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#8 in Configuration

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SLRConfig

Build Status

SLRConfig is a simple configuration format with a focus on readability and ease of editing. It supports tables (mappings of strings to elements) and arrays of elements, where an element may be a string, an array or a table. Despite only supporting arrangements of strings, it's permissive syntax and a standard implementation that preserves the location of where each element was assigned allows the programmer to add support for any other type with the same quality of error messages as if it were built in. See more motivation down below.

Documentation

Rust interface documentation

Packages

Example

Here's a sample snippet. The details of the syntax are explained further below.

# This is a comment.
# The implicit outer structure is a table, a mapping of string keys to string
# values, as well as other collections.
key = value

statement = there's no need to quote the vast majority of characters

"sometimes, you" = "need
to"

"you can always escape ☺
" = you can always escape \u263a\n

raw string for when you're tired of escaping = {{"embedded quote -> " <-"}}

there is no builtin date format = 1970/01/01
there are no bulltin integers = 1_000_000
all values are strings = -1.5

on = a, single = line

table
{
	array = [a, { b = c }, [e]]
}

# Tagged variants of tables and arrays are particularly useful when serializing
# structs and tuple/struct variants.
tagged table = tag
{
   tagged array = tag [1, 2]
}

Motivation

SLRConfig is the third evolution of my previous attempt at making configuration files, following SLConfig and DConfig. I always wanted a simple, human centric configuration format that was capable of expressing the hierarchical structure of my types. In this third attempt I cut away all that I deemed unnecessary:

  • There are no built in types except collections and strings. This was an essential point, as I felt people are too eager to add native support for types which creates an artificial divide between built-in and other types. E.g. why is one particular date format supported and not another? Why only date and not time? Why only 64 bit integers and not more? It's astonishing to me how some languages claim to be minimal, but support N varieties of time formats.

    In SLRConfig, I simplified the syntax such that you can write many strings without quotes. With a little bit of bookkeeping, I could defer the interpretation of these strings to the user while letting them still track where the strings originated for error reporting purposes.

  • As said above, I set things up such that quoting strings is rare. Strings support spaces in them without a need to quote them. The reseved characters are chosen such that writing down floating point numbers, dates, units and other common type encodings also require no quoting.

  • Compared to SLConfig, I dropped support for file includes, complicated variable expansions and docstrings. Those only seemed to complicate the specification and implementation.

I did add some less common features though:

  • Raw strings. I think they're just so useful for long text entry without escapes being present everywhere.

  • Simple variable expansions. This is probably the most extraneous feature of SLRConfig, but when it is useful it is hard to replicate. I'm happy to defer to string substitution for include files, but variable expansion really has to respect the syntax to be easy to use.

Format description.

The language grammar is described using the GOLD meta-language. Essentially, character sets are specified using set notation, terminals are specified using regular expressions and productions are specified using BNF.

The root element is defined by the <TableElements> production.

Lexical grammar

Encoding

The string representation of the format is encoded using UTF-8.

Character sets

{Raw String Chars} = {Printable} + {Whitespace}
{Escaped String Chars} = {Printable} + {Whitespace} - ["]

{String Middle} = {Printable} - {Whitespace} - [#='['']'{}$",~] + [' ']
{String Border} = {String Middle} - [' ']

Terminals

NakedEscapedString = ({String Border} | '\' {Raw String Chars})
                     ({String Middle} | '\' {Raw String Chars})*
                     ({String Border} | '\' {Raw String Chars})
                   | ({String Border} | '\' {Raw String Chars})
QuotedEscapedString = '"' {Escaped String Chars}* '"'

RawString0 = '{{"' {Raw String Chars}* '"}}'
RawString1 = '{{{"' {Raw String Chars}* '"}}}'
RawString2 = '{{{{"' {Raw String Chars}* '"}}}}'

Comments

Line comments are introduced using the # character. Line comments can start anywhere on a line and end at a newline (LF).

Parser grammar and semantics

Strings

<String> ::= NakedEscapedString
           | QuotedEscapedString
           | RawString0
           | RawString1
           | RawString2

Strings are used as keys in tables as well as one type of element that can be in tables and arrays. There are two types of strings, escaped and raw.

Escaped strings

Escaped strings can have character escapes which are resolved during parsing. The supported escapes are as follows:

  • \n - Line feed
  • \r - Carriage return
  • \t - Horizontal tab
  • \0 - NUL
  • \\ - Backslash
  • \uxxxx - Unicode character xxxx, where x's are lower-case hexadecimal digits
  • \Uxxxxxxxx - Unicode character xxxxxxxx, where x's are lower-case hexadecimal digits

Invalid escapes are replaced by (U+fffd) character. There are two types of escaped strings, naked and quoted. Naked strings do not need quotes around them, but are restricted by what characters they may contain.

Raw strings

Raw strings contain exactly the characters that are inside of them. The number of leading braces must match the number of trailing braces, but otherwise can be increased in case there's a quote character followed run of trailing braces inside the string itself.

Expressions

<Expansion> ::= '$' <String>

<Expr> ::= <String>
        |  <Expansion>
        |  <Expr> '~' <String>
        |  <Expr> '~' <Expansion>

Table and array elements are assigned the results of expressions. An expression can result in a value (a string), a table or an array. Multiple strings can be contatenated using the ~ operator.

Using the expansion syntax it is possible to fetch the value of a previously assigned element. The elements are looked up by key from tables and by index (zero based) from arrays. The lookup is performed by looking up the element in the table/array that the element being assigned to is. If nothing was found, then a higher level table/array is examined, and so on. The element being looked up must be lexically prior in the source string to the element being assigned to. The element being assigned to is never considered as an expansion element.

Tables

<OptComma> ::= ','
            |
<Table> ::= '{' <TableContents> '}'

<TableElement> ::= <String> <Table>
                |  <String> '=' <Array>
                |  <String> '=' <Expr>
                |  <String> '=' <TaggedTable>
                |  <String> '=' <TaggedArray>

<TableElements> ::= <TableElement>
                 |  <TableElements> <OptComma> <TableElement>

<TableContents> ::= <TableElements> <OptComma>
                 |

Arrays

<Array> ::= '[' <ArrayContents> ']'

<ArrayElement> ::= <Expr>
                |  <Table>
                |  <Array>
                |  <TaggedTable>
                |  <TaggedArray>

<ArrayElements> ::= <ArrayElement>
                 |  <ArrayElements> ',' <ArrayElement>

<ArrayContents> ::= <ArrayElements> <OptComma>
                 |

Tagged Tables

<TaggedTable> ::= <String> <Table>

Tagged tables are tables with a string tag attached to them. These are surprisingly useful for representing named types.

Tagged Array

<TaggedArray> ::= <String> <Array>

Tagged arrays are arrays with a string tag attached to them. These are surprisingly useful for representing named types.

Serde integration

Here is how various Rust constructs are encoded in SLRConfig.

Numeric types

These are encoded using their string representation as naked strings.

Boolean

true is encoded as true and false is encoded as false.

Strings and characters

Strings are encoded as escaped strings, and are automatically escaped.

Byte slices

Bytes are encoded as an array of integers.

Slices and tuples

These are encoded as arrays.

Maps

These are encoded as arrays of 2-element arrays (key/value pairs). These are not encoded as tables because tables must have strings as their values, while Rust mappings don't need to be. In the future, we might relax this.

Option

Some is encoded as the contained value and None is encoded as an empty string.

Units

Units are encoded as an empty string.

Unit structs and variants

These are encoded by their names as strings.

Newtype structs and variants

These are encoded by 1-element tagged arrays. The tag is the name of the struct/variant, and the array element is the value.

Tuple structs and variants

These are encoded by a tagged arrays. The tag is the name of the struct/variant, and the array elements are the tuple elements. When deserializing tuple structs, a non-tagged array is also accepted.

Structs and struct variants

These are encoded by a tagged tables. The tag is the name of the struct/variant, and the table elements are the struct elements. When deserializing tuple structs, a non-tagged table is also accepted.

Dependencies

~0.6–1MB
~24K SLoC