Indentation refers to the white space at the left of a line, which therefore determines the column in which the actual text of the line begins. Indentation is used in normal English text to indicate the beginning of paragraphs, quotations, lists, etc. and hence to indicate the logical structure of a document.
It is very important to use systematic indentation to indicate the logical structure of the source code of a computer program. Whilst the general principles of indentation are largely agreed, precise indentation conventions vary from author to author. The automatic indentation currently provided by REDUCE mode is very inflexible and reflects very much my own style of indentation. Future versions may provide more flexible and customizable indentation.
Currently all indentation is done in steps consisting of a fixed
number of spaces determined by the value of the variable
reduce-indentation, the default value of which is 3;
Options that control REDUCE IDE. This is the
indentation recommended by A. C. Hearn (the principal author of
REDUCE) for the indentation of the first line after a procedure
(header) statement.
REDUCE mode provides fairly intelligent automatic indentation. The
style used is as follows, where the indentation of a child statement is
expressed relative to the parent statement. Each top-level statement is
indented to the left margin. Procedure bodies are indented by one step.
Bodies of multi-line compound and group statements are indented by one
step and labels are exdented to match the beginning of the enclosing
block. Lines that begin with end or >> are exdented to
match the line containing the matching begin or <<.
Bodies of control structures and lines that continue a previous
statement are indented by one step. As parts of larger statements,
compound and group statements themselves are generally not indented if
they occupy multiple lines (because their bodies are indented) but they
are indented if they occupy only a single line.
When a new line that is inserted is being indented, the indentation
can be based only on the preceding code, and not on the code that will
appear in the line. Therefore, it is often necessary to re-indent a
line in order to get consistent indentation. This may seem a little
strange, but it is unavoidable given the syntax of REDUCE and the
indentation style that I have chosen. It is for this reason that the
key C-j runs the command
reindent-then-newline-and-indent rather than just
newline-and-indent.
The command to indent, or re-indent, a line of text is
reduce-indent-line, normally bound (indirectly) to the key
TAB. The execution of reduce-indent-line is
independent of the position of point within the line. It does not
move point relative to the text around it unless point was within the
indentation, in which case it is left before the first non-blank
character (i.e. at the end of the indentation), or at the end of the
line if it contains only white space. Normally, however, the most
convenient way to use automatic indentation is to terminate each line
of code with C-j rather than RET.
See Miscellaneous features.
When called with any argument, which is possible only via the direct
key binding M-i, reduce-indent-line will indent the
current line correctly and then re-indent the rest of the logical
statement containing point by the same amount that the current line
was re-indented. This is not the same as correctly
re-indenting the subsequent lines – it re-indents them rigidly,
without changing their relative indentations at all, and is much
faster.
Indent or re-indent the current line as REDUCE code. Indents to a fixed style determined by the current and previous non-blank lines. With an interactive argument, indent any additional lines of the same statement rigidly together with this one.
Re-indent the current line, insert a newline, then indent the new line.
Indentation of both lines is done using reduce-indent-line, which
is bound by default to TAB.
With the current indentation style, it is not possible in all cases to
determine the correct indentation until after some text has been
entered on a line. This applies to the terminal delimiter of a block
(end) or group (>>) and to an else clause.
Therefore, REDUCE mode can automatically re-indent the current line
once there is enough text to recognise that this is necessary. It
does this only when it is otherwise idle and only when the relevant
text has just been typed. It is not done if the cursor is later moved
onto such a line since it is assumed that the desired indentation has
been set by then. (The indentation of any text can, of course, be
changed at any time, but it will never be automatically changed
retrospectively!)
This facility can be turned on and off by the command
reduce-auto-indent-mode, and it is turned on automatically if
the value of the option reduce-auto-indent-mode-on is non-nil,
which it is by default. The length of idle time required before the
facility will operate is controlled by the option
reduce-auto-indent-delay, and whether the current line is
re-indented by this facility is controlled by the regular expression
that is the value of the option reduce-auto-indent-regexp.
Auto-indentation is on by default. See Customization of the REDUCE IDE.
Toggle REDUCE Auto Indent mode. With a prefix argument, turn the mode
on if and only if the argument is positive. When REDUCE Auto Indent
mode is enabled, after reduce-auto-indent-delay seconds of Emacs
idle time re-indent the current line if the text just typed matches
reduce-auto-indent-regexp.
A section of code can be re-indented using one command if it is first marked as the current region, or the whole buffer or a complete procedure definition can be re-indented by a single command. The latter command works by marking the procedure and then re-indenting the region. Region (and hence procedure) indenting is currently implemented inefficiently by applying the single-line indentation algorithm line-by-line, and hence is very slow for a large region or procedure. In some future version it may be re-implemented more efficiently.
Indent or re-indent the region as REDUCE source code by applying
reduce-indent-line to each line. With a prefix argument it
indents the whole buffer.
Indent or re-indent the procedure (and trailing white space) ending
after point by applying reduce-indent-line to each line.
The command reduce-indent-line-always, bound to
C-TAB, is analogous to reduce-indent-line but
always indents by precisely one additional step consisting of
reduce-indentation spaces. With an argument, it rigidly
indents by one step the current line and the rest of the logical
statement.
The command reduce-unindent-line, bound to S-TAB
(i.e. BACKTAB), is an inverse of
reduce-indent-line-always and decreases the indentation by one
step. With an argument, it rigidly unindents by one step the current
line and the rest of the logical statement.
Indent the current line as REDUCE code by adding
reduce-indentation spaces at the beginning of the line. With
an interactive argument, indent any additional lines of the same
statement rigidly along with this one.
Unindent the current line as REDUCE code by deleting
reduce-indentation spaces (or as many as possible) from the
beginning of the line. With an interactive argument, unindent any
additional lines of the same statement rigidly along with this one.