Merge pull request #378 from fsprojects/repo-assist/test-splitAt-zipWith-sideeffects-2026-04-04-821c94b644eb99e1

[Repo Assist] test: add SideEffects tests to TaskSeq.splitAt and TaskSeq.zipWith

Author: dsyme

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

@@ -95,3 +95,77 @@ module Immutable =
         let combined = Array.append prefix restArr
         combined |> should equal data
     }
+
+module SideEffects =
+    [<Theory; ClassData(typeof<TestSideEffectTaskSeq>)>]
+    let ``TaskSeq-splitAt prefix gets first n elements from side-effect seq`` variant = task {
+        let ts = Gen.getSeqWithSideEffect variant
+        let! prefix, _ = TaskSeq.splitAt 3 ts
+        prefix |> should equal [| 1; 2; 3 |]
+    }
+
+    [<Theory; ClassData(typeof<TestSideEffectTaskSeq>)>]
+    let ``TaskSeq-splitAt rest yields remaining elements from side-effect seq`` variant = task {
+        let ts = Gen.getSeqWithSideEffect variant
+        let! _, rest = TaskSeq.splitAt 3 ts
+        let! restArr = TaskSeq.toArrayAsync rest
+        restArr |> should equal [| 4..10 |]
+    }
+
+    [<Theory; ClassData(typeof<TestSideEffectTaskSeq>)>]
+    let ``TaskSeq-splitAt prefix and rest together cover all elements of side-effect seq`` variant = task {
+        let ts = Gen.getSeqWithSideEffect variant
+        let! prefix, rest = TaskSeq.splitAt 5 ts
+        let! restArr = TaskSeq.toArrayAsync rest
+        let combined = Array.append prefix restArr
+        combined |> should equal [| 1..10 |]
+    }
+
+    [<Fact>]
+    let ``TaskSeq-splitAt rest is lazy: side effects in rest not triggered until consumed`` () = task {
+        let mutable restSideEffectCount = 0
+
+        let ts = taskSeq {
+            yield 1
+            yield 2
+            yield 3
+
+            // These yields are in the "rest" portion
+            restSideEffectCount <- restSideEffectCount + 1
+            yield 4
+            restSideEffectCount <- restSideEffectCount + 1
+            yield 5
+        }
+
+        let! _prefix, rest = TaskSeq.splitAt 3 ts
+        // rest has NOT been consumed yet; the side effects in it should not have fired
+        restSideEffectCount |> should equal 0
+
+        // Now consume rest
+        let! restArr = TaskSeq.toArrayAsync rest
+        restArr |> should equal [| 4; 5 |]
+        restSideEffectCount |> should equal 2
+    }
+
+    [<Fact>]
+    let ``TaskSeq-splitAt second evaluation of side-effect seq yields next batch`` () = task {
+        let mutable i = 0
+
+        let ts = taskSeq {
+            for _ = 1 to 10 do
+                i <- i + 1
+                yield i
+        }
+
+        // First split
+        let! prefix1, rest1 = TaskSeq.splitAt 4 ts
+        let! restArr1 = TaskSeq.toArrayAsync rest1
+        prefix1 |> should equal [| 1; 2; 3; 4 |]
+        restArr1 |> should equal [| 5..10 |]
+
+        // Second split of the same 'ts' uses the same 'i' capture; i is now 10
+        let! prefix2, rest2 = TaskSeq.splitAt 4 ts
+        let! restArr2 = TaskSeq.toArrayAsync rest2
+        prefix2 |> should equal [| 11; 12; 13; 14 |]
+        restArr2 |> should equal [| 15..20 |]
+    }

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

@@ -172,3 +172,86 @@ module Immutable =
 
         viaZipWith |> should equal viaZipMap
     }
+
+module SideEffects =
+    [<Theory; ClassData(typeof<TestSideEffectTaskSeq>)>]
+    let ``TaskSeq-zipWith on two side-effect seqs combines elements correctly`` variant = task {
+        let s1 = Gen.getSeqWithSideEffect variant
+        let s2 = Gen.getSeqWithSideEffect variant
+
+        // Both sequences yield 1..10 on first iteration; side effects increment independently
+        let! result = TaskSeq.zipWith (+) s1 s2 |> TaskSeq.toArrayAsync
+
+        result
+        |> should equal (Array.init 10 (fun i -> (i + 1) + (i + 1)))
+    }
+
+    [<Theory; ClassData(typeof<TestSideEffectTaskSeq>)>]
+    let ``TaskSeq-zipWithAsync on two side-effect seqs combines elements correctly`` variant = task {
+        let s1 = Gen.getSeqWithSideEffect variant
+        let s2 = Gen.getSeqWithSideEffect variant
+
+        let! result =
+            TaskSeq.zipWithAsync (fun a b -> Task.fromResult (a * b)) s1 s2
+            |> TaskSeq.toArrayAsync
+
+        result
+        |> should equal (Array.init 10 (fun i -> (i + 1) * (i + 1)))
+    }
+
+    [<Fact>]
+    let ``TaskSeq-zipWith consumes both sequences one element at a time`` () = task {
+        let mutable count1 = 0
+        let mutable count2 = 0
+
+        let s1 = taskSeq {
+            for i in 1..5 do
+                count1 <- count1 + 1
+                yield i
+        }
+
+        let s2 = taskSeq {
+            for i in 10..14 do
+                count2 <- count2 + 1
+                yield i
+        }
+
+        let! result = TaskSeq.zipWith (+) s1 s2 |> TaskSeq.toArrayAsync
+        result |> should equal [| 11; 13; 15; 17; 19 |]
+        count1 |> should equal 5
+        count2 |> should equal 5
+    }
+
+    [<Fact>]
+    let ``TaskSeq-zipWith truncates at shorter side-effect seq, output is correct`` () = task {
+        let mutable longCount = 0
+
+        let short = taskSeq { yield! [ 1; 2 ] }
+
+        let long = taskSeq {
+            for i in 10..20 do
+                longCount <- longCount + 1
+                yield i
+        }
+
+        let! result = TaskSeq.zipWith (+) short long |> TaskSeq.toArrayAsync
+        result |> should equal [| 11; 13 |]
+        // The implementation reads one element from each sequence to check for stop condition,
+        // so the longer sequence is advanced one step beyond the last paired element.
+        longCount |> should be (greaterThanOrEqualTo 2)
+        longCount |> should be (lessThanOrEqualTo 3)
+    }
+
+    [<Theory; ClassData(typeof<TestSideEffectTaskSeq>)>]
+    let ``TaskSeq-zipWith3 on three side-effect seqs combines elements correctly`` variant = task {
+        let s1 = Gen.getSeqWithSideEffect variant
+        let s2 = Gen.getSeqWithSideEffect variant
+        let s3 = Gen.getSeqWithSideEffect variant
+
+        let! result =
+            TaskSeq.zipWith3 (fun a b c -> a + b + c) s1 s2 s3
+            |> TaskSeq.toArrayAsync
+
+        result
+        |> should equal (Array.init 10 (fun i -> 3 * (i + 1)))
+    }