domain-fintech

---

name: domain-fintech description: Use when building fintech applications in Rust. Keywords: fintech, trading, decimal, currency, financial, money, transaction, ledger, payment, exchange rate, precision, rounding, accounting, audit.

Domain: FinTech (Rust)

Triggers: fintech, trading, decimal, currency, financial, money, transaction, ledger, payment, exchange rate, precision, rounding, accounting, audit, compliance.

Domain Constraints -> Design Implications

Domain Rule	Design Constraint	Rust Implication
Audit trail	Immutable records	Arc, no mutation
Precision	No floating point	rust_decimal::Decimal
Consistency	Transaction boundaries	Clear ownership
Compliance	Complete logging	Structured tracing
Reproducibility	Deterministic execution	No race conditions

---

Critical Constraints

Financial Precision

RULE: Never use f64 for money
WHY: Floating point loses precision with cents
RUST: Use rust_decimal::Decimal for all monetary values

Audit Requirements

RULE: All transactions must be immutable and traceable
WHY: Regulatory compliance, dispute resolution
RUST: Arc<T> for sharing, event sourcing pattern

Consistency

RULE: Money can't disappear or appear
WHY: Double-entry accounting principles
RUST: Transaction types with validated totals

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Key Crates

Purpose	Crate
Decimal math	rust_decimal
Date/time	chrono, time
UUID	uuid
Serialization	serde
Validation	validator

Design Patterns

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

Code Pattern: Currency Type

use rust_decimal::Decimal;

#[derive(Clone, Debug, PartialEq, Eq)]
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))
    }

    /// Checked subtraction — returns None if result would be negative
    pub fn sub(&self, other: &Amount) -> Result<Amount, CurrencyMismatch> {
        if self.currency != other.currency {
            return Err(CurrencyMismatch);
        }
        if self.value < other.value {
            return Err(InsufficientFunds);
        }
        Ok(Amount::new(self.value - other.value, self.currency))
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Currency { Usd, Eur, Gbp, Jpy, Cny }

---

Common Mistakes

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

Layer Mapping

Constraint	Feature File
Immutable records	load features/domain-modeling.md
Type safety	load features/type-safety.md
Precision numbers	load features/numeric.md
Structured logging	load features/observability.md