Crate ucglib[−][src]

ucg, A universal configuration grammar.

Ucg defines a common grammar for describing a collection of configuration values. ucg allows you to specify configuration values with a syntax that that is immutable, composable, with copy-on-write semantics, and safe.

Example

// named bindings
let host = "mysql.internal.net";
let port = 8080

// format strings
let connstr = "mysql://@:@" % (host, port);

// tuples
let dbconf = {
    connstr = connstr,
    database = "mydb",
    // lists
    tables = ["posts", "comments", "users"],
};

Syntax

A valid ucg file is composesed of a series of statements. Stataments start with an optional keyword and terminate with a semicolon.

Reserved words

The following words are reserved in ucg and can't be used as named bindings.

  • assert
  • true
  • false
  • let
  • import
  • as
  • select
  • macro
  • env
  • map
  • filter
  • NULL
  • out

Types

Primitive types

ucg has a relatively simple syntax with just a few primitive types, Null, Boolean, Int, Float, and String.

Boolean

A Boolean is either true or false.

true;
false;

Int

An Int is any integer number.

1; // a single Integer

Float

A Float is any number with a decimal point.

1.0; // A typical float.
1. // You can leave off the 0 after the decimal point.
.1 // the leading 0 is also optional.

String

A String is any quoted text. Backslashes within a string escape the next preceding character.

"foo"; // a simple string
"I'm a \"fine\" looking string"; // escaped quotes in a string.

NULL or the Empty type.

A NULL is an empty type. It represents no value.

let empty = NULL;

Complex types

ucg has two complex data types, Lists and Tuples.

Lists

Lists are surrounded with square brackets [ ] and have comma separated elements. Trailing commas are permitted in lists.

[1, 2, 3]; // A simple list of numbers.

[[1, 2], [3, 4]] // A deep list with embedded lists inside.

Lists are 0 indexed and you can index into them using the dotted selector syntax.

let mylist = [0, 1, 2, 3];

let zero = mylist.0;

Tuple

Tuple's are an ordered collection of name, value pairs. They are bounded by curly braces { } and contain name = value pairs separated by commas. Trailing commas are permitted. The name must be a bareword without quotes.

let mytuple = {
    field1 = "value1",
    field2 = "value2",
};

Tuples can be indexed using dotted selector syntax.

let field = mytuple.fields1;

Expressions

Selectors

Selectors are references to a bound value in ucg. They can index arbitrarily deep into either tuples or lists. The head of a selector can be a tuple or list or symbol. Optionally a selector can also be followed by either a bareword to index a tuple field or an integer to index a list position.

The simplest selector is just a reference to a bound value by name.

let mytuple = {
    field1 = "a string",
    field2 = [{
        subfield1 = 1,
    }];
};

mytuple.field2.0; // descend into a deeply nested tuple and array.
The Environment Selector

The env selector is a reserved selector that always contains a tuple with any environment variables in it.

Attempting to reference an enviroment selector field that does not exist is a compile error.

Binary operators

Numeric operators

ucg supports the following numeric operators, +, -, *, / Each one is type safe and infers the types from the values they operate on. The operators expect both the left and right operands to be of the same type.

1 + 1; // result is 2
Concatenation

ucg supports concatenation using the + operator. It is typesafe expecting both sides to be of the same type. You can concatenate strings or lists but not tuples.

"foo " + "bar" // result is "foo bar"
[1,2] + [3,4]; // result is [1,2,3,4]
Comparison

ucg supports comparison using the ==, !=, >, <, >=, <= operators. They are type safe and expect both sides to be of the same type.

The >, <, >=, and >= operators are only supported on numeric types (i.e. int, and float).

1 > 2; // result is false
2 < 3; // result is true
10 > "9"; // This is a compile error.
(1+2) == 3

The equality operators == and != are supported for all types and will perform deep equal comparisons on complex types.

let tpl1 = {
  foo = "bar",
  one = 1
};
let tpl2 = tpl1{}; // copy the tpl1 tuple
tpl1 == tpl2; // returns true
let tpl3 = tpl1{duck="quack"};
tpl1 == tpl3; // returns false

Note that tuple fields are ordered so a tuple will only be equal if the fields are both in the same order and have the same values in them.

Operator Precedence

UCG binary operators follow the typical operator precedence for math. * and / are higher precendence than + and - which are higher precedence than any of the comparison operators.

Copy expressions

ucg Tuples support a form of reuse with copy on write semantics. You can copy a tuple and selectively overwrite fields or add new fields to it with the copy expression. To perform a copy first reference the tuple by a bound name and then use { field = value, ... } syntax to copy with overridden fields or add completely new fields. When replacing a preexisting field with a new value you cannot change the type of the field. This allows you to define a typed shape for a tuple with default values and then provide new values for some or all of the fields while still enforcing the same types for those fields. Adding completely new fields has no such restriction.

let base = {
    field1 = "value1",
    field2 = 100,
    field3 = 5.6,
};

let overridden = base{
    field1 = "new value"
};

let expanded = base{
    field2 = 200,
    field3 = "look ma a new field",
};

The following will cause an error because the overriden field's value does not match the original.

let bad = base{
    field1 = 300, // Error!!! must be a string.
};

Conditional data

ucg supports a limited form of conditional data selection of using the select expression. A select expression starts with the select keyword and is followed by a an expression resolving to a string naming the field to select, an expression resolving to the default value, and a tuple to select the field from. If the field selected is not in the tuple then the default value will be used.

let want = "baz";

//     field  default
select want, "quux", {
    baz = "foo",
    fuzz = "bang",
}; // result will be "foo"

//     field    default
select "quack", "quux", {
    baz = "foo",
    fuzz = "bang",
}; // result will be "quux"

Macros

Macros look like functions but they are resolved at compile time and configurations don't execute so they never appear in output. Macros do not close over their environment so they can only reference values defined in their arguments. They can't refer to bindings or other macros defined elsewhere. They are useful for constructing tuples of a certain shape or otherwise promoting data reuse. You define a macro with the macro keyword followed by the arguments in parentheses, a =>, and then a tuple.

let mymacro = macro (arg1, arg2) => {
    host = arg1,
    port = arg2,
    connstr = "couchdb://@:@" % (arg1, arg2),
}

let my_dbconf = mymacro("couchdb.example.org", "9090");

let my_dbhost = dbconf.host;

macros always resolve to a tuple. If you want to get a single value out you can use selector syntax to retrieve it.

List macros

ucg supports a couple of ways to use macros for mapping or filtering a list to a new list.

A map expression starts with the map keyword followed by the name of a macro that takes exactly one argument, a ., and the name of the output field for the macro. ucg will apply the macro to each element of the list and then take the output field from the resulting tuple and append it to the resulting list. If the output field does not exist in the macro output tuple it will be a compile error.

let list = [1, 2, 3, 4];
let mapper = macro(item) => { result = item + 1 };

// results in: [2, 3, 4, 5]
let mapped = map mapper.result list;

A filter expression starts with the filter keyword followed by the name of a macro with exactly one argument, a ., and the name of the output field for the macro. The filter will apply the macro to each element of the list and if the output field is a value that is not NULL then the list element is appended to the output list. If the output field returns a NULL Value then the element is not appended to the output list. If the output field does not exist in the macro it will be a compile error.

let list = ["foo", "bar", "foo", "bar"];
let filtrator = macro(item) => {
  ok = select item NULL {
    foo = 1
  }
};

// results in: ["foo", "foo"]
let filtered = filter filtrator.ok list;

Statements

There are several kinds of statements in a ucg file. expression statements, let statements, import statements, assert statements, and out statements. All ucg statements must be terminated by a semicolon.

Expression statements

The simplest and least useful is the expression statement. It is any valid expression followed by a semicolon.

1;
4 / 2;
"foo";
"foo" + "bar";

Despite the fact that these are valid the results are thrown away and can essentially be considered a noop. If we ever create a repl for ucg statements they may prove more useful.

Named value statements

There are two statements that can introduce a named value for a given ucg file. Let statnements and import statements. Any collisions in binding names inside a file are treated as compile errors. Bindings are immutable and once bound they can't be modified.

  • Let statements

The let statement binds the result of any valid expression to a name. It starts with the let keyword and is followed by the name of the binding, an =, and a valid ucg expression.

let name = "foo";
  • Import statement

The import statement imports the contents of another ucg file into the current file with a name. The imported files named values are exposed as a tuple in the referencing file. It starts with the import keyword and is followed by a quoted path to the ucg file, the keyword as, and a name for the imported values.

import "dbconfigs.ucg" as dbconfigs;

let mysqlconf = dbconfigs.mysql;

Statements to generate output.

Some statements in ucg exist to generate an output. Either a compiled configuration or the results of test assertions.

  • Assert statement

The assert statement defines an expression that must evaluate to either true or false. Assert statements are noops except during a validation compile. They give you a way to assert certains properties about your data and can be used as a form of unit testing for your configurations. It starts with the assert keyword followed by a valid boolean ucg expression delimited by | characters.

assert host == "www.example.com";
assert |select qa, 443, {
  qa = 80,
  prod = 443,
} == 443|;

When _test.ucg files are run in a validation run then ucg will output a log of all the assertions to stdout. Giving you a simple test harness for your ucg configs.

  • Out statement

The out statement defined the output for a UCG file. It identifies an expression that will be output as a compiled artifact by the UCG compiler was well as the artifact type. The artifact type is expected to be one of the registered converters (e.g. json, exec) and the artifact file will take the same name as the ucg file with the extension replaced by the defined extension for that converter.

Converters

Converters convert the ucg intermediate format into an output artifact. Converters define two different things. A conversion for UCG's Intermediate Representation Val's and a file extension for output artifacts.

  • json: json, Serialized json datastructure.
  • flags: txt, command line flags suitable for appending to a command line as arguments.
  • env: env, a list of environment variables posix shell style.
  • exec: sh, a bash script that will exec an executable with arguments and environment variables set.

Re-exports

pub use ast::Expression;
pub use ast::Statement;
pub use ast::Value;
pub use build::Builder;
pub use build::Val;
pub use parse::parse;

Modules

ast

The definitions of the ucg AST and Tokens.
build

The build stage of the ucg compiler.
convert

The conversion stage of the ucg compiler.
error

Errors for use by the ucg compiler.
parse

The Parsing stage of the ucg compiler.
tokenizer

The tokenization stage of the ucg compiler.