Your First Configuration File

This page walks through creating a WCL file, evaluating it, adding a schema, and validating it.

Create a File

Create a file called hello.wcl with a simple server block:

server web {
    host = "localhost"
    port = 3000
    workers = 2
}

A block in WCL has a type (server), an optional ID (web), and a body containing attribute assignments.

Evaluate It

The eval command runs the WCL pipeline and outputs the result as JSON:

wcl eval hello.wcl

Output:

{
  "server": {
    "web": {
      "host": "localhost",
      "port": 3000,
      "workers": 2
    }
  }
}

WCL evaluates expressions, resolves references, and expands macros before producing output. For a simple file like this, the output mirrors the input structure.

Add a Schema

Extend hello.wcl to include a schema:

server web {
    host = "localhost"
    port = 3000
    workers = 2
}

schema "server" {
    host    : string
    port    : int
    workers : int
}

The schema block declares the expected type for each attribute. WCL matches schemas to blocks automatically by block type name — a schema named "server" validates all server blocks.

Validate It

wcl validate hello.wcl

If the configuration is valid, the command exits with code 0 and no output. If there is a type mismatch or a missing required field, you will see a diagnostic:

error[E071]: type mismatch for field `port`: expected int, got string
  --> hello.wcl:3:12
   |
 3 |     port = "3000"
   |            ^^^^^^ expected int

JSON Output with a Schema

Validation does not change the JSON output — wcl eval hello.wcl produces the same JSON structure as before. The schema is purely a validation constraint.

Key Concepts

Blocks: the primary unit of configuration. A block has a type, an optional ID, and a set of attributes.
Attributes: key-value pairs inside a block. Values can be literals, expressions, function calls, or references to other values.
Schemas: declare the expected type (and optionally constraints) for each attribute in a block. Schemas are matched to blocks automatically by block type name.
Decorators: @name or @name(args) annotations on blocks or attributes. Built-in decorators include @sensitive, @optional, @partial, and more.

From here, explore the CLI Quickstart to learn about the other commands available, or jump into the language reference to learn about expressions, macros, and the query engine.

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