Expr Reference

Expr is a lightweight expression language for simple transformations. An Expr step evaluates a single expression and returns its value as the step output.

Use Expr when you don't need loops or custom functions. Use Lark when you do.

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The Basics

An Expr step contains a single expression — an object, array, scalar, or function call:

- expr: build_response
  code: |
    {
      "total": len(step_0),
      "has_data": len(step_0) > 0
    }

The result of the expression becomes the step output. There is no result = assignment — the expression itself is the result.

---

Building Objects

- expr: shape_response
  code: |
    {
      "user_id":   input.user_id,
      "org_id":    input.org_id,
      "row_count": len(step_0),
      "first_row": step_0
    }

Nest objects freely:

- expr: nested
  code: |
    {
      "meta": {
        "total": len(step_0),
        "page":  input.page
      },
      "data": step_0
    }

---

Accessing Variables

Variable	Description
input.field	Field from request body or query param
step_0	Result of first logic step
step_1	Result of second logic step
step_N	Result of Nth step (zero-indexed)

- expr: example
  code: |
    {
      "requested_by": input.user_id,
      "results":      step_0,
      "count":        len(step_0)
    }

---

Operators

Comparison

Operator	Meaning	Example
==	Equal	input.role == "admin"
!=	Not equal	input.status != "inactive"
>	Greater than	len(step_0) > 0
<	Less than	len(step_0) < 10
>=	Greater than or equal	input.age >= 18
<=	Less than or equal	input.count <= 50

Logical

Operator	Meaning	Example
&&	AND	len(step_0) > 0 && input.active == true
||	OR	input.role == "admin" || input.role == "power"
!	NOT	!input.disabled

Arithmetic

Operator	Example
+	input.count + 1
-	input.total - input.used
*	input.price * input.qty
/	input.total / input.count

---

Built-in Functions

Function	Description	Example
len(x)	Length of array or string	len(step_0) → 42

---

Conditional Expression

Use a ternary expression for inline conditionals:

- expr: check
  code: |
    len(step_0) > 0 ? step_0 : {}

- expr: status
  code: |
    {
      "status": len(step_0) > 0 ? "found" : "not_found",
      "data":   len(step_0) > 0 ? step_0 : []
    }

---

Accessing Nested Fields

# If step_0 = {"user": {"id": 1, "name": "Alice"}}
- expr: nested_access
  code: |
    {
      "user_name": step_0["user"]["name"]
    }

---

Common Patterns

Return all rows with metadata

- expr: with_meta
  code: |
    {
      "total": len(step_0),
      "data":  step_0
    }

Return first row only

- expr: first_only
  code: |
    len(step_0) > 0 ? step_0 : {}

Pass input fields into response

- expr: echo_input
  code: |
    {
      "requested_by": input.user_id,
      "filter_used":  input.status,
      "results":      step_0
    }

Boolean check

- expr: availability
  code: |
    {
      "has_results": len(step_0) > 0,
      "count":       len(step_0)
    }

---

Expr vs Lark

Need	Use
Build a response object	Expr
Count or check length	Expr
Inline conditional	Expr
Loop over rows	Lark
Define a function	Lark
String manipulation	Lark
Complex multi-step transform	Lark

When in doubt — if it fits in one expression, use Expr. If you need more than one line of logic, use Lark.