NFun as data language (JSON/TOML alternative)

[tmteam]

Idea

NFun expressions already look like data literals. Structs, arrays, numbers, strings — all are valid NFun expressions. Essentially, NFun is already a superset of JSON in expressiveness.

The proposal is to formalize and promote NFun as a data language — an alternative to JSON/TOML/YAML for configs, data, and templates.

Three usage modes

Mode	Capabilities	Analogue
Constant	Only literals, structs, arrays. No computations	JSON, TOML
Computable	+ expressions, variables, functions, if/else	Jsonnet, Dhall
With external variables	+ host provides context (user, order, ...)	Current main NFun use case

All three modes are the same NFun — the host decides what's available.

Example: NFun document

# ===== Server configuration =====

appName = 'MyService'
version = '2.1.0'
debug   = true

# ----- Network -----

host = 'localhost'
port = if (debug) 8080 else 443
baseUrl = 'http://{host}:{port}/api/v1'

# ----- Database -----

db = {
  host     = 'db.internal'
  port     = 5432
  name     = 'myservice'
  pool     = if (debug) 5 else 50
  connStr  = 'Host={db.host};Port={db.port};Database={db.name};Pooling=true;MaxPool={db.pool}'
}

# ----- Limits -----

maxUploadMb    = 50
maxUploadSize  = maxUploadMb * 1024 * 1024
requestTimeout = 30
retryDelays    = [1, 2, 5, 10, 30]

# ----- Logging -----

logging = {
  level   = if (debug) 'trace' else 'warning'
  console = debug
  file    = '/var/log/{appName}.log'
  maxSize = 100 * 1024 * 1024
  rotate  = 5
}

# ----- External services -----

services = [
  { name = 'auth',    url = 'http://auth.internal:3001',    timeout = 5  }
  { name = 'storage', url = 'http://storage.internal:3002', timeout = 30 }
  { name = 'notify',  url = 'http://notify.internal:3003',  timeout = 10 }
]

# ----- CORS -----

cors = {
  origins = if (debug)
    ['http://localhost:3000', 'http://localhost:5173']
  else
    ['https://myservice.com', 'https://app.myservice.com']

  methods = ['GET', 'POST', 'PUT', 'DELETE']
  maxAge  = 24 * 60 * 60
}

Key advantage: embeddability

Unlike Jsonnet/Dhall/CUE — NFun is not an external tool, but a library:

// Constant / computable — result is a value
var config = Funny.Calc<ServerConfig>(configText);

// With external variables — host provides context
var rule = Funny.ForCalc<Context, Result>(ruleText);

The host application controls the sandbox. No filesystem, no network — only what was explicitly provided.

What needs to be done

• Schema validation API — Funny.Validate<T>(text) returns errors without execution
• JSON interop — Funny.FromJson() / .ToJson() conversion
• Multiline strings (Triple-quoted multiline strings with trim margin: '''...''' #98) — critical for templates
• Raw strings with escape levels (Escape level prefix for string interpolation: $'...' $$'...' #99) — for embedded JSON/regex
• Import/include — for config composition (NFun-Lang)

Comparison with JSON

The same config in JSON:

{
  "appName": "MyService",
  "version": "2.1.0",
  "debug": true,
  "host": "localhost",
  "port": 8080,
  "baseUrl": "http://localhost:8080/api/v1",
  "db": {
    "host": "db.internal",
    "port": 5432,
    "name": "myservice",
    "pool": 5,
    "connStr": "Host=db.internal;Port=5432;Database=myservice;Pooling=true;MaxPool=5"
  },
  "maxUploadMb": 50,
  "maxUploadSize": 52428800,
  "requestTimeout": 30,
  "retryDelays": [1, 2, 5, 10, 30],
  "logging": {
    "level": "trace",
    "console": true,
    "file": "/var/log/MyService.log",
    "maxSize": 104857600,
    "rotate": 5
  },
  "services": [
    { "name": "auth",    "url": "http://auth.internal:3001",    "timeout": 5  },
    { "name": "storage", "url": "http://storage.internal:3002", "timeout": 30 },
    { "name": "notify",  "url": "http://notify.internal:3003",  "timeout": 10 }
  ],
  "cors": {
    "origins": ["http://localhost:3000", "http://localhost:5173"],
    "methods": ["GET", "POST", "PUT", "DELETE"],
    "maxAge": 86400
  }
}

Key differences:

• No comments — impossible to explain why port is 8080 or what maxSize means
• No computations — 52428800 instead of 50 * 1024 * 1024, 86400 instead of 24 * 60 * 60
• No variables — "MyService" repeated in file, baseUrl must be hardcoded with duplicated host/port
• No conditionals — need separate config.dev.json / config.prod.json files
• Mandatory double quotes on keys — visual noise
• connStr must be manually assembled with all values hardcoded and duplicated

Related issues

• Triple-quoted multiline strings with trim margin: '''...''' #98 — Multiline strings
• Escape level prefix for string interpolation: $'...' $$'...' #99 — Escape levels for string interpolation