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# YQL SQL Syntax Highlighting Specfication
## Overview
This document specifies the syntax highlighting system for the YQL. The system specifies how to identify and categorize different syntactic elements in queries for highlighting porposes.
## Terms
- `Highlighting` is a _list_ of `Highlighting Unit`s that define how to recognize different parts of SQL syntax.
- `Highlighting Unit` is a language construction to be highlighted (e.g., keywords, identifiers, literals).
- `Highlighting Token` is a text fragment matched with a `Highlighting Unit`.
- `Highlighter` is an function parametrized by `Highlighting` transforming a text into a stream of `Highlighting Token`s.
- `Theme` is a mapping from `Highlighting Unit` to a `Color`.
## Highlighting Unit
Here are examples of `Highlighting Unit`s. They will evolve and should be taken from the JSON `Highlighting` programmatically. Only comments are always present, as they may require special processing.
- `keyword`: SQL reserved words (e.g., `SELECT`, `INSERT`, `FROM`).
- `punctuation`: Syntactic symbols (e.g., `.`, `;`, `(`, `)`).
- `identifier`: Unquoted names (e.g., table or column names).
- `quoted-identifier`: Backtick-quoted names (e.g., ``` `table` ```).
- `bind-parameter-identifier`: Parameter references (e.g., `$param`).
- `type-identifier`: Type names (e.g., `Int32`, `String`).
- `function-identifier`: Function names (e.g., `MIN`, `Math::Sin`).
- `literal`: Numeric constants (e.g., `123`, `1.23`).
- `string-literal`: Quoted strings (e.g., `"example"`).
- `comment`: Single-line (`--`) or multi-line (`/* */`) comments.
- `ws`: Spaces, tabs, newlines.
- `error`: Unrecognized syntax.
Each `Highlighting Unit` contains one or more `Patterns` that define how to recognize the unit in text.
## Pattern Matching
A `Pattern` consists of:
- `body`: The main regex pattern to match.
- `after`: A lookahead pattern.
- `is-case-insensitive`: Whether matching should be case-insensitive.
The matching behavior is equivalent to the regex: `body(?=after)`.
## Highlighter Algorithm
The highlighter algorithm can be described with the following pseudocode.
```python
# Consume matched tokens until empty.
# For each iteration:
# 1. Find the next token match (or error)
# 2. Emit the token
# 3. Continues with the remaining text
highlight(text) =
if text is not empty do
token = match(text)
emit token
highlight(text[token.length:])
# Select the longest match from all possible
# patterns. Leftmost is chosen. If no match,
# emits a 1-character error token for as a
# recovery.
match(text) =
max of matches(text) by length
or error token with length = 1
# For each highlighting unit and its patterns,
# attempt to match.
matches(text) = do
unit <- highlighting.units
pattern <- unit.patterns
content <- match(text, pattern)
yield token with unit, content
# Match both the pattern body and lookahead
# (after) portion with case sensitivity settings.
match(text, pattern) = do
body <- (
regex pattern.body
matches text prefix
with pattern.case_sensivity)
after <- (
regex pattern.body
matches text[body.length:] prefix
with pattern.case_sensivity)
yield body + after
# Special ANSI Comment handling.
# Recursively process nested multiline comments.
match(text, Comment if ANSI) =
if text not starts with "/*" do
return match(text, Comment if Default)
text = text after "/*"
loop do
if text starts with "*/" do
return text after "/*"
if text starts with "/*" do
budget = text before last "*/"
match = match(budget, Comment if ANSI)
text = text after match
if match:
continue
if text is empty:
return Nothing
text = text[1:]
```
## Highlighting JSON Example
The highlighting can be generated using the `yql_highlight` tool in JSON format.
```json
{
"units": [
...
{
"kind":"type-identifier",
"patterns": [
{
"body":"([a-z]|[A-Z]|_)([a-z]|[A-Z]|_|[0-9])*",
"after":"\\<"
},
{
"body":"Int32|Int16|Utf8|...",
"is-case-insensitive":true
}
]
},
...
]
}
```
## Test Suite
The reference implementation includes a comprehensive test suite that verifies correct highlighting behavior. The test suite is defined in JSON format with the following structure:
```json
{
"SQL": [
["SELECT id, alias from users", "KKKKKK#_#II#P#_#IIIII#_#KKKK#_#IIIII"],
],
"TypeIdentifier": [
["Bool(value)", "TTTT#P#IIIII#P"]
]
}
```
Where the first element is the SQL text to highlight and the second one is a string where each character represents the highlighting unit kind for each character in the input.
Here's the table representation of the unit kind to character mapping:
| Unit Kind | Character |
| ------------------------- | --------- |
| keyword | K |
| punctuation | P |
| identifier | I |
| quoted-identifier | Q |
| bind-parameter-identifier | B |
| type-identifier | T |
| function-identifier | F |
| literal | L |
| string-literal | S |
| comment | C |
| ws | _ |
| error | E |
Note: The `#` is used to make tokens visually distinct from other.
The test driver pseudocode:
```cpp
run_test_suite =
let
highlighting = load_highlighting()
highlighter = make_highlighter(highlighting)
suite = load_sest_suite()
in do
scenario <- suite
test <- scenario
(input, expected) = test
tokens = highlighter.highlight(input)
actual = to_pattern(tokens)
assert actual == expected
```
## Implementation Guidelines
- The module `yql/essentials/sql/v1/highlight` is a reference implementation of the `YQL` highlighting. Module includes a comprehensive test suite to check an implementation compliance with the specification. Also this module contains this specification document.
- The module `yql/essentials/tools/yql_highlight` contains a tool to play with the reference highlighting implementation and to generate various representation of highlighting (e.g. in JSON).
- The test suite data can be found at `yql/essentials/sql/v1/highlight/ut/suite.json`.
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