Option types for C# with LINQ support and rich fluent syntax for many popular uses.
To add this to your project, execute this command in the directory with your csproj
file:
dotnet add package Mackiovello.Maybe
Alternatively, add this directly to your csproj
file:
<PackageReference Include="Mackiovello.Maybe" Version="1.0.0" />
Then, add using Mackiovello.Maybe
to the top of all the files where you want to use this library.
All these examples require that you have the following using statement:
using Mackiovello.Maybe
Maybe<string> maybeGood = "hello".ToMaybe();
Maybe<string> maybeJunk = Maybe<string>.Nothing;
var concat = from good in maybeGood
from junk in maybeJunk
select good + junk;
if (concat.IsNothing())
Console.WriteLine("One of the strings was bad, could not concat");
LINQ terminates the computation if there's a Nothing
at any point in the
computation.
string nullString = null;
nullString.ToMaybe().Do(str =>
{
// str will never be null, ToMaybe guards against null and Do unwraps the value
});
You can check a condition on a maybe type and guard against them:
string name = "Bill Casarin";
Maybe<string> maybeName = from n in name.ToMaybe()
where n.StartsWith("Bill")
select n;
If the name didn't start with Bill, maybeName
would be Maybe<string>.Nothing
Maybe has an operator similar to the null coalescing operator ??
. We achieve
optional short-circuit evaluation with lambdas:
Maybe<string> name1 = Maybe<string>.Nothing;
Maybe<string> name2 = "Some Name".ToMaybe();
Maybe<string> goodNameLazy = name1.Or(() => name2);
// this works too:
Maybe<string> goodName = name1.Or(name2);
// and this:
Maybe<string> goodName = name1.Or("goodName");
You can also convert value-kinded maybe types to Nullable<T>
s:
Maybe<int> maybeNumber = Maybe<int>.Nothing;
Maybe<int> maybeAnotherNumber = (4).ToMaybe();
int? ok = maybeNumber.ToNullable() ?? maybeAnotherNumber.ToNullable();
Sometime you want to pull out a value with a default value in case of Nothing
:
Maybe<string> possibleString = Maybe<string>.Nothing;
string goodString = possibleString.OrElse("default");
The default parameter can also be lazy:
string goodString = possibleString.OrElse(() => doHeavyComputationForString());
Or you can throw an exception instead:
string val = null;
try
{
val = (Maybe<string>.Nothing).OrElse(() => new Exception("no value"));
}
catch (Exception)
{
// exception will be thrown
}
Or, finally, you can just get the default value for that type:
string val = maybeString.OrElseDefault();
Nullable only works on value types. Maybe works on both value and reference types. It also has LINQ support.
public static Maybe<T> Head<T>(this IEnumerable<T> xs)
{
foreach(var x in xs)
return x.ToMaybe();
return Maybe<T>.Nothing;
}
Now lets get a bunch of heads!
var result = from h1 in list1.Head()
from h2 in list2.Head()
from h3 in list3.Head()
select ConsumeHeads(h1, h2, h3);
ConsumeHeads will never run unless all Head() calls return valid results.
Here's a function for getting a value out of a dictionary:
public static Maybe<T2> Lookup<T, T2>(this IDictionary<T, T2> d, T key)
{
var getter = MaybeFunctionalWrappers.Wrap(d.TryGetValue);
return getter(key);
}
public static Maybe<int> ParseInt(string s)
{
var parser = MaybeFunctionalWrappers.Wrap(int.TryParse);
return parser(s);
}
var parsedFromDict = from val in d.Lookup("key")
from parsedVal in ParseInt(val)
select parsedVal;
To build this library, you need to install .NET Core SDK 2.1. If it's installed, you should be able to do this:
$ dotnet --version
2.1.301
If your version is higher or equal to 2.1, you are good to go.
To build, execute dotnet build
at the root of the repository. Similarly, to test, execute dotnet test
at the root:
$ dotnet test
Build started, please wait...
Build completed.
Test run for /home/user/Mackiovello.Maybe/test/Mackiovello.Maybe.Tests/bin/Debug/netcoreapp2.1/Mackiovello.Maybe.Tests.dll(.NETCoreApp,Version=v2.1)
Microsoft (R) Test Execution Command Line Tool Version 15.7.0
Copyright (c) Microsoft Corporation. All rights reserved.
Starting test execution, please wait...
Total tests: 19. Passed: 19. Failed: 0. Skipped: 0.
Test Run Successful.
Test execution time: 1.1823 Seconds
The tests are xUnit tests, so you can execute them with any xUnit test runner.