CXL Overview

CXL (Clinker Expression Language) is a per-record expression language designed for ETL transformations. Every CXL program operates on one record at a time, producing output fields, filtering records, or computing derived values.

CXL is not SQL. There are no SELECT, FROM, or WHERE keywords. CXL programs are sequences of statements – emit, let, filter, distinct – that execute top to bottom against the current record.

Key differences from SQL

Boolean operators are keywords

CXL uses English keywords for boolean logic, not symbols:

$ cxl eval -e 'emit result = true and false' --field dummy=1

{
  "result": false
}

The operators &&, ||, and ! are syntax errors in CXL. Always use and, or, and not.

System namespaces use $ prefix

CXL provides built-in namespaces for accessing pipeline state, metadata, and window functions. All system namespaces are prefixed with $:

• $pipeline.* – pipeline execution context (name, counters, provenance)
• $meta.* – per-record metadata
• $window.* – window function calls
• $vars.* – user-defined pipeline variables

$ cxl eval -e 'emit name = $pipeline.name'

{
  "name": "cxl-eval"
}

Compile-time type checking

CXL catches type errors before data processing begins. The compilation pipeline runs four phases:

1. Parse – tokenize and build an AST from CXL source text
2. Resolve – bind field references, validate method names, check arity
3. Typecheck – infer types, validate operator compatibility, check method receiver types
4. Eval – execute the typed program against each record

Errors at any phase produce rich diagnostics with source locations and fix suggestions via miette.

$ cxl check transform.cxl
ok: transform.cxl is valid

If there are type errors, the checker reports them with spans:

error[typecheck]: cannot apply '+' to String and Int (at transform.cxl:12)
  help: convert one operand — use .to_int() or .to_string()

A minimal CXL program

emit greeting = "hello"
emit doubled = amount * 2
filter amount > 0

This program:

1. Emits a constant string field greeting
2. Emits doubled as twice the input amount
3. Filters out records where amount is not positive

Try it:

$ cxl eval -e 'emit greeting = "hello"' -e 'emit doubled = amount * 2' \
    --field amount=5

{
  "greeting": "hello",
  "doubled": 10
}

Statement order matters

CXL statements execute sequentially. Later statements can reference fields produced by earlier emit or let statements:

$ cxl eval -e 'let tax_rate = 0.21' -e 'emit tax = price * tax_rate' \
    --field price=100

{
  "tax": 21.0
}

A filter statement short-circuits execution – if the condition is false, remaining statements do not run and the record is excluded from output.