class YahooWeatherInCityInUnits
{
string City; string Units; string AppID;

services such as query optimization, which implement rewrite rules such as xs.Union(ys).Where(p) = xs.Where(p). Union(ys.Where(p)) that can be reused across many LINQ providers.

TaskAwaiter<ForeCast> GetAwaiter()
{
var www = new WebClient();
var response =
from xml in www.DownloadStringTaskAsync(…City, AppID…)
let woeid = …fish WOEID from result…
from rss in www.DownloadStringTaskAsync(…woeid…)
let forecast = …deserialize forecast from rss…
select forecast;

 

the city as a WOEID (where on earth ID), we need to make two service calls under the hood in order to retrieve the weather forecast. The first service call retrieves the WOEID of a requested city via http://where.yahooapis.com/v1/ places.q(city)?appid=XXXX. If that successfully returns, then a second call is made to retrieve the weather forecast for that location. The calls to the Web server are performed asynchronously and both return a Task<T> (in Java you would use java.util.concurrent. Future<T> to represent the result of an asynchronous operation). Since we can consider a Task<T> as a kind of collection that contains (at most) one element, it also supports the LINQ query operators, and we have turtles all the way down; the LINQ implementation for Weather is defined using the LINQ implementation of Task<T> (see Figure 5).

Though this is an extremely small and limited example, it clearly illustrates many of the techniques used to create real-world LINQ providers such as LINQ to Objects, LINQ to SharePoint, LINQ to Active Directory, LINQ to Twitter, LINQ to Netflix, and many more.

LinQ-friendly aPis

All examples so far have dealt with implementing particular LINQ providers. An orthogonal aspect of LINQ is APIs that leverage particular LINQ implementations, often LINQ to Objects. For example, LINQ to XML is an API for manipulating XML documents that has been designed specifically with LINQ in mind, which eliminates the need for a DSL such as XQuery or XPath to query and transform XML.

The Google Chart API is a Web
service that lets you dynamically cre-
ate attractive-looking charts, using a
simple URI (Uniform Resource iden-
tifier) scheme. The URI syntax for
Google charts, however, is not very
sequence friendly. For example, the
URI for the earlier sample pie chart
looks like this:
http://chart.apis.google.com/chart
?cht=p3&chtt=Top+ 5+words&chs=
500x200

Generic Query Providers
The weather service query provider ex-
ample is structured as an internal DSL.
While this provides a great user experi-
ence with maximum static typing, it al-
lows little room for reusing the actual
implementation of the provider. It is
custom built for the particular target
top-to-bottom. At the other end of the
spectrum we can create a completely
generic query provider that records a
complete query “as is,” using a little bit
of meta-programming magic.