Parsing Expressions

The syntax of OCaml expressions consists of a set of rules that dictate what input string makes up a valid OCaml expression. Some of the rules might look something like the following in the BNF notation.

<expr> ::= 
  | <number>
  | <unop> <expr>
  | <expr> binop expr
  | if <expr> then <expr> else <expr>
  | ... 

<number> ::= 0 | 1 | 2 | ...
<unop> ::= not | ...
<binop> ::= + | - | * | ...
...

Since the OCaml compiler implements all these rules, it can accept an input string as syntactically valid if it satisfies the syntactic rules. For instance, 1 + 2 follows the rule, <expr> binop <expr>, which is accepted by the OCaml compiler.

However, the OCaml compiler might reject input strings that do not follow its syntactic rules. For instance, the OCaml compiler will reject 1xyz + 2 with the following error message:

Error: Line 1, characters 0-4:
Error: Invalid literal 1xyz

The error message indicates that the OCaml compiler cannot recognize 1xyz as a valid literal. Similarly, the OCaml compiler also reject 1 + with an error message.

Error: Syntax error

This error message is different from the one above. While the former ones say Invalid literal, ...