DESIGN_IDEAS.md

Exploring Various Patterns

Rule Expressions

A Rule is a string expressing a bit of logic.

Typically it includes 3 parts: a left-hand side (LHS), an operator (+, *, =, ==), and a right-hand side (RHS).

Example Rules

• Assignment Operator Examples
  • account.balance -= 100
  • discount ??= 10 - this is modern ES2019 syntax. It sets the discount to 10 only if it is unset (null-ish coalescing.)
  • invoice.total += cart.subtotal
• Logical Operators (Boolean Expressions)
  • ==
  • !=
  • >=
  • >
  • etc.

Logical AND, OR

// {"and": ["price >= 25", "price <= 50"]}
[
  {"if": {"and": ["price >= 25", "price <= 50"]}, "then": "discount = 5"},
  {"if": "price >= 100", "then": "discount = 20"},
  {"return": "discount"},
]

// Possible recursive structure?
[
  {
    "if":
      {"or": [
        "price >= 25",
        {"and": [
          "price <= 50",
          "discountApplied == false"
        ]}
      ]},
    "then": "discount = 5"
  },
  {"if": "price >= 100", "then": "discount = 20"},
  {"return": "discount"},
]

// "price >= 25 || price <= 50"
[
  {"if": "price >= 25 || price <= 50", "then": "discount = 5"},
  {"if": "price >= 100", "then": "discount = 20"},
  {"return": "discount"},
]

Syntax Ideas

Scenario: Determine customer discount: if price >= 100 then discount = 20, else price >= 25, discount = 5

Option #1

[
  { if: 'price >= 25', then: 'discount = 5' },
  { if: 'price >= 100', then: 'discount = 20' },
  { return: 'discount' },
];

// Idea: execution could map to functional-style operators
ifThen(condition, thenLogic);
ifElse(condition, thenLogic, elseLogic);

Option #2

• Pro: Familiar JS Syntax.
• Con: More complicated & error-prone parsing

['if (price >= 25) discount = 5', 'if (price >= 100) discount = 20'];

Option 3: Reference Style: JSON-Rules-Engine (Microsoft)

• IMHO, this is convoluted stylistically & difficult to write.

let microsoftRule = {
  conditions: {
    all: [
      {
        fact: 'account-information',
        operator: 'equal',
        value: 'microsoft',
        path: '$.company', // access the 'company' property of "account-information"
      },
      {
        fact: 'account-information',
        operator: 'in',
        value: ['active', 'paid-leave'], // 'status' can be active or paid-leave
        path: '$.status', // access the 'status' property of "account-information"
      },
      {
        fact: 'account-information',
        operator: 'contains', // the 'ptoDaysTaken' property (an array) must contain '2016-12-25'
        value: '2016-12-25',
        path: '$.ptoDaysTaken', // access the 'ptoDaysTaken' property of "account-information"
      },
    ],
  },
};