Merge pull request #419 from fsprojects/repo-assist/improve-trymax-trymin-20260508-f1717f1ca85019fb

[Repo Assist] feat: add TaskSeq.tryMax and TaskSeq.tryMin

Author: dsyme

release-notes.txt

@@ -2,6 +2,7 @@
 Release notes:
 
 Unreleased
+    - adds TaskSeq.tryMax and TaskSeq.tryMin: safe variants of TaskSeq.max and TaskSeq.min that return None instead of raising ArgumentException when the input sequence is empty
     - test: rename `SideEffect` module to `SideEffects` in TaskSeq.Concat.Tests.fs, TaskSeq.Delay.Tests.fs, and TaskSeq.Item.Tests.fs for consistency with the rest of the test suite (50+ files already use the plural form)
     - test: add SideEffects module to TaskSeq.Using.Tests.fs; 7 new tests verify Dispose/DisposeAsync call counts, re-iteration semantics, and early-termination disposal for use and use! CE bindings
     - perf: pairwise, distinctUntilChanged, distinctUntilChangedWith, distinctUntilChangedWithAsync now use explicit enumerator + while! instead of ValueOption tracking + for-in loop, eliminating per-element struct match overhead

src/FSharp.Control.TaskSeq.Test/TaskSeq.MaxMin.Tests.fs

@@ -10,6 +10,8 @@ open FSharp.Control
 //
 // TaskSeq.max
 // TaskSeq.min
+// TaskSeq.tryMax
+// TaskSeq.tryMin
 // TaskSeq.maxBy
 // TaskSeq.minBy
 // TaskSeq.maxByAsync
@@ -316,3 +318,87 @@ module SideEffects =
             do! test (MinMax.getByFunction minMax) 20
             do! test (MinMax.getByFunction minMax) 30
         }
+
+
+module TryMaxMin =
+    [<Fact>]
+    let ``TaskSeq-tryMax returns None for null source`` () =
+        assertNullArg
+        <| fun () -> TaskSeq.tryMax (null: TaskSeq<int>)
+
+    [<Fact>]
+    let ``TaskSeq-tryMin returns None for null source`` () =
+        assertNullArg
+        <| fun () -> TaskSeq.tryMin (null: TaskSeq<int>)
+
+    [<Theory; ClassData(typeof<TestEmptyVariants>)>]
+    let ``TaskSeq-tryMax returns None on empty`` variant = task {
+        let! result = Gen.getEmptyVariant variant |> TaskSeq.tryMax
+        result |> should equal None
+    }
+
+    [<Theory; ClassData(typeof<TestEmptyVariants>)>]
+    let ``TaskSeq-tryMin returns None on empty`` variant = task {
+        let! result = Gen.getEmptyVariant variant |> TaskSeq.tryMin
+        result |> should equal None
+    }
+
+    [<Fact>]
+    let ``TaskSeq-tryMax returns Some for singleton`` () = task {
+        let! result = TaskSeq.singleton 42 |> TaskSeq.tryMax
+        result |> should equal (Some 42)
+    }
+
+    [<Fact>]
+    let ``TaskSeq-tryMin returns Some for singleton`` () = task {
+        let! result = TaskSeq.singleton 42 |> TaskSeq.tryMin
+        result |> should equal (Some 42)
+    }
+
+    [<Theory; ClassData(typeof<TestImmTaskSeq>)>]
+    let ``TaskSeq-tryMax returns Some max of sequence`` variant = task {
+        let! result = Gen.getSeqImmutable variant |> TaskSeq.tryMax
+        result |> should equal (Some 10)
+    }
+
+    [<Theory; ClassData(typeof<TestImmTaskSeq>)>]
+    let ``TaskSeq-tryMin returns Some min of sequence`` variant = task {
+        let! result = Gen.getSeqImmutable variant |> TaskSeq.tryMin
+        result |> should equal (Some 1)
+    }
+
+    [<Fact>]
+    let ``TaskSeq-tryMax and max agree on non-empty sequence`` () = task {
+        let ts = TaskSeq.ofList [ 3; 1; 4; 1; 5; 9; 2; 6 ]
+        let! viaMax = ts |> TaskSeq.max
+        let ts2 = TaskSeq.ofList [ 3; 1; 4; 1; 5; 9; 2; 6 ]
+        let! viaTryMax = ts2 |> TaskSeq.tryMax
+        viaTryMax |> should equal (Some viaMax)
+    }
+
+    [<Fact>]
+    let ``TaskSeq-tryMin and min agree on non-empty sequence`` () = task {
+        let ts = TaskSeq.ofList [ 3; 1; 4; 1; 5; 9; 2; 6 ]
+        let! viaMin = ts |> TaskSeq.min
+        let ts2 = TaskSeq.ofList [ 3; 1; 4; 1; 5; 9; 2; 6 ]
+        let! viaTryMin = ts2 |> TaskSeq.tryMin
+        viaTryMin |> should equal (Some viaMin)
+    }
+
+    [<Theory; ClassData(typeof<TestSideEffectTaskSeq>)>]
+    let ``TaskSeq-tryMax re-iteration reflects side effects`` variant = task {
+        let ts = Gen.getSeqWithSideEffect variant
+        let! first = ts |> TaskSeq.tryMax
+        let! second = ts |> TaskSeq.tryMax
+        first |> should equal (Some 10)
+        second |> should equal (Some 20)
+    }
+
+    [<Theory; ClassData(typeof<TestSideEffectTaskSeq>)>]
+    let ``TaskSeq-tryMin re-iteration reflects side effects`` variant = task {
+        let ts = Gen.getSeqWithSideEffect variant
+        let! first = ts |> TaskSeq.tryMin
+        let! second = ts |> TaskSeq.tryMin
+        first |> should equal (Some 1)
+        second |> should equal (Some 11)
+    }

src/FSharp.Control.TaskSeq/TaskSeq.fs

@@ -303,6 +303,8 @@ type TaskSeq private () =
 
     static member max source = Internal.maxMin max source
     static member min source = Internal.maxMin min source
+    static member tryMax source = Internal.tryMaxMin max source
+    static member tryMin source = Internal.tryMaxMin min source
     static member maxBy projection source = Internal.maxMinBy (<) projection source // looks like 'less than', is 'greater than'
     static member minBy projection source = Internal.maxMinBy (>) projection source
     static member maxByAsync projection source = Internal.maxMinByAsync (<) projection source // looks like 'less than', is 'greater than'

src/FSharp.Control.TaskSeq/TaskSeq.fsi

@@ -224,6 +224,26 @@ type TaskSeq =
     /// <exception cref="T:ArgumentException">Thrown when the input task sequence is empty.</exception>
     static member min: source: TaskSeq<'T> -> Task<'T> when 'T: comparison
 
+    /// <summary>
+    /// Returns <c>Some</c> with the greatest of all elements of the task sequence, compared via <see cref="Operators.max" />,
+    /// or <c>None</c> if the sequence is empty. For sequences that should never be empty, prefer <see cref="TaskSeq.max" />.
+    /// </summary>
+    ///
+    /// <param name="source">The input task sequence.</param>
+    /// <returns>The largest element of the sequence wrapped in <c>Some</c>, or <c>None</c> if the sequence is empty.</returns>
+    /// <exception cref="T:ArgumentNullException">Thrown when the input task sequence is null.</exception>
+    static member tryMax: source: TaskSeq<'T> -> Task<'T option> when 'T: comparison
+
+    /// <summary>
+    /// Returns <c>Some</c> with the smallest of all elements of the task sequence, compared via <see cref="Operators.min" />,
+    /// or <c>None</c> if the sequence is empty. For sequences that should never be empty, prefer <see cref="TaskSeq.min" />.
+    /// </summary>
+    ///
+    /// <param name="source">The input task sequence.</param>
+    /// <returns>The smallest element of the sequence wrapped in <c>Some</c>, or <c>None</c> if the sequence is empty.</returns>
+    /// <exception cref="T:ArgumentNullException">Thrown when the input task sequence is null.</exception>
+    static member tryMin: source: TaskSeq<'T> -> Task<'T option> when 'T: comparison
+
     /// <summary>
     /// Returns the greatest of all elements of the task sequence, compared via <see cref="Operators.max" />
     /// on the result of applying the function <paramref name="projection" /> to each element.

src/FSharp.Control.TaskSeq/TaskSeqInternal.fs

@@ -238,6 +238,24 @@ module internal TaskSeqInternal =
             return acc
         }
 
+    let inline tryMaxMin ([<InlineIfLambda>] maxOrMin) (source: TaskSeq<_>) =
+        checkNonNull (nameof source) source
+
+        task {
+            use e = source.GetAsyncEnumerator CancellationToken.None
+            let! hasFirst = e.MoveNextAsync()
+
+            if not hasFirst then
+                return None
+            else
+                let mutable acc = e.Current
+
+                while! e.MoveNextAsync() do
+                    acc <- maxOrMin e.Current acc
+
+                return Some acc
+        }
+
     // 'compare' is either `<` or `>` (i.e, less-than, greater-than resp.)
     let inline maxMinBy ([<InlineIfLambda>] compare) ([<InlineIfLambda>] projection) (source: TaskSeq<_>) =
         checkNonNull (nameof source) source