From IFWiki

See also parser-based interactive fiction. For the German publication, see The Parser.

A parser is a computer program or portion of a program that analyzes a string of characters in order to recognize grammatical units. Joseph Weizenbaum's Eliza (1966) and Terry Winograd's SHRDLU (c.1970) are two of the best-known early interactive programs that had parsers.

Early IF, including the original Adventure, used a two-word parser that only looked at the first few characters of each word. More recent IF parsers, following the lead of Dungeon aka Zork, have an Infocom-type parser that can understand longer words and more complex commands, but the genre is still a long way from using natural language processing to achieve complete understanding of all English inputs.

Modern IF programming languages include a parser as part of the authoring system, removing the need for the programmer to write a homebrew parser. Authoring systems for writing parser-based interactive fiction include Inform 7, Quest, TADS, and ADRIFT, among others.