iBIOS Customization Example - Currency Exchange 4

currencyExample4

/**
 * Obtain a converted currency value, using a cached for exchange rates
 * The cached value of the exchange rate is used (if available) so long as it is no older than ageSeconds
 * @arg ageSeconds the maximum age of the cached value of the exchange rate that will be used in the conversion
 * @arg fromCurrency the originating currency ISO 4217 trigraph abbreviation in any case (e.g. EUR, cad, usd, Gbp)
 * @arg toCurrency the target currency ISO 4217 trigraph abbrebiation in any case (e.g. Eur, CAD, Usd, GBP)
 * @arg amount the amount to convert, denominated in the originating currency unit
 * @return the equivalent amount, denominated in the target currency unit
 */
cachedCurrencyConversion :: Double -> String -> String -> Double -> Double;
public cachedCurrencyConversion ageSeconds fromCurrency toCurrency amount =
    let
        present = now;

        oldConversion :: Maybe CachedConversion;
        oldConversion = fetch "exchangeRates" (fromCurrency ++ toCurrency); 

        newConversion :: CachedConversion;
        newConversion = store "exchangeRates" (fromCurrency ++ toCurrency) $ CachedConversion (unitExchangeRate fromCurrency toCurrency) present;
        converted cachedValue amount = amount * cachedValue.CachedConversion.rate;    
    in
        case oldConversion of
        Nothing -> converted newConversion amount;
        Just cachedConversion -> 
            if (durationToSeconds $ subtractTimeFromTime present cachedConversion.CachedConversion.time) > ageSeconds then
                // Timed out
                converted newConversion amount
            else
                // Cache still valid
                converted cachedConversion amount;
        ;