The standard way to describe the syntax of a language is with a mathematical notation called Backus-Naur form (BNF), named for its inventors, John Backus and Peter Naur. There are many variants of BNF. Here, we won't be too picky about adhering to one variant or another. Our goal is just to have a reasonably good notation for describing language syntax.

BNF uses a set of derivation rules to describe the syntax of a language. Let's start with an example. Here's the BNF description of a tiny language of expressions that include just the integers and addition:

e ::= i | e + e
i ::= <integers>

These rules say that an expression e is either an integer i, or two expressions with the symbol + appearing between them. The syntax of "integers" is left unspecified by these rules.

Each rule has the form

metavariable ::= symbols | ... | symbols

A metavariable is variable used in the BNF rules, rather than a variable in the language being described. The ::= and | that appear in the rules are metasyntax: BNF syntax used to describe the language's syntax. Symbols are sequences that can include metavariables (such as i and e) as well as tokens of the language (such as +). Whitespace is not relevant in these rules.

Sometimes we might want to easily refer to individual occurrences of metavariables. We do that by appending some distinguishing mark to the metavariable(s). For example, we could rewrite the first rule above as

e ::= i | e1 + e2

or as

e ::= i | e + e'

Now we can talk about e2 or e' rather than having to say "the e on the right-hand side of +".

If the language itself contains either of the tokens ::= or |—and OCaml does contain the latter—then writing BNF can become a little confusing. Some BNF notations attempt to deal with that by using additional delimiters to distinguish syntax from metasyntax. We will be more relaxed and assume that the reader can distinguish them.

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