Merge pull request #348 from fsprojects/repo-assist/improve-fold-tests-20260316-10feb1bd15349016

[Repo Assist] test: expand TaskSeq.fold tests with call-count, ordering, and edge case coverage

Author: dsyme

release-notes.txt

@@ -5,6 +5,7 @@ Release notes:
     - adds TaskSeq.distinctUntilChangedWith and TaskSeq.distinctUntilChangedWithAsync, #345
     - adds TaskSeq.withCancellation, #167
     - adds docs/ with fsdocs-based documentation site covering generating, transforming, consuming, combining and advanced operations
+    - test: adds 70 new tests to TaskSeq.Fold.Tests.fs covering call-count assertions, folder-not-called-on-empty, ordering, null initial state, and fold/foldAsync equivalence
 
 0.7.0
     - performance: TaskSeq.exists, existsAsync, contains no longer allocate an intermediate Option value

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

@@ -39,6 +39,181 @@ module EmptySeq =
         alphabet |> should equal '_'
     }
 
+    [<Theory; ClassData(typeof<TestEmptyVariants>)>]
+    let ``TaskSeq-fold does not call folder function when empty`` variant = task {
+        let mutable called = false
+
+        let! _ =
+            Gen.getEmptyVariant variant
+            |> TaskSeq.fold
+                (fun state _ ->
+                    called <- true
+                    state)
+                0
+
+        called |> should be False
+    }
+
+    [<Theory; ClassData(typeof<TestEmptyVariants>)>]
+    let ``TaskSeq-foldAsync does not call folder function when empty`` variant = task {
+        let mutable called = false
+
+        let! _ =
+            Gen.getEmptyVariant variant
+            |> TaskSeq.foldAsync
+                (fun state _ -> task {
+                    called <- true
+                    return state
+                })
+                0
+
+        called |> should be False
+    }
+
+module Functionality =
+    [<Fact>]
+    let ``TaskSeq-fold calls folder exactly N times for N elements`` () = task {
+        let mutable callCount = 0
+
+        let! _ =
+            TaskSeq.ofList [ 1; 2; 3; 4; 5 ]
+            |> TaskSeq.fold
+                (fun acc item ->
+                    callCount <- callCount + 1
+                    acc + item)
+                0
+
+        callCount |> should equal 5
+    }
+
+    [<Fact>]
+    let ``TaskSeq-foldAsync calls folder exactly N times for N elements`` () = task {
+        let mutable callCount = 0
+
+        let! _ =
+            TaskSeq.ofList [ 1; 2; 3; 4; 5 ]
+            |> TaskSeq.foldAsync
+                (fun acc item -> task {
+                    callCount <- callCount + 1
+                    return acc + item
+                })
+                0
+
+        callCount |> should equal 5
+    }
+
+    [<Fact>]
+    let ``TaskSeq-fold over singleton calls folder once`` () = task {
+        let mutable callCount = 0
+
+        let! result =
+            TaskSeq.singleton 42
+            |> TaskSeq.fold
+                (fun acc item ->
+                    callCount <- callCount + 1
+                    acc + item)
+                0
+
+        result |> should equal 42
+        callCount |> should equal 1
+    }
+
+    [<Fact>]
+    let ``TaskSeq-fold over two elements calls folder twice`` () = task {
+        let mutable callCount = 0
+
+        let! result =
+            taskSeq {
+                yield 10
+                yield 20
+            }
+            |> TaskSeq.fold
+                (fun acc item ->
+                    callCount <- callCount + 1
+                    acc + item)
+                0
+
+        result |> should equal 30
+        callCount |> should equal 2
+    }
+
+    [<Fact>]
+    let ``TaskSeq-fold is left-associative: applies folder left-to-right`` () = task {
+        // For non-commutative ops like string concat, order matters.
+        // fold f s [a;b;c] = f (f (f s a) b) c
+        let! result =
+            TaskSeq.ofList [ "b"; "c"; "d" ]
+            |> TaskSeq.fold (fun acc item -> acc + item) "a"
+
+        result |> should equal "abcd"
+    }
+
+    [<Fact>]
+    let ``TaskSeq-foldAsync is left-associative: applies folder left-to-right`` () = task {
+        let! result =
+            TaskSeq.ofList [ "b"; "c"; "d" ]
+            |> TaskSeq.foldAsync (fun acc item -> task { return acc + item }) "a"
+
+        result |> should equal "abcd"
+    }
+
+    [<Fact>]
+    let ``TaskSeq-fold with null initial state works for reference types`` () = task {
+        let! result =
+            TaskSeq.ofList [ "hello"; " "; "world" ]
+            |> TaskSeq.fold
+                (fun acc item ->
+                    match acc with
+                    | null -> item
+                    | _ -> acc + item)
+                null
+
+        result |> should equal "hello world"
+    }
+
+    [<Fact>]
+    let ``TaskSeq-foldAsync and fold return the same result for pure functions`` () = task {
+        let input = [ 1..10 ]
+
+        let! syncResult =
+            TaskSeq.ofList input
+            |> TaskSeq.fold (fun acc item -> acc + item) 0
+
+        let! asyncResult =
+            TaskSeq.ofList input
+            |> TaskSeq.foldAsync (fun acc item -> task { return acc + item }) 0
+
+        syncResult |> should equal asyncResult
+    }
+
+    [<Fact>]
+    let ``TaskSeq-fold accumulates a list in correct order`` () = task {
+        let! result =
+            TaskSeq.ofList [ 1; 2; 3; 4; 5 ]
+            |> TaskSeq.fold (fun acc item -> acc @ [ item ]) []
+
+        result |> should equal [ 1; 2; 3; 4; 5 ]
+    }
+
+    [<Theory; ClassData(typeof<TestImmTaskSeq>)>]
+    let ``TaskSeq-fold sum over immutable variants`` variant = task {
+        // items are 1..10; sum = 55
+        let! result =
+            Gen.getSeqImmutable variant
+            |> TaskSeq.fold (fun acc item -> acc + item) 0
+
+        result |> should equal 55
+    }
+
+    [<Theory; ClassData(typeof<TestImmTaskSeq>)>]
+    let ``TaskSeq-foldAsync sum over immutable variants`` variant = task {
+        let! result =
+            Gen.getSeqImmutable variant
+            |> TaskSeq.foldAsync (fun acc item -> task { return acc + item }) 0
+
+        result |> should equal 55
+    }
+
 module Immutable =
     [<Theory; ClassData(typeof<TestImmTaskSeq>)>]
     let ``TaskSeq-fold folds with every item`` variant = task {