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, comoposable 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

ucg is a safe language with type inference that tries to guarantee that it will halt. A valid ucg file is composesed of a series of statements. Statements are any valid ucg expression terminated by a semicolon.

Reserved words

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

  • let
  • import
  • as
  • select
  • macro

Primitive types

ucg has a relatively simple syntax with 3 primitive types, Int, Float, and String.

  • Int is any integer number.
1; // a single Integer
  • 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 is any quoted text. backslashes within a string escape the next preceding character.
"foo"; // a smiple string
"I'm a \"fine\" looking string"; // escaped quotes in a string.

Complex types

ucg has two complex data types, Lists and Tuples.

  • Lists start are surrounded with square brackets [ ] and have comma separated elements.
[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'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 any expression that resolves to a tuple or list. 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.

Binary operators

ucg supports the following 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. All of the operators are valid on integers and floats. The + operator can additionally concatenate strings or arrays.

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

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. 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 and then a tuple.

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

let my dbconf = myfunc("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.

Statements

There are 3 kinds of statements in a ucg configuration file. expression statements, let statements, and import 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.

  • Let statements

The let expression 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 bound 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;

Reexports

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

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.