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<!--
Copyright (c) ONNX Project Contributors
SPDX-License-Identifier: Apache-2.0
-->
# ONNX Textual Syntax
## Overview
This document describes a textual syntax for ONNX models, which is currently an experimental feature.
The syntax enables a compact and readable representation of ONNX models. It is motivated by a couple
of use-cases. One is to enable compact description of test-cases and its use in CI (both in the ONNX
repo as well as in other dependent repos such as ONNX-MLIR). The second is to help simplify the
definition of ONNX functions. Several of the existing function-definitions are verbose, and the
use of this syntax will lead to more compact, readable, and easier-to-maintain function definitions.
Efficient representation and efficient parsing of very large tensor-constants is *not* a goal.
Alternative methods should be used for that.
## The API
The key parser methods are the ```OnnxParser::Parse``` methods, used as below.
```cpp
const char* code = R"ONNX(
<
ir_version: 7,
opset_import: [ "" : 10 ]
>
agraph (float[N, 128] X, float[128, 10] W, float[10] B) => (float[N, 10] C)
{
T = MatMul(X, W)
S = Add(T, B)
C = Softmax(S)
}
)ONNX";
ModelProto model;
OnnxParser::Parse(model, code);
checker::check_model(model);
```
See the [test-cases](../onnx/test/cpp/parser_test.cc) for more examples illustrating the API and syntax.
## The Syntax
The grammar below describes the syntax:
```
id-list ::= id (',' id)*
tensor-dim ::= '?' | id | int-constant
tensor-dims ::= tensor-dim (',' tensor-dim)*
tensor-type ::= prim-type | prim-type '[' ']' | prim-type '[' tensor-dims ']'
type ::= tensor-type | 'seq' '(' type ')' | 'map' '(' prim-type ',' type ')'
| 'optional' '(' type ')' | 'sparse_tensor' '(' tensor-type ')'
value-info ::= type id
value-infos ::= value-info (',' value-info)*
value-info-list ::= '(' value-infos? ')
id-or-value-info ::= type? id
id-or-value-infos ::= id-or-value-info (',' id-or-value-info)*
quoted-str :== '"' ([^"])* '"'
str-str :== quoted-str ':' quoted-str
str-str-list :== '[' str-str (',' str-str)* ']'
internal-data ::= '{' prim-constants '}'
external-data ::= str-str-list
constant-data ::= internal-data | external-data
value-info-or-initializer ::= type id [ '=' constant-data ]
value-info-or-initializers ::= value-info-or-initializer (',' value-info-or-initializer)*
input-list ::= '(' value-info-or-initializers? ')'
output-list ::= '(' value-infos? ')'
initializer-list ::= '<' value-info-or-initializers? '>'
prim-constants ::= prim-constant (',' prim-constant)*
tensor-constant ::= tensor-type (id)? ('=')? '{' prim-constants '}'
attr-ref ::= '@' id
single-attr-value ::= tensor-constant | graph | prim-constant | attr-ref
attr-value-list ::= '[' single-attr-value (',' single-attr-value)* ']'
attr-value ::= single-attr-value | attr-value-list
attr-type ::= ':' id
attr ::= id attr-type? '=' attr-value
attr-list ::= '<' attr (',' attr)* '>'
node ::= id-list? '=' qualified-id attr-list? '(' id-list? ')'
| id-list? '=' qualified-id '(' id-list? ')' attr-list
node-list ::= '{' node* '}'
graph ::= id input-list '=>' output-list initializer-list node-list
other-data ::= id ':' value
other-data-list ::= '<' other-data (',' other-data)* '>'
fun-attr-list ::= '<' id | attr (',' id | attr)* '>'
fun-input-list ::= '(' id-or-value-infos ')'
fun-output-list ::= '(' id-or-value-infos ')'
fun-value-infos ::= ( '<' value-infos '>' )?
function ::= other-data-list? id fun-attr-list? fun-input-list '=>' fun-output-list fun-value-infos node-list
model ::= other-data-list? graph function*
```
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