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102 lines
4.4 KiB
Markdown
102 lines
4.4 KiB
Markdown
# GBNF Guide
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GBNF (GGML BNF) is a format for defining [formal grammars](https://en.wikipedia.org/wiki/Formal_grammar) to constrain model outputs in `llama.cpp`. For example, you can use it to force the model to generate valid JSON, or speak only in emojis. GBNF grammars are supported in various ways in `examples/main` and `examples/server`.
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## Background
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[Bakus-Naur Form (BNF)](https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form) is a notation for describing the syntax of formal languages like programming languages, file formats, and protocols. GBNF is an extension of BNF that primarily adds a few modern regex-like features.
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## Basics
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In GBNF, we define *production rules* that specify how a *non-terminal* (rule name) can be replaced with sequences of *terminals* (characters, specifically Unicode [code points](https://en.wikipedia.org/wiki/Code_point)) and other non-terminals. The basic format of a production rule is `nonterminal ::= sequence...`.
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## Example
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Before going deeper, let's look at some of the features demonstrated in `grammars/chess.gbnf`, a small chess notation grammar:
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```
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# `root` specifies the pattern for the overall output
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root ::= (
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# it must start with the characters "1. " followed by a sequence
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# of characters that match the `move` rule, followed by a space, followed
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# by another move, and then a newline
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"1. " move " " move "\n"
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# it's followed by one or more subsequent moves, numbered with one or two digits
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([1-9] [0-9]? ". " move " " move "\n")+
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)
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# `move` is an abstract representation, which can be a pawn, nonpawn, or castle.
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# The `[+#]?` denotes the possibility of checking or mate signs after moves
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move ::= (pawn | nonpawn | castle) [+#]?
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pawn ::= ...
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nonpawn ::= ...
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castle ::= ...
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```
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## Non-Terminals and Terminals
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Non-terminal symbols (rule names) stand for a pattern of terminals and other non-terminals. They are required to be a dashed lowercase word, like `move`, `castle`, or `check-mate`.
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Terminals are actual characters ([code points](https://en.wikipedia.org/wiki/Code_point)). They can be specified as a sequence like `"1"` or `"O-O"` or as ranges like `[1-9]` or `[NBKQR]`.
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## Characters and character ranges
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Terminals support the full range of Unicode. Unicode characters can be specified directly in the grammar, for example `hiragana ::= [ぁ-ゟ]`, or with escapes: 8-bit (`\xXX`), 16-bit (`\uXXXX`) or 32-bit (`\UXXXXXXXX`).
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Character ranges can be negated with `^`:
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```
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single-line ::= [^\n]+ "\n"`
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```
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## Sequences and Alternatives
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The order of symbols in a sequence matters. For example, in `"1. " move " " move "\n"`, the `"1. "` must come before the first `move`, etc.
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Alternatives, denoted by `|`, give different sequences that are acceptable. For example, in `move ::= pawn | nonpawn | castle`, `move` can be a `pawn` move, a `nonpawn` move, or a `castle`.
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Parentheses `()` can be used to group sequences, which allows for embedding alternatives in a larger rule or applying repetition and optional symbols (below) to a sequence.
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## Repetition and Optional Symbols
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- `*` after a symbol or sequence means that it can be repeated zero or more times.
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- `+` denotes that the symbol or sequence should appear one or more times.
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- `?` makes the preceding symbol or sequence optional.
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## Comments and newlines
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Comments can be specified with `#`:
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```
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# defines optional whitespace
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ws ::= [ \t\n]+
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```
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Newlines are allowed between rules and between symbols or sequences nested inside parentheses. Additionally, a newline after an alternate marker `|` will continue the current rule, even outside of parentheses.
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## The root rule
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In a full grammar, the `root` rule always defines the starting point of the grammar. In other words, it specifies what the entire output must match.
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```
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# a grammar for lists
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root ::= ("- " item)+
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item ::= [^\n]+ "\n"
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```
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## Next steps
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This guide provides a brief overview. Check out the GBNF files in this directory (`grammars/`) for examples of full grammars. You can try them out with:
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```
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./main -m <model> --grammar-file grammars/some-grammar.gbnf -p 'Some prompt'
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```
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## Troubleshooting
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Grammars currently have performance gotchas (see https://github.com/ggerganov/llama.cpp/issues/4218).
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### Efficient optional repetitions
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A common pattern is to allow repetitions of a pattern `x` up to N times.
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While semantically correct, the syntax `x? x? x?.... x?` (with N repetitions) will result in extremely slow inference. Instead, you can write `(x (x (x ... (x)?...)?)?)?` (w/ N-deep nesting)
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