24.20. Fortran Mode

Fortran mode provides special motion commands for Fortran statements and subprograms, and indentation commands that understand Fortran conventions of nesting, line numbers and continuation statements. Fortran mode has its own Auto Fill mode that breaks long lines into proper Fortran continuation lines.

Special commands for comments are provided because Fortran comments are unlike those of other languages. Built-in abbrevs optionally save typing when you insert Fortran keywords.

Use M-x fortran-mode to switch to this major mode. This command runs the hook fortran-mode-hook (Section 32.2.3).

Fortan mode is meant for editing Fortran77 "fixed format" source code. For editing the modern Fortran90 "free format" source code, use F90 mode (f90-mode). Emacs normally uses Fortran mode for files with extension .f, .F or .for, and F90 mode for the extension .f90. GNU Fortran supports both kinds of format.

24.20.1. Motion Commands

In addition to the normal commands for moving by and operating on "defuns" (Fortran subprograms--functions and subroutines), Fortran mode provides special commands to move by statements.

C-c C-n

Move to beginning of current or next statement (fortran-next-statement).

C-c C-p

Move to beginning of current or previous statement (fortran-previous-statement).

24.20.2. Fortran Indentation

Special commands and features are needed for indenting Fortran code in order to make sure various syntactic entities (line numbers, comment line indicators and continuation line flags) appear in the columns that are required for standard Fortran.

24.20.2.1. Fortran Indentation and Filling Commands

C-M-j

Break the current line and set up a continuation line (fortran-split-line).

M-^

Join this line to the previous line (fortran-join-line).

C-M-q

Indent all the lines of the subprogram point is in (fortran-indent-subprogram).

M-q

Fill a comment block or statement.

The key C-M-q runs fortran-indent-subprogram, a command to reindent all the lines of the Fortran subprogram (function or subroutine) containing point.

The key C-M-j runs fortran-split-line, which splits a line in the appropriate fashion for Fortran. In a non-comment line, the second half becomes a continuation line and is indented accordingly. In a comment line, both halves become separate comment lines.

M-^ or C-c C-d runs the command fortran-join-line, which joins a continuation line back to the previous line, roughly as the inverse of fortran-split-line. The point must be on a continuation line when this command is invoked.

M-q in Fortran mode fills the comment block or statement that point is in. This removes any excess statement continuations.

24.20.2.2. Continuation Lines

Most modern Fortran compilers allow two ways of writing continuation lines. If the first non-space character on a line is in column 5, then that line is a continuation of the previous line. We call this fixed format. (In GNU Emacs we always count columns from 0.) The variable fortran-continuation-string specifies what character to put on column 5. A line that starts with a tab character followed by any digit except 0 is also a continuation line. We call this style of continuation tab format.

Fortran mode can make either style of continuation line, but you must specify which one you prefer. The value of the variable indent-tabs-mode controls the choice: nil for fixed format, and non-nil for tab format. You can tell which style is presently in effect by the presence or absence of the string Tab in the mode line.

If the text on a line starts with the conventional Fortran continuation marker $, or if it begins with any non-whitespace character in column 5, Fortran mode treats it as a continuation line. When you indent a continuation line with TAB, it converts the line to the current continuation style. When you split a Fortran statement with C-M-j, the continuation marker on the newline is created according to the continuation style.

The setting of continuation style affects several other aspects of editing in Fortran mode. In fixed format mode, the minimum column number for the body of a statement is 6. Lines inside of Fortran blocks that are indented to larger column numbers always use only the space character for whitespace. In tab format mode, the minimum column number for the statement body is 8, and the whitespace before column 8 must always consist of one tab character.

When you enter Fortran mode for an existing file, it tries to deduce the proper continuation style automatically from the file contents. The first line that begins with either a tab character or six spaces determines the choice. The variable fortran-analyze-depth specifies how many lines to consider (at the beginning of the file); if none of those lines indicates a style, then the variable fortran-tab-mode-default specifies the style. If it is nil, that specifies fixed format, and non-nil specifies tab format.

24.20.2.3. Line Numbers

If a number is the first non-whitespace in the line, Fortran indentation assumes it is a line number and moves it to columns 0 through 4. (Columns always count from 0 in GNU Emacs.)

Line numbers of four digits or less are normally indented one space. The variable fortran-line-number-indent controls this; it specifies the maximum indentation a line number can have. Line numbers are indented to right-justify them to end in column 4 unless that would require more than this maximum indentation. The default value of the variable is 1.

Simply inserting a line number is enough to indent it according to these rules. As each digit is inserted, the indentation is recomputed. To turn off this feature, set the variable fortran-electric-line-number to nil. Then inserting line numbers is like inserting anything else.

24.20.2.4. Syntactic Conventions

Fortran mode assumes that you follow certain conventions that simplify the task of understanding a Fortran program well enough to indent it properly:

  • Two nested do loops never share a continue statement.

  • Fortran keywords such as if, else, then, do and others are written without embedded whitespace or line breaks.

    Fortran compilers generally ignore whitespace outside of string constants, but Fortran mode does not recognize these keywords if they are not contiguous. Constructs such as else if or end do are acceptable, but the second word should be on the same line as the first and not on a continuation line.

If you fail to follow these conventions, the indentation commands may indent some lines unaesthetically. However, a correct Fortran program retains its meaning when reindented even if the conventions are not followed.

24.20.2.5. Variables for Fortran Indentation

Several additional variables control how Fortran indentation works:

fortran-do-indent

Extra indentation within each level of do statement (default 3).

fortran-if-indent

Extra indentation within each level of if statement (default 3). This value is also used for extra indentation within each level of the Fortran 90 where statement.

fortran-structure-indent

Extra indentation within each level of structure, union, or map statements (default 3).

fortran-continuation-indent

Extra indentation for bodies of continuation lines (default 5).

fortran-check-all-num-for-matching-do

If this is nil, indentation assumes that each do statement ends on a continue statement. Therefore, when computing indentation for a statement other than continue, it can save time by not checking for a do statement ending there. If this is non-nil, indenting any numbered statement must check for a do that ends there. The default is nil.

fortran-blink-matching-if

If this is t, indenting an endif statement moves the cursor momentarily to the matching if statement to show where it is. The default is nil.

fortran-minimum-statement-indent-fixed

Minimum indentation for fortran statements when using fixed format continuation line style. Statement bodies are never indented less than this much. The default is 6.

fortran-minimum-statement-indent-tab

Minimum indentation for fortran statements for tab format continuation line style. Statement bodies are never indented less than this much. The default is 8.

24.20.3. Fortran Comments

The usual Emacs comment commands assume that a comment can follow a line of code. In Fortran, the standard comment syntax requires an entire line to be just a comment. Therefore, Fortran mode replaces the standard Emacs comment commands and defines some new variables.

Fortran mode can also handle the Fortran90 comment syntax where comments start with ! and can follow other text. Because only some Fortran77 compilers accept this syntax, Fortran mode will not insert such comments unless you have said in advance to do so. To do this, set the variable comment-start to "!" (Section 32.2).

M-;

Align comment or insert new comment (fortran-comment-indent).

C-x ;

Applies to nonstandard ! comments only.

C-c ;

Turn all lines of the region into comments, or (with argument) turn them back into real code (fortran-comment-region).

M-; in Fortran mode is redefined as the command fortran-comment-indent. Like the usual M-; command, this recognizes any kind of existing comment and aligns its text appropriately; if there is no existing comment, a comment is inserted and aligned. But inserting and aligning comments are not the same in Fortran mode as in other modes.

When a new comment must be inserted, if the current line is blank, a full-line comment is inserted. On a non-blank line, a nonstandard ! comment is inserted if you have said you want to use them. Otherwise a full-line comment is inserted on a new line before the current line.

Nonstandard ! comments are aligned like comments in other languages, but full-line comments are different. In a standard full-line comment, the comment delimiter itself must always appear in column zero. What can be aligned is the text within the comment. You can choose from three styles of alignment by setting the variable fortran-comment-indent-style to one of these values:

fixed

Align the text at a fixed column, which is the sum of fortran-comment-line-extra-indent and the minimum statement indentation. This is the default.

The minimum statement indentation is fortran-minimum-statement-indent-fixed for fixed format continuation line style and fortran-minimum-statement-indent-tab for tab format style.

relative

Align the text as if it were a line of code, but with an additional fortran-comment-line-extra-indent columns of indentation.

nil

Don't move text in full-line comments automatically at all.

In addition, you can specify the character to be used to indent within full-line comments by setting the variable fortran-comment-indent-char to the single-character string you want to use.

Fortran mode introduces two variables comment-line-start and comment-line-start-skip, which play for full-line comments the same roles played by comment-start and comment-start-skip for ordinary text-following comments. Normally these are set properly by Fortran mode, so you do not need to change them.

The normal Emacs comment command C-x ; has not been redefined. If you use ! comments, this command can be used with them. Otherwise it is useless in Fortran mode.

The command C-c ; (fortran-comment-region) turns all the lines of the region into comments by inserting the string C$$$ at the front of each one. With a numeric argument, it turns the region back into live code by deleting C$$$ from the front of each line in it. The string used for these comments can be controlled by setting the variable fortran-comment-region. Note that here we have an example of a command and a variable with the same name; these two uses of the name never conflict because in Lisp and in Emacs it is always clear from the context which one is meant.

24.20.4. Fortran Auto Fill Mode

Fortran Auto Fill mode is a minor mode which automatically splits Fortran statements as you insert them when they become too wide. Splitting a statement involves making continuation lines using fortran-continuation-string (Section 24.20.2.2). This splitting happens when you type SPC, RET, or TAB, and also in the Fortran indentation commands.

M-x fortran-auto-fill-mode turns Fortran Auto Fill mode on if it was off, or off if it was on. This command works the same as M-x auto-fill-mode does for normal Auto Fill mode (Section 23.5). A positive numeric argument turns Fortran Auto Fill mode on, and a negative argument turns it off. You can see when Fortran Auto Fill mode is in effect by the presence of the word Fill in the mode line, inside the parentheses. Fortran Auto Fill mode is a minor mode, turned on or off for each buffer individually. Section 32.1.

Fortran Auto Fill mode breaks lines at spaces or delimiters when the lines get longer than the desired width (the value of fill-column). The delimiters that Fortran Auto Fill mode may break at are ,, ', +, -, /, *, =, and ). The line break comes after the delimiter if the variable fortran-break-before-delimiters is nil. Otherwise (and by default), the break comes before the delimiter.

By default, Fortran Auto Fill mode is not enabled. If you want this feature turned on permanently, add a hook function to fortran-mode-hook to execute (fortran-auto-fill-mode 1). Section 32.2.3.

24.20.5. Checking Columns in Fortran

C-c C-r

Display a "column ruler" momentarily above the current line (fortran-column-ruler).

C-c C-w

Split the current window horizontally temporarily so that it is 72 columns wide (fortran-window-create-momentarily). This may help you avoid making lines longer than the 72-character limit that some Fortran compilers impose.

C-u C-c C-w

Split the current window horizontally so that it is 72 columns wide (fortran-window-create). You can then continue editing.

M-x fortran-strip-sequence-nos

Delete all text in column 72 and beyond.

The command C-c C-r (fortran-column-ruler) shows a column ruler momentarily above the current line. The comment ruler is two lines of text that show you the locations of columns with special significance in Fortran programs. Square brackets show the limits of the columns for line numbers, and curly brackets show the limits of the columns for the statement body. Column numbers appear above them.

Note that the column numbers count from zero, as always in GNU Emacs. As a result, the numbers may be one less than those you are familiar with; but the positions they indicate in the line are standard for Fortran.

The text used to display the column ruler depends on the value of the variable indent-tabs-mode. If indent-tabs-mode is nil, then the value of the variable fortran-column-ruler-fixed is used as the column ruler. Otherwise, the variable fortran-column-ruler-tab is displayed. By changing these variables, you can change the column ruler display.

C-c C-w (fortran-window-create-momentarily) temporarily splits the current window horizontally, making a window 72 columns wide, so you can see which lines that is too long. Type a space to restore the normal width.

You can also split the window horizontally and continue editing with the split in place. To do this, use C-u C-c C-w (M-x fortran-window-create). By editing in this window you can immediately see when you make a line too wide to be correct Fortran.

The command M-x fortran-strip-sequence-nos deletes all text in column 72 and beyond, on all lines in the current buffer. This is the easiest way to get rid of old sequence numbers.

24.20.6. Fortran Keyword Abbrevs

Fortran mode provides many built-in abbrevs for common keywords and declarations. These are the same sort of abbrev that you can define yourself. To use them, you must turn on Abbrev mode. Chapitre 26.

The built-in abbrevs are unusual in one way: they all start with a semicolon. You cannot normally use semicolon in an abbrev, but Fortran mode makes this possible by changing the syntax of semicolon to "word constituent."

For example, one built-in Fortran abbrev is ;c for continue. If you insert ;c and then insert a punctuation character such as a space or a newline, the ;c expands automatically to continue, provided Abbrev mode is enabled.

Type ;? or ;C-h to display a list of all the built-in Fortran abbrevs and what they stand for.