# Lexical Structure and Syntax

This chapter summarizes the surface syntax.
See the EBNF in [Grammar](./grammar.md) for the precise grammar.

## Language name and extension

The project is named **Decodal**.
Its full descriptive name is **Deferred Constraint Data Language**, abbreviated **DCDL**.
Source files use the `.dcdl` extension.

```text
config.dcdl
schema.dcdl
service.dcdl
```

## Module source

An entire file can contain a single expression.
A sequence of field definitions at the top level is treated as an implicit object.

```dcdl
host = String;
port = Int default 8080;
```

The source above is equivalent to:

```dcdl
{
    host = String;
    port = Int default 8080;
}
```

## Comments

A comment begins with `#` and continues to the end of the line.

```dcdl
# comment
host = "127.0.0.1"; # trailing comment
```

## Semicolons

Semicolons separate object fields, `let` bindings, and `match` arms.
A trailing semicolon is allowed.

```dcdl
{
    host = "127.0.0.1";
    port = 8000;
}
```

## Identifiers

An identifier begins with an ASCII letter and may continue with ASCII letters, digits, or `_`.
Naming conventions can use `lower_snake`, `lowerCamel`, or `UpperCamel`.

```dcdl
my_config
mkConfig
IPv4Address
```

## Path references

Use a dot to reference a field.

```dcdl
config.host
config.feature_hoge.enable
```

Fields inside an object can also be defined with a dot path.

```dcdl
{
    feature_hoge.enable = false;
}
```

This represents the same structure as:

```dcdl
{
    feature_hoge = {
        enable = false;
    };
}
```

## Reserved words

The following words are reserved:

```text
let
in
match
import
default
as
true
false
```

## Operators

The primary operators are:

```text
+ - * /       arithmetic
++            array concatenation
! && ||       logical operations
== != < <= > >= comparisons
&             constraint-preserving composition
//            patch composition
default       fallback specification
as            range containment check and refinement from left to right
=>            function
.             field reference / dot-path definition
...           value constraint for an array or associative array
```

Operator precedence is defined in [Operators](./operators.md).