This chapter describes the OCaml core library, which is composed of declarations for built-in types and exceptions, plus the module Stdlib that provides basic operations on these built-in types. The Stdlib module is special in two ways:
The following built-in types and predefined exceptions are always defined in the compilation environment, but are not part of any module. As a consequence, they can only be referred by their short names.
The type of integer numbers.
The type of characters.
The type of (writable) byte sequences.
The type of (read-only) character strings.
The type of floating-point numbers.
type bool = false | true
The type of booleans (truth values).
type unit = ()
The type of the unit value.
The type of exception values.
type 'a array
The type of arrays whose elements have type 'a.
type 'a list =  | :: of 'a * 'a list
The type of lists whose elements have type 'a.
type 'a option = None | Some of 'a
The type of optional values of type 'a.
The type of signed 32-bit integers. Literals for 32-bit integers are suffixed by l. See the Int32 module.
The type of signed 64-bit integers. Literals for 64-bit integers are suffixed by L. See the Int64 module.
The type of signed, platform-native integers (32 bits on 32-bit processors, 64 bits on 64-bit processors). Literals for native integers are suffixed by n. See the Nativeint module.
type ('a, 'b, 'c, 'd, 'e, 'f) format6
The type of format strings. 'a is the type of the parameters of the format, 'f is the result type for the printf-style functions, 'b is the type of the first argument given to %a and %t printing functions (see module Printf), 'c is the result type of these functions, and also the type of the argument transmitted to the first argument of kprintf-style functions, 'd is the result type for the scanf-style functions (see module Scanf), and 'e is the type of the receiver function for the scanf-style functions.
type 'a lazy_t
This type is used to implement the Lazy module. It should not be used directly.
exception Match_failure of (string * int * int)
Exception raised when none of the cases of a pattern-matching apply. The arguments are the location of the match keyword in the source code (file name, line number, column number).
exception Assert_failure of (string * int * int)
Exception raised when an assertion fails. The arguments are the location of the assert keyword in the source code (file name, line number, column number).
exception Invalid_argument of string
Exception raised by library functions to signal that the given arguments do not make sense. The string gives some information to the programmer. As a general rule, this exception should not be caught, it denotes a programming error and the code should be modified not to trigger it.
exception Failure of string
Exception raised by library functions to signal that they are undefined on the given arguments. The string is meant to give some information to the programmer; you must not pattern match on the string literal because it may change in future versions (use
Exception raised by search functions when the desired object could not be found.
Exception raised by the garbage collector when there is insufficient memory to complete the computation. (Not reliable for allocations on the minor heap.)
Exception raised by the bytecode interpreter when the evaluation stack reaches its maximal size. This often indicates infinite or excessively deep recursion in the user’s program. Before 4.10, it was not fully implemented by the native-code compiler.
exception Sys_error of string
Exception raised by the input/output functions to report an operating system error. The string is meant to give some information to the programmer; you must not pattern match on the string literal because it may change in future versions (use
Exception raised by input functions to signal that the end of file has been reached.
Exception raised by integer division and remainder operations when their second argument is zero.
A special case of Sys_error raised when no I/O is possible on a non-blocking I/O channel.
exception Undefined_recursive_module of (string * int * int)
Exception raised when an ill-founded recursive module definition is evaluated. (See section 12.2.) The arguments are the location of the definition in the source code (file name, line number, column number).