= TeX input state
{c}
A \b[TeX input state] is the N, S or M state used by the <TeX input processor> while reading character input: new line, skipping blanks or middle of line. It helps determine whether a space or end-of-line character becomes a <TeX token>.<TeX source code by Donald E. Knuth>[Input stacks: mid_line, skip_blanks and new_line; get_next whitespace cases]{ref}
Assume ordinary <TeX category codes>. In N, leading spaces are ignored. An ordinary letter emits its character token and enters M. The first space in M emits one space token and enters S; further spaces in S are ignored. An end-of-line character emits a space from M, emits `\par` from N and emits nothing from S. The next line starts in N.<TeX source code by Donald E. Knuth>[Cases where character is ignored; Handle situations involving spaces, braces, changes of state]{ref}
For an original trace, take a line containing two leading spaces, `A`, three spaces and `B`, followed by a space-only line. The first two spaces disappear in N. `A` enters M; one space survives and the next two disappear in S; `B` returns to M. The first line end supplies another space. The blank line then supplies `\par`. The resulting sequence is `A`, space, `B`, space, `\par`.
Thus source whitespace is processed before <word spacing> and <text paragraph> layout. This state records part of scanning, not whether <TeX> is currently building a text line or a mathematical formula; <TeX control sequences>, comments and changed <TeX category codes> have further scanning rules.
Back to article page
