Next: SMIE Indentation Helpers, Previous: SMIE Tricks, Up: SMIE
Based on the provided grammar, SMIE will be able to provide automatic indentation without any extra effort. But in practice, this default indentation style will probably not be good enough. You will want to tweak it in many different cases.
SMIE indentation is based on the idea that indentation rules should be
as local as possible. To this end, it relies on the idea of
virtual indentation, which is the indentation that a particular
program point would have if it were at the beginning of a line.
Of course, if that program point is indeed at the beginning of a line,
its virtual indentation is its current indentation. But if not, then
SMIE uses the indentation algorithm to compute the virtual indentation
of that point. Now in practice, the virtual indentation of a program
point does not have to be identical to the indentation it would have if
we inserted a newline before it. To see how this works, the SMIE rule
for indentation after a { in C does not care whether the
{ is standing on a line of its own or is at the end of the
preceding line. Instead, these different cases are handled in the
indentation rule that decides how to indent before a {.
Another important concept is the notion of parent: The
parent of a token, is the head token of the nearest enclosing
syntactic construct. For example, the parent of an else is the
if to which it belongs, and the parent of an if, in turn,
is the lead token of the surrounding construct. The command
backward-sexp jumps from a token to its parent, but there are
some caveats: for openers (tokens which start a construct, like
if), you need to start with point before the token, while for
others you need to start with point after the token.
backward-sexp stops with point before the parent token if that is
the opener of the token of interest, and otherwise it stops with
point after the parent token.
SMIE indentation rules are specified using a function that takes two arguments method and arg where the meaning of arg and the expected return value depend on method.
method can be:
:after, in which case arg is a token and the function
should return the offset to use for indentation after arg.
:before, in which case arg is a token and the function
should return the offset to use to indent arg itself.
:elem, in which case the function should return either the offset
to use to indent function arguments (if arg is the symbol
arg) or the basic indentation step (if arg is the symbol
basic).
:list-intro, in which case arg is a token and the function
should return non-nil if the token is followed by a list of
expressions (not separated by any token) rather than an expression.
When arg is a token, the function is called with point just before
that token. A return value of nil always means to fallback on the
default behavior, so the function should return nil for arguments it
does not expect.
offset can be:
nil: use the default indentation rule.
(column . column): indent to column column.
:after and its parent for :before.