FinTech Development

Domain Constraints

Critical Rules

• Never use f64 for money — floating point loses precision. Use rust_decimal::Decimal.
• All transactions must be immutable and traceable — regulatory compliance requires event sourcing.
• Money can’t disappear or appear — double-entry accounting with validated totals.

Key Crates

Currency Type Pattern

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))
    }
}

Design Patterns

Common Mistakes