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+---
+title: "Interface Description Language (IDL)"
+kind: doc
+---
+## Thrift interface description language
+The Thrift interface definition language (IDL) allows for the definition of [Thrift Types](/docs/types). A Thrift IDL file is processed by the Thrift code generator to produce code for the various target languages to support the defined structs and services in the IDL file.
+
+## Description
+
+*Under construction*
+
+Here is a description of the Thrift IDL.
+
+## Document
+
+Every Thrift document contains 0 or more headers followed by 0 or more definitions.
+
+    [1]  Document        ::=  Header* Definition*
+
+## Header
+
+A header is either a Thrift include, a C++ include, or a namespace declaration.
+
+    [2]  Header          ::=  Include | CppInclude | Namespace
+
+### Thrift Include
+
+An include makes all the symbols from another file visible (with a prefix) and adds corresponding include statements into the code generated for this Thrift document.
+
+    [3]  Include         ::=  'include' Literal
+
+### C++ Include
+
+A C++ include adds a custom C++ include to the output of the C++ code generator for this Thrift document. 
+
+    [4]  CppInclude      ::=  'cpp_include' Literal
+
+### Namespace
+
+A namespace declares which namespaces/package/module/etc. the type definitions in this file will be declared in for the target languages. The namespace scope indicates which language the namespace applies to; a scope of '*' indicates that the namespace applies to all target languages.
+
+    [5]  Namespace       ::=  ( 'namespace' ( NamespaceScope Identifier ) |
+                                            ( 'smalltalk.category' STIdentifier ) |
+                                            ( 'smalltalk.prefix' Identifier ) ) |
+                              ( 'php_namespace' Literal ) |
+                              ( 'xsd_namespace' Literal )
+
+    [6]  NamespaceScope  ::=  '*' | 'cpp' | 'java' | 'py' | 'perl' | 'rb' | 'cocoa' | 'csharp'
+
+N.B.: Smalltalk has two distinct types of namespace commands.
+*Can someone who knows Smalltalk explain why Smalltalk needs two different kinds of namespaces?*
+
+N.B.: The `php_namespace` directive will be deprecated at some point in the future in favor of the scoped syntax, but the scoped syntax is not yet supported for PHP.
+
+N.B.: The `xsd_namespace` directive has some purpose internal to Facebook but serves no purpose in Thrift itself. Use of this feature is strongly discouraged
+
+## Definition
+
+    [7]  Definition      ::=  Const | Typedef | Enum | Senum | Struct | Union | Exception | Service
+
+### Const
+
+    [8]  Const           ::=  'const' FieldType Identifier '=' ConstValue ListSeparator?
+
+### Typedef
+
+A typedef creates an alternate name for a type.
+
+    [9]  Typedef         ::=  'typedef' DefinitionType Identifier
+
+### Enum
+
+An enum creates an enumerated type, with named values. If no constant value is supplied, the value is either 0 for the first element, or one greater than the preceding value for any subsequent element. Any constant value that is supplied must be non-negative.
+
+    [10] Enum            ::=  'enum' Identifier '{' (Identifier ('=' IntConstant)? ListSeparator?)* '}'
+
+### Senum
+
+Senum (and Slist) are now deprecated and should both be replaced with String.
+
+    [11] Senum           ::=  'senum' Identifier '{' (Literal ListSeparator?)* '}'
+
+### Struct
+
+Structs are the fundamental compositional type in Thrift. The name of each field must be unique within the struct.
+
+    [12] Struct          ::=  'struct' Identifier 'xsd_all'? '{' Field* '}'
+
+N.B.: The `xsd_all` keyword has some purpose internal to Facebook but serves no purpose in Thrift itself. Use of this feature is strongly discouraged
+
+### Union
+
+Unions are similar to structs, except that they provide a means to transport exactly one field of a possible set of fields, just like union {} in C++. Consequently, union members cannot be required fields.
+
+    [13] Union          ::=  'union' Identifier 'xsd_all'? '{' Field* '}'
+
+N.B.: The `xsd_all` keyword has some purpose internal to Facebook but serves no purpose in Thrift itself. Use of this feature is strongly discouraged
+
+### Exception
+
+Exceptions are similar to structs except that they are intended to integrate with the native exception handling mechanisms in the target languages. The name of each field must be unique within the exception.
+
+    [14] Exception       ::=  'exception' Identifier '{' Field* '}'
+
+### Service
+
+A service provides the interface for a set of functionality provided by a Thrift server. The interface is simply a list of functions. A service can extend another service, which simply means that it provides the functions of the extended service in addition to its own.
+
+    [15] Service         ::=  'service' Identifier ( 'extends' Identifier )? '{' Function* '}'
+
+## Field
+
+    [16] Field           ::=  FieldID? FieldReq? FieldType Identifier ('= ConstValue)? XsdFieldOptions ListSeparator?
+
+### Field ID
+
+    [17] FieldID         ::=  IntConstant ':'
+
+### Field Requiredness
+
+    [18] FieldReq        ::=  'required' | 'optional'
+
+### XSD Options
+
+N.B.: These have  some internal purpose at Facebook but serve no current purpose in Thrift. Use of these options is strongly discouraged.
+
+    [19] XsdFieldOptions ::=  'xsd_optional'? 'xsd_nillable'? XsdAttrs?
+
+    [20] XsdAttrs        ::=  'xsd_attrs' '{' Field* '}'
+
+## Functions
+
+    [21] Function        ::=  'oneway'? FunctionType Identifier '(' Field* ')' Throws? ListSeparator?
+
+    [22] FunctionType    ::=  FieldType | 'void'
+
+    [23] Throws          ::=  'throws' '(' Field* ')'
+
+## Types
+
+    [24] FieldType       ::=  Identifier | BaseType | ContainerType
+
+    [25] DefinitionType  ::=  BaseType | ContainerType
+
+    [26] BaseType        ::=  'bool' | 'byte' | 'i16' | 'i32' | 'i64' | 'double' | 'string' | 'binary' | 'slist'
+
+    [27] ContainerType   ::=  MapType | SetType | ListType
+
+    [28] MapType         ::=  'map' CppType? '<' FieldType ',' FieldType '>'
+
+    [29] SetType         ::=  'set' CppType? '<' FieldType '>'
+
+    [30] ListType        ::=  'list' '<' FieldType '>' CppType?
+
+    [31] CppType         ::=  'cpp_type' Literal
+
+## Constant Values
+
+    [32] ConstValue      ::=  IntConstant | DoubleConstant | Literal | Identifier | ConstList | ConstMap
+
+    [33] IntConstant     ::=  ('+' | '-')? Digit+
+
+    [34] DoubleConstant  ::=  ('+' | '-')? Digit* ('.' Digit+)? ( ('E' | 'e') IntConstant )?
+
+    [35] ConstList       ::=  '[' (ConstValue ListSeparator?)* ']'
+
+    [36] ConstMap        ::=  '{' (ConstValue ':' ConstValue ListSeparator?)* '}'
+
+## Basic Definitions
+
+### Literal
+
+    [37] Literal         ::=  ('"' [^"]* '"') | ("'" [^']* "'")
+
+### Identifier
+
+    [38] Identifier      ::=  ( Letter | '_' ) ( Letter | Digit | '.' | '_' )*
+
+    [39] STIdentifier    ::=  ( Letter | '_' ) ( Letter | Digit | '.' | '_' | '-' )*
+
+### List Separator
+
+    [40] ListSeparator   ::=  ',' | ';'
+
+### Letters and Digits
+
+    [41] Letter          ::=  ['A'-'Z'] | ['a'-'z']
+
+    [42] Digit           ::=  ['0'-'9']
+
+## Examples
+
+Here are some examples of Thrift definitions, using the Thrift IDL:
+
+ * [ThriftTest.thrift][] used by the Thrift TestFramework
+ * Thrift [tutorial][]
+ * Facebook's [fb303.thrift][]
+ * [Apache Cassandra's][] Thrift IDL: [cassandra.thrift][]
+ * [Evernote API][]
+
+ [ThriftTest.thrift]:  https://git-wip-us.apache.org/repos/asf?p=thrift.git;a=blob_plain;f=test/ThriftTest.thrift;hb=HEAD
+ [tutorial]:           /tutorial/
+ [fb303.thrift]:       https://git-wip-us.apache.org/repos/asf?p=thrift.git;a=blob_plain;f=contrib/fb303/if/fb303.thrift;hb=HEAD
+ [Apache Cassandra's]: http://cassandra.apache.org/
+ [cassandra.thrift]:   http://svn.apache.org/viewvc/cassandra/trunk/interface/cassandra.thrift?view=co
+ [Evernote API]:       http://www.evernote.com/about/developer/api/
+
+## To Do/Questions
+
+Initialization of Base Types for all Languages?
+
+ * Do all Languages initialize them to 0, bool=false and string=""? or null, undefined?
+
+Why does position of `CppType` vary between `SetType` and `ListType`?
+
+ * std::set does sort the elements automatically, that's the design. see [Thrift Types](/docs/types) or the [C++ std:set reference][] for further details
+ * The question is, how other languages are doing that? What about custom objects, do they have a Compare function the set the order correctly?
+
+ [C++ std:set reference]: http://www.cplusplus.com/reference/stl/set/
+
+Why can't `DefinitionType` be the same as `FieldType` (i.e. include `Identifier`)?
+
+Examine the `smalltalk.prefix` and `smalltalk.category` status (esp `smalltalk.category`, which takes `STIdentifier` as its argument)...
+
+What to do about `ListSeparator`? Do we really want to be as lax as we currently are?
+
+Should `Field*` really be `Field+` in `Struct`, `Enum`, etc.?
+