Use when building fintech apps. Keywords: fintech, trading, decimal, currency, financial, money, transaction, ledger, payment, exchange rate, precision, rounding, accounting, 金融, 交易系统, 货币, 支付
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Layer 3: Domain Constraints
| Domain Rule | Design Constraint | Rust Implication |
|---|---|---|
| Audit trail | Immutable records | Arc<T>, no mutation |
| Precision | No floating point | rust_decimal |
| Consistency | Transaction boundaries | Clear ownership |
| Compliance | Complete logging | Structured tracing |
| Reproducibility | Deterministic execution | No race conditions |
RULE: Never use f64 for money
WHY: Floating point loses precision
RUST: Use rust_decimal::Decimal
RULE: All transactions must be immutable and traceable
WHY: Regulatory compliance, dispute resolution
RUST: Arc<T> for sharing, event sourcing pattern
RULE: Money can't disappear or appear
WHY: Double-entry accounting principles
RUST: Transaction types with validated totals
From constraints to design (Layer 2):
"Need immutable transaction records"
↓ m09-domain: Model as Value Objects
↓ m01-ownership: Use Arc for shared immutable data
"Need precise decimal math"
↓ m05-type-driven: Newtype for Currency/Amount
↓ rust_decimal: Use Decimal type
"Need transaction boundaries"
↓ m12-lifecycle: RAII for transaction scope
↓ m09-domain: Aggregate boundaries
| Purpose | Crate |
|---|---|
| Decimal math | rust_decimal |
| Date/time | chrono, time |
| UUID | uuid |
| Serialization | serde |
| Validation | validator |
| Pattern | Purpose | Implementation |
|---|---|---|
| Currency newtype | Type safety | struct Amount(Decimal); |
| Transaction | Atomic operations | Event sourcing |
| Audit log | Traceability | Structured logging with trace IDs |
| Ledger | Double-entry | Debit/credit balance |
use rust_decimal::Decimal;
#[derive(Clone, Debug, PartialEq)]
pub struct Amount {
value: Decimal,
currency: Currency,
}
impl Amount {
pub fn new(value: Decimal, currency: Currency) -> Self {
Self { value, currency }
}
pub fn add(&self, other: &Amount) -> Result<Amount, CurrencyMismatch> {
if self.currency != other.currency {
return Err(CurrencyMismatch);
}
Ok(Amount::new(self.value + other.value, self.currency))
}
}
| Mistake | Domain Violation | Fix |
|---|---|---|
| Using f64 | Precision loss | rust_decimal |
| Mutable transaction | Audit trail broken | Immutable + events |
| String for amount | No validation | Validated newtype |
| Silent overflow | Money disappears | Checked arithmetic |
| Constraint | Layer 2 Pattern | Layer 1 Implementation |
|---|---|---|
| Immutable records | Event sourcing | Arc<T>, Clone |
| Transaction scope | Aggregate | Owned children |
| Precision | Value Object | rust_decimal newtype |
| Thread-safe sharing | Shared immutable | Arc (not Rc) |
| When | See |
|---|---|
| Value Object design | m09-domain |
| Ownership for immutable | m01-ownership |
| Arc for sharing | m02-resource |
| Error handling | m13-domain-error |
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
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Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).