Structured Output

Devlish programs can return structured data to their caller using Respond with for success and Fail with for errors. This is the primary mechanism for programs called as tools by LLMs or other systems.

Respond With (Success)

Respond with serializes a value as JSON, writes it to the program’s output, and stops execution.

result equals record with "completed" as status and 42 as score
Respond with result

Output:

{
  "status": "completed",
  "score": 42
}

Any value type works: records, lists, strings, numbers, or booleans.

Respond with list of "tax_credit", "depreciation", and "bonus"

Fail With (Structured Error)

When Fail with receives a record, it serializes as JSON and exits with a non-zero code:

Fail with record with "awaiting_input" as status and "Cannot classify document" as message

Output (exit 1):

{
  "status": "awaiting_input",
  "message": "Cannot classify document"
}

When Fail with receives a plain string, the error message is returned directly:

Fail with "Missing required field: customer_email"

Response Format via MCP

When a Devlish program runs as an MCP tool, the response is wrapped in the MCP result envelope:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "{\"status\":\"completed\",\"score\":42}"
      }
    ]
  }
}

The calling LLM receives the JSON string in the tool result and can parse it for further reasoning.

Building Records for Response

Compose complex responses by building records step by step:

# Gather data
Read JSON from "analysis.json" as raw_data
findings equals filter items of raw_data where severity equals "high"
finding_count equals count of findings

# Build response record
response equals record with "success" as status and finding_count as total_findings and findings as high_severity_items

Respond with response

Conditional Responses

Use branching to return different shapes based on results:

If validation_passed:
  Respond with record with "pass" as result and score as final_score
Otherwise:
  Fail with record with "fail" as result and errors as validation_errors

Response in Methods

In class-style programs, respond with serves as both the method return value and the tool output:

Finance's Credit Analyzer:

analyze credit using project_id and credit_type:
  # ... computation ...
  result equals record with project_id as id and calculated_value as value and "ITC" as type
  respond with result

Full Tool Example

# Permissions:
#   Read files from "/data/"

Read JSON from "/data/projects.json" as projects
active equals filter projects where status equals "active"
total_value equals reduce active starting at 0 with sum and item to sum plus value of item

Respond with record with count of active as project_count and total_value as portfolio_value