Fix language-ext Properties & add tests

src/LeviySoft.Visor.LanguageExt.Tests/PropertyTests.cs

@@ -18,4 +18,37 @@ public void AtIndexTest()
         sut.Update(i => i * 2)(seq).ShouldBe(Seq<int?>(1, 4, 3));
         sut.Update(i => i * 2)(Seq<int?>()).ShouldBe(Seq<int?>());
     }
+
+    [Fact]
+    public void FirstTest()
+    {
+        var seq = Seq<string?>("a", "bb", "ccc");
+        var sut = Property.First<string?>(s => (s?.Length % 2) == 0);
+
+        sut.MaybeGet(seq).ShouldBe("bb");
+        sut.MaybeGet(Seq<string?>()).ShouldBeNull();
+
+        sut.Update(s => s + s?.Length)(seq).ShouldBe(Seq<string?>("a", "bb2", "ccc"));
+        sut.Update(s => s + s?.Length)(Seq<string?>()).ShouldBe(Seq<string?>());
+    }
+
+    [Fact]
+    public void AtKeyTest()
+    {
+        var map = Map(("a", 1), ("b", 2));
+        var sut = Property.AtKey<string, int>("b");
+        var sut2 = Property.AtKey("b", 42);
+
+        sut.MaybeGet(map).ShouldBe(2);
+        sut.MaybeGet(Map<string, int>()).ShouldBe(0);
+
+        sut2.MaybeGet(map).ShouldBe(2);
+        sut2.MaybeGet(Map<string, int>()).ShouldBe(42);
+
+        sut.Update(v => v * 2)(map)["b"].ShouldBe(4);
+        sut.Update(v => v * 2)(Map<string, int>()).ShouldBe(Map<string, int>());
+
+        sut2.Update(v => v * 2)(map)["b"].ShouldBe(4);
+        sut2.Update(v => v * 2)(Map<string, int>())["b"].ShouldBe(84);
+    }
 }

src/LeviySoft.Visor.LanguageExt/Internal/Utils.cs

@@ -0,0 +1,22 @@
+using LanguageExt;
+using System;
+
+namespace LeviySoft.Visor.LanguageExt.Internal;
+
+internal static class Utils
+{
+    public static int FindIndex<T>(this Seq<T> source, Func<T, bool> predicate)
+    {
+        int index = 0;
+        foreach (T elem in source)
+        {
+            if (predicate(elem))
+            {
+                return index;
+            }
+            ++index;
+        }
+
+        return -1;
+    }
+}

src/LeviySoft.Visor.LanguageExt/Property.cs

@@ -1,5 +1,6 @@
 using LanguageExt;
 using System;
+using LeviySoft.Visor.LanguageExt.Internal;
 
 namespace LeviySoft.Visor.LanguageExt;
 
@@ -13,21 +14,24 @@ public static IProperty<Seq<T>, T> AtIndex<T>(int index) =>
 
     public static IProperty<Seq<T>, T> First<T>() where T : class? => AtIndex<T>(0);
 
+    public static IProperty<Seq<T>, T> First<T>(Func<T, bool> predicate) where T : class? =>
+        Property<Seq<T>, T>.New(
+            seq => seq.FirstOrDefault(predicate),
+            upd => seq => {
+                var index = seq.FindIndex(predicate);
+                return Seq.map(seq, (i, v) => i == index ? upd(v) : v);
+            } 
+        );
+
     public static IProperty<Map<K, V>, V> AtKey<K, V>(K key) =>
         Property<Map<K, V>, V>.New(
-            map => map[key] ?? default,
+            map => map.ContainsKey(key) ? map[key] : default,
             upd => map => Map.map(map, (k, v) => Equals(k, key) ? upd(v) : v)
         );
 
     public static IProperty<Map<K, V>, V> AtKey<K, V>(K key, V defaultVal) =>
         Property<Map<K, V>, V>.New(
-            map => map[key] ?? default,
+            map => map.ContainsKey(key) ? map[key] : defaultVal,
             upd => map => map.AddOrUpdate(key, upd, upd(defaultVal))
         );
-
-    public static IProperty<Seq<T>, T> First<T>(Func<T, bool> predicate) where T : class? =>
-        Property<Seq<T>, T>.New(
-            seq => seq.First(predicate),
-            upd => seq => Seq.map(seq, (_, v) => predicate(v) ? upd(v) : v)
-        );
 }