Asynchronous Pattern Matching

Starting with version 3.0 asynchronous pattern matching is also available.

Async Patterns

Async patterns are objects which implement the Matchmaker.Patterns.Async.IAsyncPattern<TInput, TMatchResult> interface. They are very similar to normal patterns, except they are, well, matched asynchronously. The interface contains two members:

string Description { get; }

Task<MatchResult<TMatchResult>> MatchAsync(Tinput input);

The predefined patterns (contained in the Matchmaker.Patterns.Async.AsyncPattern class) and extension methods from MatchMaker.Linq are basically the same. There are also extensions for Task<MatchResult<TMatchResult>>.

The only difference is that the Cached extension method returns a pattern which caches its inputs in a thread-safe manner.

Creating custom async patterns is also basically the same as creating custom patterns: you can use the CreatePattern methods from the AsyncPattern class, extend the Matchmaker.Patterns.Async.AsyncPattern<TInput, TMatchResult> class to get the Description property for free, or implement the IAsyncPattern<TInput, TMatchResult> directly (which is not recommended).

Normal patterns can be turned into async patterns by calling the AsAsync() extension (defined in the Matchmaker.Linq namespace).

Async Match Expressions

Async match expressions are also very similar to normal match expressions. There are two types of async match expressions: AsyncMatch<TInput, TOutput> which yields a result and AsyncMatch<TInput> which doesn't. They can be created using methods in the AsyncMatch class.

The Case methods of async match expressions are overloaded to take either async patterns or normal patterns (which are turned into async patterns using the AsAsync() extension) and to take either async or sync actions to executed when a pattern is matched successfully (sync actions are turned into pseudo-async actions).

Async match expressions can be executed using the ExecuteAsync, ExecuteNonStrictAsync and ExecuteWithFallthroughAsync methods. The ToFunction method and its variations are also available. ExecuteWithFallthroughAsync returns an IAsyncEnumerable which enable lazy async execution of match expressions.

The Matchmaker.Linq namespace contains the EnumerateAsync extension method for IEnumerable<T> which enumerates it and ignores the result. You can use it if you just want to execute the match statement with fall-through.

Static async match expressions are also available. Use the AsyncMatch.CreateStatic methods to create them, just like normal match expressions. The methods accept an action on either AsyncMatchBuilder<TInput, TOutput> or AsyncMatchBuilder<TInput>. Static match expressions are globally cached and the caching process is thread-safe. Caches can be cleared with the ClearCache methods in AsyncMatch.

Next article: Discriminated unions