------->| oak | o------->| maple | nil |
| | | | | | | | | |
-- | --------- -------------- --------------
|
|
| -------------- ----------------
| | car | cdr | | car | cdr |
------>| pine | o------->| needles | nil |
| | | | | |
-------------- ----------------
@end group
@end example
@node Dotted Pair Notation
@comment node-name, next, previous, up
@subsubsection Dotted Pair Notation
@cindex dotted pair notation
@cindex @samp{.} in lists
@dfn{Dotted pair notation} is an alternative syntax for cons cells
that represents the @sc{car} and @sc{cdr} explicitly. In this syntax,
@dfn{Dotted pair notation} is a general syntax for cons cells that
represents the @sc{car} and @sc{cdr} explicitly. In this syntax,
@code{(@var{a} .@: @var{b})} stands for a cons cell whose @sc{car} is
the object @var{a}, and whose @sc{cdr} is the object @var{b}. Dotted
pair notation is therefore more general than list syntax. In the dotted
pair notation, the list @samp{(1 2 3)} is written as @samp{(1 . (2 . (3
. nil)))}. For @code{nil}-terminated lists, you can use either
notation, but list notation is usually clearer and more convenient.
When printing a list, the dotted pair notation is only used if the
@sc{cdr} of a cons cell is not a list.
pair notation is more general than list syntax because the @sc{cdr}
does not have to be a list. However, it is more cumbersome in cases
where list syntax would work. In dotted pair notation, the list
@samp{(1 2 3)} is written as @samp{(1 . (2 . (3 . nil)))}. For
@code{nil}-terminated lists, you can use either notation, but list
notation is usually clearer and more convenient. When printing a
list, the dotted pair notation is only used if the @sc{cdr} of a cons
cell is not a list.
Here's an example using boxes to illustrate dotted pair notation.
This example shows the pair @code{(rose . violet)}:
......@@ -860,8 +909,9 @@ Once an array is created, its length is fixed.
All Emacs Lisp arrays are one-dimensional. (Most other programming
languages support multidimensional arrays, but they are not essential;
you can get the same effect with an array of arrays.) Each type of
array has its own read syntax; see the following sections for details.
you can get the same effect with nested one-dimensional arrays.) Each
type of array has its own read syntax; see the following sections for
details.
The array type is contained in the sequence type and
contains the string type, the vector type, the bool-vector type, and the
......@@ -1661,6 +1711,9 @@ with references to further information.
@item functionp
@xref{Functions, functionp}.
@item hash-table-p
@xref{Other Hash, hash-table-p}.
@item integer-or-marker-p
@xref{Predicates on Markers, integer-or-marker-p}.
......
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