Getting Started

Overview

@veridtools/feel-runner is a full-featured FEEL (Friendly Enough Expression Language) engine for DMN 1.5. It evaluates FEEL expressions with:

• All 80+ standard builtin functions
• Decimal128 arithmetic precision
• Complete temporal type support (date, time, date and time, duration)
• Unary tests for DMN decision tables
• Typed contexts and closures

Install

pnpm

pnpm add @veridtools/feel-runner

npm

npm install @veridtools/feel-runner

yarn

yarn add @veridtools/feel-runner

Quickstart

import { evaluate, unaryTest } from '@veridtools/feel-runner'

// Evaluate a FEEL expression
const result = evaluate('sum([1, 2, 3]) * 2')
console.log(result.value) // 12

// With context variables
const price = evaluate('price * quantity - discount', {
  price: 100,
  quantity: 3,
  discount: 50,
})
console.log(price.value) // 250

// Unary test (for DMN decision table cells)
const test = unaryTest('[18..65]', { '?': 30 })
console.log(test.value) // true

Return type

evaluate() and unaryTest() return an EvaluationResult:

interface EvaluationResult {
  value: FeelValue        // the result (null if error)
  warnings: EvaluationWarning[]  // non-fatal issues
}

Warnings include things like undefined variables or unknown functions — the expression still evaluates, returning null for those parts.

CLI

feel-runner ships with a command-line interface.

Installed

pnpm

pnpm add -g @veridtools/feel-runner

npm

npm install -g @veridtools/feel-runner

npx

npx @veridtools/feel-runner "1 + 2 * 3"

# Evaluate a FEEL expression
feel-runner "(1 + 2) * 3 - 4 / 2"

# With context variables
feel-runner "price * qty" --ctx '{"price":10,"qty":5}'

# Unary test
feel-runner "[18..65]" --unary --input 30

feel-runner --help

Try it locally (from the repo)

git clone https://github.com/veridtools/feel-runner
cd feel-runner
pnpm install

# Run without building (via tsx)
npx tsx bin/feel-runner.ts "(1 + 2) * 3 - 4 / 2"
npx tsx bin/feel-runner.ts "sum([1,2,3,4,5])"
npx tsx bin/feel-runner.ts "price * qty" --ctx '{"price":10,"qty":5}'
npx tsx bin/feel-runner.ts 'date("2024-06-15").month'
npx tsx bin/feel-runner.ts "[18..65]" --unary --input 30
npx tsx bin/feel-runner.ts '"A","B","C"' --unary --ctx '{"?":"B"}'
npx tsx bin/feel-runner.ts --help

# Or build first and run directly
pnpm build
node dist/bin/feel-runner.js "some x in [1,2,3] satisfies x > 2"

Options

feel-runner <expression> [options]

  -u, --unary         Evaluate as unary test (requires "?" in context)
  -i, --input <v>     Input value for unary test (shorthand for --ctx '{"?": v}')
  -c, --ctx <json>    Context variables as JSON object
  -n, --no-color      Disable ANSI colors
  -h, --help          Show help

Examples

# Arithmetic
npx tsx bin/feel-runner.ts "(1 + 2) * 3 - 4 / 2"
# → 7

# With context variables
npx tsx bin/feel-runner.ts "price * qty" --ctx '{"price":10,"qty":5}'
# → 50

# Builtin functions
npx tsx bin/feel-runner.ts "sum([1,2,3,4,5])"
# → 15

# Temporal
npx tsx bin/feel-runner.ts 'date("2024-06-15").month'
# → 6

# Unary test — value in range
npx tsx bin/feel-runner.ts "[18..65]" --unary --input 30
# → true

# Unary test — value in list
npx tsx bin/feel-runner.ts '"A","B","C"' --unary --ctx '{"?":"B"}'
# → true

# Warnings are shown in yellow with code
npx tsx bin/feel-runner.ts "price * 2"
# ⚠ [NO_VARIABLE_FOUND] Variable not found: price
# → null

FEEL in DMN

In DMN decision tables, FEEL is used in two places:

1. Input cells — evaluated as unary tests against the current input value
2. Output cells / expressions — evaluated as full FEEL expressions

Use evaluate() for output columns and full expressions. Use unaryTest() for input column cells.