GetRates(IObservable <string> pairs)
{
var retrievedRemotely = new Subject <(string Pair, decimal Rate)>();
var cacheStates = retrievedRemotely
.Scan(CcyCache.Empty, (cache, result) => cache.Add(result.Pair, result.Rate))
.StartWith(CcyCache.Empty); // must give it a starting value, otherwise inputs never signals
var inputs = pairs.WithLatestFrom(cacheStates
, (pair, cache) => (Pair: pair, Cache: cache));
var(finds, misses) = inputs.Partition(t => t.Cache.ContainsKey(t.Pair)
, t => Exceptional((Pair: t.Pair, Rate: t.Cache[t.Pair])) // create a successful Exceptional for pairs whose rate is found in the cache
, t => t.Pair); // keep the names of the pairs we must look up remotely
var remoteLookups = misses.SelectMany(pair =>
Observable.FromAsync(() =>
Yahoo.GetRate(pair).Map(
ex => ex,
rate => Exceptional((Pair: pair, Rate: rate)))));
return(remoteLookups
.Do(exc => exc.ForEach(result => retrievedRemotely.OnNext(result)))
.Merge(finds));
}
public static void _main()
{
var inputs = new Subject <string>();
var accumulator = (Cache : CcyCache.Empty, Message : string.Empty);
var lookups = inputs.Scan(accumulator
, (tpl, pair) => tpl.Cache.ContainsKey(pair)
? (tpl.Cache, tpl.Cache[pair].ToString())
: Yahoo.GetRate(pair)
.Map(rate => (tpl.Cache.Add(pair, rate), rate.ToString()))
.Recover(ex => (tpl.Cache, ex.Message))
.Result);
// verbose version of the above
//var main = inputs.Scan(Tuple(ImmutableDictionary.Create<string, decimal>(), string.Empty)
// , (tpl, pair) =>
// {
// if (tpl.Item1.ContainsKey(pair))
// {
// WriteLine("reusing cached value");
// return (tpl.Item1, tpl.Item1[pair].ToString());
// }
// else
// {
// WriteLine("fetching remotely...");
// return Yahoo.GetRate(pair)
// .Map(rate => (tpl.Item1.Add(pair, rate), rate.ToString()))
// .Recover(ex => (tpl.Item1, ex.Message))
// .Result;
// }
// });
var outputs = lookups.Select(t => t.Message)
.StartWith("Enter a currency pair like 'EURUSD' to get a quote, or 'q' to quit");
using (outputs.Subscribe(WriteLine))
for (string input; (input = ReadLine().ToUpper()) != "Q";)
{
inputs.OnNext(input);
}
}
public static void Run()
{
var inputs = new Subject <string>();
// var rates =
// from pair in inputs
// from rate in Observable.FromAsync(() => Yahoo.GetRate(pair))
// select rate;
var rates =
from pair in inputs
from rate in Yahoo.GetRate(pair)
select rate;
var outputs = from r in rates select r.ToString();
using (inputs.Trace("inputs"))
using (rates.Trace("rates"))
using (outputs.Trace("outputs"))
for (string input; (input = ReadLine().ToUpper()) != "Q";)
{
inputs.OnNext(input);
}
}