diff --git a/Xledger.Collections.Test/TestImmArray.cs b/Xledger.Collections.Test/TestImmArray.cs
index 0771ac7..3c6fe71 100644
--- a/Xledger.Collections.Test/TestImmArray.cs
+++ b/Xledger.Collections.Test/TestImmArray.cs
@@ -172,5 +172,16 @@ public void TestSpan() {
     }
 
     public record Employee(int Id, string Name, decimal Salary);
+
+    [Fact]
+    public void TestToImmArrayWithFunc() {
+        var arr = Enumerable.Range(-1, 100).Select(i => $"{i}: {2*i}").ToArray();
+        var imm = Enumerable.Range(-1, 100).ToImmArray(i => $"{i}: {2 * i}");
+        Assert.Equal(arr.Length, imm.Length);
+        Assert.Equal("-1: -2", imm[0]);
+        for (int i = 0; i < imm.Length; ++i) {
+            Assert.Equal(arr[i], imm[i]);
+        }
+    }
 }
 
diff --git a/Xledger.Collections.Test/TestImmSet.cs b/Xledger.Collections.Test/TestImmSet.cs
index ced1dcc..b9a360d 100644
--- a/Xledger.Collections.Test/TestImmSet.cs
+++ b/Xledger.Collections.Test/TestImmSet.cs
@@ -107,6 +107,15 @@ public void TestNoOps() {
         Assert.ThrowsAny<NotSupportedException>(() => iset.Remove(4));
         Assert.ThrowsAny<NotSupportedException>(() => ((System.Collections.ICollection)iset).CopyTo((Array)null, 0));
     }
+
+    [Fact]
+    public void TestSelect() {
+        var imm = Enumerable.Range(-10, 21).ToImmSet(Math.Abs);
+        Assert.Equal(11, imm.Count);
+        for (int i = 0; i < 10; ++i) {
+            Assert.Contains(i, (ISet<int>)imm);
+        }
+    }
 }
 
 
diff --git a/Xledger.Collections/Extensions.cs b/Xledger.Collections/Extensions.cs
index e5caab7..31338bd 100644
--- a/Xledger.Collections/Extensions.cs
+++ b/Xledger.Collections/Extensions.cs
@@ -12,6 +12,16 @@ public static ImmArray<T> ToImmArray<T>(this IEnumerable<T> xs) {
         };
     }
 
+    public static ImmArray<U> ToImmArray<T, U>(this IEnumerable<T> xs, Func<T, U> selector) {
+        return xs switch {
+            null => ImmArray<U>.Empty,
+            T[] arr => new ImmArray<U>(ArrayOf(arr.Length, arr, selector)),
+            ICollection<T> coll => new ImmArray<U>(ArrayOf(coll.Count, coll, selector)),
+            IReadOnlyCollection<T> roColl => new ImmArray<U>(ArrayOf(roColl.Count, roColl, selector)),
+            _ => new ImmArray<U>(ArrayOf(xs, selector)),
+        };
+    }
+
 #if NET
     public static ImmArray<T> ToImmArray<T>(this ReadOnlySpan<T> xs) {
         return new ImmArray<T>(xs.ToArray());
@@ -26,6 +36,13 @@ public static ImmSet<T> ToImmSet<T>(this IEnumerable<T> xs) {
         };
     }
 
+    public static ImmSet<U> ToImmSet<T, U>(this IEnumerable<T> xs, Func<T, U> selector) {
+        return xs switch {
+            null => ImmSet<U>.Empty,
+            _ => new ImmSet<U>(SetOf(xs, selector)),
+        };
+    }
+
 #if NET
     public static ImmSet<T> ToImmSet<T>(this ReadOnlySpan<T> xs) {
         return new ImmSet<T>(xs.ToArray());
@@ -64,6 +81,7 @@ Func<T, V> selectVal
             _ => new ImmDict<K, V>(xs.ToDictionary(selectKey, selectVal)),
         };
     }
+
     internal static T[] ArrayOf<T>(T[] arr) {
         return (T[])arr.Clone();
     }
@@ -92,10 +110,32 @@ internal static T[] ArrayOf<T>(IEnumerable<T> xs) {
         return lst.ToArray();
     }
 
+    internal static U[] ArrayOf<T, U>(int n, IEnumerable<T> xs, Func<T, U> f) {
+        var arr = new U[n];
+        var i = 0;
+        foreach (var x in xs) {
+            arr[i] = f(x);
+            ++i;
+        }
+        return arr;
+    }
+
+    internal static U[] ArrayOf<T, U>(IEnumerable<T> xs, Func<T, U> f) {
+        var lst = new List<U>();
+        foreach (var x in xs) {
+            lst.Add(f(x));
+        }
+        return lst.ToArray();
+    }
+
     internal static HashSet<T> SetOf<T>(IEnumerable<T> xs) {
         return new HashSet<T>(xs);
     }
 
+    internal static HashSet<U> SetOf<T, U>(IEnumerable<T> xs, Func<T, U> f) {
+        return new HashSet<U>(xs.Select(f));
+    }
+
     internal static Dictionary<K, V> DictOf<K, V>(IEnumerable<KeyValuePair<K, V>> xs) {
 #if NET6_0_OR_GREATER
         return new Dictionary<K, V>(xs, null);