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skills/domain-fintech/SKILL.md

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+---
+name: domain-fintech
+description: "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"
+user-invocable: false
+---
+
+# FinTech Domain
+
+> **Layer 3: Domain Constraints**
+
+## Domain Constraints → Design Implications
+
+| 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 |
+
+---
+
+## Critical Constraints
+
+### Financial Precision
+
+```
+RULE: Never use f64 for money
+WHY: Floating point loses precision
+RUST: Use rust_decimal::Decimal
+```
+
+### 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
+```
+
+---
+
+## Trace Down ↓
+
+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
+```
+
+---
+
+## 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
+
+```rust
+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))
+    }
+}
+```
+
+---
+
+## 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 |
+
+---
+
+## Trace to Layer 1
+
+| 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) |
+
+---
+
+## Related Skills
+
+| When | See |
+|------|-----|
+| Value Object design | m09-domain |
+| Ownership for immutable | m01-ownership |
+| Arc for sharing | m02-resource |
+| Error handling | m13-domain-error |