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|
/// Demystify Property-Based Testing
///
/// Arnaud Bailly
///
/// @abailly.bsky.social - @abailly@piaille.fr - @dr_c0d3 (X)
///
/// https://www.linkedin.com/in/arnaudbailly/
///
/// https://abailly.github.io
///
/// An experiment and an experience
///
/// https://app.radicle.xyz/nodes/seed.hydra.bzh/rad:z28SSH7hyCTu387guFhtPXYxcwtUs/tree/pbt/README.md
///
/// 1. Introduction
///
/// 2. Real world use cases
///
/// 3. Developing a mini-quickcheck from a "concrete" use case
///
import {Option, fromNullable, matchW, toNullable} from 'fp-ts/lib/Option.js';
// keep it simple
export type Account = string;
/// A basic "Adapter" to a banking system
///
/// NOTE: we make the functions Promise-aware in order to support
/// async/await easily.
export interface Bank {
/// Retrieve the balance of given account.
///
/// @param account The account to retrieve balance of
/// @return: nothing if account is unknown, its balance otherwise
balance: (account: Account) => Promise<Option<number>>;
/// Deposit amount into account
///
/// This function increases the balance of the given account by the
/// specified amount. It's a `void` function hence implicitly
/// stateful: It's expected to change some internal state of the
/// implementing class.
///
/// @param account The account to deposit into
/// @param amt The amount to deposit
deposit: (account: Account, amt: number) => Promise<void>;
/// Withdraw amount from given `account`.
///
/// This function decreases the balance of the given account by the
/// specified amount.
///
/// @param account The account to deposit into
/// @param amt The amount to withdraw
withdraw: (account: Account, amt: number) => Promise<void>;
}
/// All possible operations
export type Operation = GetBalance | Withdraw | Deposit;
export type GetBalance = {op: 'GetBalance'; account: Account};
export type Withdraw = {op: 'Withdraw'; account: Account; amount: number};
export type Deposit = {op: 'Deposit'; account: Account; amount: number};
//
//
//
//
//
//
/// A model interpreting bank operations on a simple in-memory map.
/// Interpreting a sequence of operations on the model will return an
/// updated state (map of accounts) and a sequence of observed outputs.
export function model(
accounts: Map<Account, number>,
): (ops: Operation[]) => [Map<Account, number>, (number | null)[]] {
return (ops: Operation[]) => {
const current = new Map(accounts);
const outputs: (number | null)[] = [];
for (const op of ops) {
switch (op.op) {
case 'GetBalance': {
outputs.push(current.get(op.account) || null);
break;
}
case 'Deposit': {
const bal = current.get(op.account);
if (bal) {
current.set(op.account, bal + op.amount);
}
break;
}
case 'Withdraw': {
const bal = current.get(op.account);
if (bal) {
current.set(op.account, bal - op.amount);
}
break;
}
}
}
return [current, outputs];
};
}
/// A predicate asserting that some `Bank` implementation matches
/// our model.
export const bankImplementationMatchesModel =
(
initialAccounts: Map<Account, number>, // Some initial dataset to work with
) =>
(
bank: Bank, // The actual bank implementation we want to test
) =>
async (
ops: Operation[], // an (arbitrary) sequence of operations
): Promise<boolean> => {
// run operations on bank implementation
const observedOutputs: (number | null)[] = [];
for (const op of ops) {
switch (op.op) {
case 'Deposit':
await bank.deposit(op.account, op.amount);
break;
case 'Withdraw':
await bank.withdraw(op.account, op.amount);
break;
case 'GetBalance':
observedOutputs.push(await bank.balance(op.account).then(toNullable));
break;
}
}
// evaluate operations on model
const [endBalances, outputs] = model(initialAccounts)(ops);
// compare outputs
for (const [expected, actual] of outputs.map((v, i) => [
v,
observedOutputs[i],
])) {
if (expected !== actual) {
console.log(`Output mismatch: expected ${expected}, got ${actual}`);
return false;
}
}
// compare end balances
for (const [account, expected] of endBalances.entries()) {
const actual = await bank.balance(account).then(toNullable);
if (actual !== expected) {
console.log(
`Final balance mismatch for account ${account}: expected ${expected}, got ${actual}`,
);
return false;
}
}
return true;
};
//
//
//
//
//
//
//
/// A "simulation" of Bank interface for integration testing purpose.
/// This
export class SimpleBank implements Bank {
private accounts: Map<Account, number>;
constructor(accounts: Map<Account, number>) {
// clone the content of the map
this.accounts = new Map(accounts);
}
balance(account: Account): Promise<Option<number>> {
return Promise.resolve(fromNullable(this.accounts.get(account)));
}
deposit(account: Account, amt: number): Promise<void> {
const current = fromNullable(this.accounts.get(account));
matchW(
() => {
throw new Error(`Unknown account ${account}`);
},
(bal: number) => {
this.accounts.set(account, bal + amt);
},
)(current);
return Promise.resolve();
}
withdraw(account: Account, amt: number): Promise<void> {
const current = fromNullable(this.accounts.get(account));
matchW(
() => {
throw new Error(`Unknown account ${account}`);
},
(bal: number) => {
this.accounts.set(account, bal - amt);
},
)(current);
return Promise.resolve();
}
}
|
