The following characters are considered as blanks: space, newline, horizontal tabulation, carriage return, line feed and form feed. Blanks are ignored, but they separate adjacent identifiers, literals and keywords that would otherwise be confused as one single identifier, literal or keyword.
Comments are introduced by the two characters (*, with no intervening blanks, and terminated by the characters *), with no intervening blanks. Comments are treated as blank characters. Comments do not occur inside string or character literals. Nested comments are correctly handled.
ident:
letter {letter | 0...9 | _}
letter:
A...Z | a...z
Identifiers are sequences of letters, digits and _ (the underscore character), starting with a letter. Letters contain at least the 52 lowercase and uppercase letters from the ASCII set. Implementations can recognize as letters other characters from the extended ASCII set. Identifiers cannot contain two adjacent underscore characters (__). Implementation may limit the number of characters of an identifier, but this limit must be above 256 characters. All characters in an identifier are meaningful.
integer-literal:
[-] {0...9}+
| [-] (0x | 0X) {0...9 | A...F | a...f}+
| [-] (0o | 0O) {0...7}+
| [-] (0b | 0B) {0...1}+
An integer literal is a sequence of one or more digits, optionally preceded by a minus sign. By default, integer literals are in decimal (radix 10). The following prefixes select a different radix:
| Prefix | Radix |
|---|---|
| 0x, 0X | hexadecimal (radix 16) |
| 0o, 0O | octal (radix 8) |
| 0b, 0B | binary (radix 2) |
float-literal:
[-] {0...9}+ [. {0...9}] [(e | E) [+ | -] {0...9}+]
Floating-point decimals consist in an integer part, a decimal part and an exponent part. The integer part is a sequence of one or more digits, optionally preceded by a minus sign. The decimal part is a decimal point followed by zero, one or more digits. The exponent part is the character e or E followed by an optional + or - sign, followed by one or more digits. The decimal part or the exponent part can be omitted, but not both to avoid ambiguity with integer literals.
char-literal:
` regular-char `
| ` \ (\ | ` | n | t | b | r) `
| ` \ (0...9) (0...9) (0...9) `
Character literals are delimited by ` (backquote) characters. The two backquotes enclose either one character different from ` and \, or one of the escape sequences below:
| Sequence | Character denoted |
|---|---|
| \\ | backslash (\) |
| \` | backquote (`) |
| \n | newline (LF) |
| \r | return (CR) |
| \t | horizontal tabulation (TAB) |
| \b | backspace (BS) |
| \ddd | the character with ASCII code ddd in decimal |
string-literal:
" {string-character} "
string-character:
regular-char
| \ (\ | " | n | t | b | r)
| \ (0...9) (0...9) (0...9)
String literals are delimited by " (double quote) characters. The two double quotes enclose a sequence of either characters different from " and \, or escape sequences from the table below:
| Sequence | Character denoted |
|---|---|
| \\ | backslash (\) |
| \" | double quote (") |
| \n | newline (LF) |
| \r | return (CR) |
| \t | horizontal tabulation (TAB) |
| \b | backspace (BS) |
| \ddd | the character with ASCII code ddd in decimal |
Implementations must support string literals up to 2^{16}-1 characters in length (65535 characters).
The identifiers below are reserved as keywords, and cannot be employed otherwise:
and as begin do done downto
else end exception for fun function
if in let match mutable not
of or prefix rec then to
try type value where while with
The following character sequences are also keywords:
# ! != & ( ) * *. + +.
, - -. -> . .( / /. : ::
:= ; ;; < <. <- <= <=. <> <>.
= =. == > >. >= >=. @ [ [|
] ^ _ __ { | |] } '
Lexical ambiguities are resolved according to the ``longest match'' rule: when a character sequence can be decomposed into two tokens in several different ways, the decomposition retained is the one with the longest first token.