[Repo Assist] feat: add LazyList.consLazy for constructing from Lazy(LazyList<'T)> by github-actions[bot] · Pull Request #239 · fsprojects/FSharpx.Collections

github-actions · 2026-03-09T01:09:18Z

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Implements the enhancement requested in #134.

Summary

LazyList.consDelayed accepts unit -> LazyList<'T> (a thunk). When a Lazy(LazyList<'T)> value is already available, wrapping it in fun () -> tail.Value works but is unnecessarily wasteful — it allocates an extra closure and forgoes .NET's built-in Lazy caching.

consLazy provides the direct construction:

val consLazy : 'T -> Lazy(LazyList<'T)> -> LazyList<'T>

Usage example (infinite list of 1s, as shown in #134):

let rec ones: LazyList(int) = LazyList.consLazy 1 (lazy ones)

Implementation

let consLazy x (l: Lazy(LazyList<'T)>) =
    lzy(fun () -> consc x l.Value)

The outer lzy keeps the head cell lazy until the list is consumed. l.Value is the standard .NET Lazy(T) evaluation, which caches the result and is evaluated at most once.

Changes

src/FSharpx.Collections/LazyList.fs — add consLazy
src/FSharpx.Collections/LazyList.fsi — add signature + doc comment
tests/FSharpx.Collections.Tests/LazyListTests.fs — 4 new tests: head access, toList, lazy divergence check (tail not evaluated on construction), and infinite list

Test Status

✅ All 710 tests pass (Passed: 710, Skipped: 6, Failed: 0). The 6 skipped tests are pre-existing skips unrelated to this change.

Related to #134

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Implements the enhancement requested in #134. consLazy takes a head value and a Lazy<LazyList<'T>> tail, providing a more efficient alternative to consDelayed when a Lazy value is already available. Example use: let rec ones: LazyList<int> = LazyList.consLazy 1 (lazy ones) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

gdziadkiewicz · 2026-03-14T13:13:12Z

/repo-assist resolve conflicts

Copilot

Pull request overview

Adds a new LazyList.consLazy constructor to build a LazyList<'T> from a head value and an existing Lazy<LazyList<'T>> tail, aligning the public API with the underlying representation discussed in #134.

Changes:

Add LazyList.consLazy implementation in LazyList.fs.
Expose consLazy in the public signature file LazyList.fsi with documentation.
Add new unit tests covering basic consLazy behavior and an infinite-list scenario.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
`src/FSharpx.Collections/LazyList.fs`	Introduces the new `consLazy` constructor implementation.
`src/FSharpx.Collections/LazyList.fsi`	Exposes `consLazy` publicly and updates signature formatting in this file.
`tests/FSharpx.Collections.Tests/LazyListTests.fs`	Adds tests for `consLazy` behavior (head, toList, divergence-on-construction, infinite list).

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src/FSharpx.Collections/LazyList.fsi

    ///O(1). Test if a list is empty.  Forces the evaluation of
    /// the first element of the stream if it is not already evaluated.
-    member IsEmpty : bool
+    member IsEmpty: bool

    ///O(1). Return the first element of the list.  Forces the evaluation of
    /// the first cell of the list if it is not already evaluated.
-    member Head : 'T
+    member Head: 'T

    ///O(n). Return the length of the list
-    member Length : unit -> int
+    member Length: unit -> int



src/FSharpx.Collections/LazyList.fsi


-    val (|Cons|Nil|) : LazyList<'T> -> Choice<('T * LazyList<'T>),unit>
-
+    val (|Cons|Nil|): LazyList<'T> -> Choice<('T * LazyList<'T>), unit>


src/FSharpx.Collections/LazyList.fs

        lzy(fun () -> (consc x (lzy(fun () -> (force(l()))))))

+    let consLazy x (l: Lazy<LazyList<'T>>) =
+        lzy(fun () -> consc x l.Value)


gdziadkiewicz · 2026-03-15T17:44:59Z

tests/FSharpx.Collections.Tests/LazyListTests.fs

+                  // tail is not evaluated unless the tail is consumed
+                  Expect.isTrue "consLazy divergence" (let _ = LazyList.consLazy 1 (lazy (failwith "diverge")) in true)


@copilot can you use mutable for that?

…939b68642ec5

Copilot · 2026-03-15T17:45:05Z

@gdziadkiewicz I've opened a new pull request, #255, to work on those changes. Once the pull request is ready, I'll request review from you.

Copilot

Pull request overview

This PR adds a new LazyList.consLazy API to construct a LazyList<'T> from a head value and an existing Lazy<LazyList<'T>>, addressing the allocation/caching drawbacks of wrapping Lazy via consDelayed.

Changes:

Add LazyList.consLazy implementation in LazyList.fs.
Expose consLazy in LazyList.fsi with documentation.
Add unit tests covering basic behavior and an infinite-list usage pattern.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
`src/FSharpx.Collections/LazyList.fs`	Implements `consLazy` construction logic.
`src/FSharpx.Collections/LazyList.fsi`	Adds public signature + doc comment for `consLazy`.
`tests/FSharpx.Collections.Tests/LazyListTests.fs`	Adds tests for `consLazy` behavior (head/toList/laziness/infinite).

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src/FSharpx.Collections/LazyList.fs

        lzy(fun () -> (consc x (lzy(fun () -> (force(l()))))))

+    let consLazy x (l: Lazy<LazyList<'T>>) =
+        lzy(fun () -> consc x l.Value)


tests/FSharpx.Collections.Tests/LazyListTests.fs

+
+              test "consLazy lazy divergence" {
+                  // tail is not evaluated unless the tail is consumed
+                  Expect.isTrue "consLazy divergence" (let _ = LazyList.consLazy 1 (lazy (failwith "diverge")) in true)


Previously consLazy wrapped the entire cell construction in lzy, meaning forcing the head cell (e.g. via LazyList.head) also forced l.Value — defeating the laziness guarantee of consLazy. Fix: use notlazy for the outer CellCons so the head is immediately available, wrapping only the tail evaluation in lzy. Test: replace the failwith divergence test with an explicit mutable flag asserting that neither construction nor head access forces the tail. Incorporates fix from PR #255. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

github-actions · 2026-03-16T01:17:15Z

Commit pushed: ff79a70

Generated by Repo Assist

github-actions bot added automation repo-assist labels Mar 9, 2026

ci: trigger checks

cb8c840

This was referenced Mar 9, 2026

[Repo Assist] Monthly Activity 2026-03 #230

Open

Add LazyList.consLazy #134

Open

gdziadkiewicz requested review from Copilot and gdziadkiewicz and removed request for Copilot March 14, 2026 13:13

gdziadkiewicz self-assigned this Mar 14, 2026

gdziadkiewicz requested a review from Copilot March 14, 2026 13:13

Copilot started reviewing on behalf of gdziadkiewicz March 14, 2026 13:13 View session

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Merge branch 'master' into repo-assist/improve-lazylist-conslazy-23c4…

c35ccbb

…939b68642ec5

Copilot AI mentioned this pull request Mar 15, 2026

Fix consLazy laziness: tail must not be forced when accessing head #255

Open

gdziadkiewicz requested a review from Copilot March 15, 2026 17:45

Copilot started reviewing on behalf of gdziadkiewicz March 15, 2026 17:46 View session

Copilot AI reviewed Mar 15, 2026

View reviewed changes

ci: trigger checks

21fb88e

dsyme marked this pull request as ready for review March 16, 2026 13:45

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[Repo Assist] feat: add LazyList.consLazy for constructing from Lazy(LazyList<'T)>#239

[Repo Assist] feat: add LazyList.consLazy for constructing from Lazy(LazyList<'T)>#239
github-actions[bot] wants to merge 5 commits intomasterfrom
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		val (\|Cons\|Nil\|) : LazyList<'T> -> Choice<('T * LazyList<'T>),unit>

		val (\|Cons\|Nil\|): LazyList<'T> -> Choice<('T * LazyList<'T>), unit>

		// tail is not evaluated unless the tail is consumed
		Expect.isTrue "consLazy divergence" (let _ = LazyList.consLazy 1 (lazy (failwith "diverge")) in true)

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github-actions bot commented Mar 9, 2026

Summary

Usage example (infinite list of 1s, as shown in #134):

Implementation

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