Data.Maybe

Option types for C# with LINQ support

Examples

Computing on maybe types

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 will terminate the computation if there is a Nothing at any point in the computation.

Running a computation with a maybe type:

string nullString = null;

nullString.ToMaybe().Run(str => {
  // str will never be null, ToMaybe guards against them and Run unwraps them
});

Guarding

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 coalescing

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 Nullables:

Maybe<int> maybeNumber = Maybe<int>.Nothing;
Maybe<int> maybeAnotherNumber = (4).ToMaybe();

int? ok = maybeNumber.ToNullable() ?? maybeAnotherNumber.ToNullable();

Extracting values

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.FromMaybe("default");

The default parameter can also be lazy:

string goodString = possibleString.FromMaybe(() => doHeavyComputationForString());

Or you can throw an exception instead:

string val = null;
try {
  val = (Maybe<string>.Nothing).FromMaybe(() => 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.FromMaybe();

Why not use Nullable instead?

Nullable only works on value types. Maybe works on both value and reference types. It also has LINQ support.

More interesting examples

Getting the first element of a list

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.

Lookups

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) {
  T2 outTest;
  var has = d.TryGetValue(key, out outTest);
  if (!has) return Maybe<T2>.Nothing;
  return outTest.ToMaybe();
}

Parsing

public static Maybe<int> ParseInt(string s) {
  int o;
  return int.TryParse(s, out o) ? o.ToMaybe() : Maybe<int>.Nothing;
}

Lookup + Parsing!

var parsedFromDict = from val in d.Lookup("key")
                     from parsedVal in ParseInt(val)
                     select parsedVal;