Answer
A language is a set of valid sentences. What makes a sentence
valid? You can break validity down into two things: syntax and
semantics. The term syntax refers to grammatical structure
whereas the term semantics refers to the meaning of the vocabulary
symbols arranged with that structure. Grammatical (syntactically
valid) does not imply sensible (semantically valid), however. For
example, the grammatical sentence "cows flow supremely" is
grammatically ok (subject verb adverb) in English, but makes no sense.
Similarly, in a programming language, your grammar (syntax rules) may
allow ID EQUALS ID but the language may only allow the sentence if the
text matched for ID does not represent a type name (a semantic rule).
When you write an ANTLR grammar, you are specifying the set of syntax
rules obeyed by your language. ANTLR uses this to generate a
recognizer for sentences in that language. To apply semantic rules,
you must add actions or semantic predicates to the grammar. The
actions test the "value" of the various tokens and their relationships
to determine semantic validity. For example, if you look up a
type name in a symbol table to ensure it's a type not a variable, you
are applying a semantic rule.
See the chapter on language in Terence's new (in-progress) book.
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