# Chapter 21 The core library

- Conventions
- Built-in types and predefined exceptions
- Module Pervasives: the initially opened module

This chapter describes the OCaml core library, which is composed of declarations for built-in types and exceptions, plus the module Pervasives that provides basic operations on these built-in types. The Pervasives module is special in two ways:

- It is automatically linked with the user’s object code files by the ocamlc command (chapter 8).
- It is automatically “opened” when a compilation starts, or when the toplevel system is launched. Hence, it is possible to use unqualified identifiers to refer to the functions provided by the Pervasives module, without adding a open Pervasives directive.

## Conventions

The declarations of the built-in types and the components of module Pervasives are printed one by one in typewriter font, followed by a short comment. All library modules and the components they provide are indexed at the end of this report.

## 21.1 Built-in types and predefined exceptions

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.

### Built-in types

     type int

> The type of integer numbers.

     type char

> The type of characters.

     type bytes

> The type of (writable) byte sequences.

     type string

> The type of (read-only) character strings.

     type float

> The type of floating-point numbers.

     type bool = false | true

> The type of booleans (truth values).

     type unit = ()

> The type of the unit value.

     type exn

> 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.

    type int32

> The type of signed 32-bit integers. See the Int32[Int32] module.

    type int64

> The type of signed 64-bit integers. See the Int64[Int64] module.

    type nativeint

> The type of signed, platform-native integers (32 bits on 32-bit processors, 64 bits on 64-bit processors). See the Nativeint[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[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[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[Lazy] module. It should not be used directly.

### Predefined exceptions

    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 `Failure _` instead).

    exception Not_found

> Exception raised by search functions when the desired object could not be found.

    exception Out_of_memory

> Exception raised by the garbage collector when there is insufficient memory to complete the computation.

    exception Stack_overflow

> 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. (Not fully implemented by the native-code compiler; see section 11.5.)

    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 `Sys_error _` instead).

    exception End_of_file

> Exception raised by input functions to signal that the end of file has been reached.

    exception Division_by_zero

> Exception raised by integer division and remainder operations when their second argument is zero.

    exception Sys_blocked_io

> 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 7.4.) The arguments are the location of the definition in the source code (file name, line number, column number).

## 21.2 Module Pervasives: the initially opened module

- Module Pervasives: the initially opened module