btv.indent

btv.indent.expr(src)

btv.indent.expr(src): compute a line’s indent, or clear the expression with nil.

bemtvi indents from tree-sitter’s indents.scm where a grammar has one; this is the escape hatch for a filetype that has no grammar, or whose indent query gets a construct wrong. It sits below the tree-sitter verdict — structure beats a hand-written rule — and above smartindent/autoindent.

src is a string of Lua source — an expression, not a function value — because it runs in the bounded compute sandbox: a second, pure VM with a wall-clock deadline, no editor state and no btv.*. That means it cannot read the buffer; everything it needs is passed in:

prev         the previous non-blank line ("" when there is none)
line         the line being indented
lnum         its 1-based number
sw           the effective 'shiftwidth'
previndent   the previous non-blank line's indent, in columns

Return the target indent in columns, or nil to decline and let smartindent/autoindent answer:

btv.indent.expr([[
  line:match("^%s*end") and previndent - sw
    or prev:match("then%s*$") and previndent + sw
    or nil
]])

The sandbox is stateless: nothing carries from one call to the next, and assigning a global raises. That is deliberate — no call shape is a clean once-per-item traversal (:s re-runs on every keystroke of the live preview, a foldexpr sees only the rows an edit touched, the picker scorer only the top survivors, foldtext is memoized), so an accumulator would be quietly wrong. It is consulted per line, so a =G over a large range calls it once per line; an expression that errors, exceeds its deadline, or returns a non-number reports once and is then uninstalled rather than repeating per line.

_{Defined in indent.lua.}