David Arno edited this page May 17, 2018 · 33 revisions


Discriminated unions, pattern matching and partial applications for C#

Current release: v3.2.0

Release Notes

The current release of Succinc<T> is 3.2.0, which is available as a nuget package that supports .NET 4.5+, .Net Core and Universal Windows Platform (Windows 10 appstore apps).

This release also includes SuccincT.Json v3.2.0, which is available as a separate nuget package that supports .NET 4.5+, .Net Core and Universal Windows Platform (Windows 10 appstore apps). SuccincT.Json is dependent on Succinc<T>, so will pull that package in as part of the install. Also, please note that this nuget package is also dependent on the Newtonsoft.JSON v4.0.30319 or later nuget package.

This release includes:

Release notes for previous versions


Discriminated Unions

Succinc<T> provides a set of union types (Union<T1, T2>, Union<T1, T2, T3> and Union<T1, T2, T3, T4>) where an instance will hold exactly one value of one of the specified types. In addition, the following more specific union types are provided, built-in to Succinc<T>:

Succinc<T> uses Option<T> to provide replacements for the .NET basic types' TryParse() methods and Enum.Parse(). In all cases, these are extension methods to string and they return Some<T> on a successful parse and None when the string is not a valid value for that type. No more out parameters! See the Option Parsers guide for more details.

In addition, Succinc<T> uses Option<T> to provide replacements for the XxxxOrDefault LINQ extension methods on IEnumerable<T>, as well as a replacement for IDictionary<,>.TryGetValue. In all cases, these new extension methods, eg TryFirst<T>() return an option with a value if a match occurred, or none if not.

Finally, Succinc<T> provides extension methods on general types, for handling null and casting. See Null and casting extensions for general types, using Option<T> for details.

Pattern Matching

Succinc<T> can pattern match values, tuples, unions etc in a way similar to F#'s pattern matching features. It uses a fluent (method chaining) syntax to achieve this. Some examples of its abilities:

public static void PrintColorName(Color color) =>
         .With(Color.Red).Do(x => Console.WriteLine("Red"))
         .With(Color.Green).Do(x => Console.WriteLine("Green"))
         .With(Color.Blue).Do(x => Console.WriteLine("Blue"))

public static string SinglePositiveOddDigitReporter(Option<int> data) =>
        .Some().Of(0).Do(x => "0 isn't positive or negative")
        .Some().Where(x => x == 1 || x == 3 || x == 5 || x == 7 || x == 9).Do(x => x.ToString())
        .Some().Where(x => x > 9).Do(x => string.Format("{0} isn't 1 digit", x))
        .Some().Where(x => x < 0).Do(i => string.Format("{0} isn't positive", i))
        .Some().Do(x => string.Format("{0} isn't odd", x))
        .None().Do(() => string.Format("There was no value"))

See the Succinc<T> pattern matching guide for more details.

Partial Applications

Succinc<T> supports partial function applications. A parameter can be supplied to a multi-parameter method and a new function will be returned that takes the remaining parameters. For example:

var times = Lambda<double>((p1, p2) => p1 * p2);
var times8 = times.Apply(8);
var result = times8(9); // <- result == 72

See the Succinc<T> partial applications guide for more details.

"Implicitly" Typed Lambdas

C# doesn't support implicitly typed lambdas, meaning it's not possible to declare something like:

var times = (p1, p2) => p1 * p2;

Normally, times would have to explicitly typed:

Func<double, double, double> times = (p1, p2) => p1 * p2;

Succinc<T> offers an alternative approach, taking advantage of the fact that var can be used if the result of a method is assigned to the variable. Using the Func, Action, Transform and Lambda set of methods, the above can be expressed more simply as:

var times = Lambda<double>((p1, p2) => p1 * p2);

For functions, the Lambda methods can be used when the parameters and return type all have the same value, as above. This means the type parameter need only be specified once. Transform can be used when all the parameters are of one type and the return value, another. The Func methods are used when the parameters and/or return type are of different values. For actions, Lambda can also be used when only one type is involved and the Action methods do a similar job to the Func methods.

This is explained in detail in the Succinc<T> typed lambdas guide.

Action/Func conversions

The ToUnitFunc methods supplied by Succinc<T> can be used to cast an Action lambda or method (from 0 to 4 parameters) to a Func delegate that returns unit, allowing void methods to be treated as functions and thus eg used in ternary oprators. In addition, Succinc<T> provides an Ignore method that can be used to explicitly ignore the result of any expression, effectively turning that expression into a void statement.

These methods are explained in detail in the Succinc<T> Action/Func conversions guide.

IEnumerable<T> cons

In many functional languages, collections of data are (can be) treated as linked lists and they have the built in pattern of treating that list as the head (first element) and tail (the rest of the list). This pattern is referred to as "cons". It allows recursive composition and decomposition of the list via a simple syntax, such as:

let newList = newItem :: existingList
let head :: tail = newList

The first line adds newItem to the list, exstingItem, to create a new list, which is assigned to newList.

The second, splits newList into the value held in the first element, and assigns it to head. In addition, the remainder of newList is assigned to tail. So newItem and head have the same value and tail and existingList contain the same elements.

Succinc<T> introduces the same idea to C#, but for all types that implement IEnumerable<T>. Following the immutable-by-default conventions of functional languages, items can be added and removed from the head of an enumeration without affecting the original collection. Also, in keeping with the basic tenet of IEnumerable<T> (that it should only be enumerated once), this is also done in a way that avoids re-enumerating the collection. This is achieved by caching the collection as it is enumerated.

So for an IEnumerable<T>, enumeration, the above lines with Succinc<T> would be:

var newList = enumeration.Cons(newItem);
var (head, tail) = newList;

For more details, see the Cons help page.

Indexed IEnumerable<T>

Sometimes, when enumerating a collection, it is useful to have access to both the item itself, as well as the element's index in the collection. Succinc<T> provides an Indexed() extension method that provides an IEnumerable<(int index, T item)> enumeration:

var list = new List<int> { 1, 2, 3 };
var indexedList = list.Indexed();
foreach (var (index, item) in indexedList)
    Console.WriteLine($"Item {index} = {item}.");

For more details, see the Indexed enumerations page.

Cycle() methods

Sometimes, with declarative programming, it's useful to be able to treat a set of data as an endlessly repeating set. Haskell's solution to this is the cycle functions, which endlessly repeat a list. Succinc<T> provides Cycle methods too, as either an extension method for IEnumerable<T>, or as a method that takes a list of values as it's parameters.

An example of their use is in solving the "fizzbuzz" problem, as shown below:

var fizzes = Cycle("", "", "Fizz");
var buzzes = Cycle("", "", "", "", "Buzz");
var words = fizzes.Zip(buzzes, (f, b) => f + b);
var numbers = Range(1, 100);
var fizzBuzz = numbers.Zip(words, (n, w) => w == "" ? n.ToString() : w);

In the above code, fizzBuzz is a 100 element enumeration of the form "1", "2", "Fizz", "4", "Buzz", etc. All achieved without using a single if or loop.

For more details, see the Cycle help page.

Pipe operators

Another feature in many functional languages is the concept of the pipe operator. The idea behind it is that, rather than expressing a series of functions - where each returns a value which is passed to the next function - as:

let result = f4(f3(f2(f1(value))))

The expression can instead be written in an easier to read form:

let result = value |> f1 |> f2 |> f3 |> f4

Succinc<T> offers similar functionality. Custom operators aren't supported in C#, so the syntax isn't as neat as the above, but by using an extension method, something close can be achieved:

var result = value.Into(f1).Into(f2).Into(f3).Into(f4);

For more details, see the Forward Pipe Operator page.


Succinc<T> provides JSON.Net serialization support for its types. Using the Succinc<T> serializers, all types can now be correctly serialized/deserialized to JSON.

For details, see the Serializing to JSON page.


The following types are defined by Succinc<T>.

You can’t perform that action at this time.
You signed in with another tab or window. Reload to refresh your session. You signed out in another tab or window. Reload to refresh your session.
Press h to open a hovercard with more details.