add map extension #9

src/Optional/OptionalExtensions.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace DeskDirector.Text.Json
+{
+    public static class OptionalExtensions
+    {
+        public static Optional<TTarget> Map<T, TTarget>(this Optional<T> input, Func<T, TTarget> map)
+        {
+            ArgumentNullException.ThrowIfNull(map);
+
+            if (!input.IsSet(out T? value)) {
+                return Optional<TTarget>.Undefined;
+            }
+
+            if (input.IsNull() || value is null) {
+                return Optional<TTarget>.Null;
+            }
+
+            TTarget target = map(value);
+            if (target is null) {
+                throw new InvalidOperationException(
+                    $"Map function converted {typeof(T).Name} to {typeof(TTarget).Name} as NULL."
+                );
+            }
+
+            return new Optional<TTarget>(target);
+        }
+
+        public static OptionalCollection<TTarget> Map<T, TTarget>(this OptionalCollection<T> input,
+            Func<T, TTarget> map)
+        {
+            ArgumentNullException.ThrowIfNull(map);
+
+            if (!input.IsSet(out ICollection<T>? value)) {
+                return OptionalCollection<TTarget>.Undefined;
+            }
+
+            if (input.IsNull() || value is null) {
+                return OptionalCollection<TTarget>.Null;
+            }
+
+            TTarget[] targets = value.Select(map).ToArray();
+
+            return new OptionalCollection<TTarget>(targets);
+        }
+    }
+}