Halliday & Matthiessen (1999: 422):
In its relation to syntax, the formal approach to semantics is compositional, being formulated in terms of syntagmatic organisation: the meaning of larger syntagmatic units such as sentences is built up as functions of smaller units such as nouns and verbs. This semantic composition is in tandem with the syntax, so what becomes important is the nature of the syntactic rules themselves, not just the result of their application. Montague used a form of categorial grammar in formalising the rules of syntax for this purpose.