[Repo Assist] feat: add LazyList.consLazy for constructing from Lazy(LazyList<'T)>#239
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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>
This was referenced Mar 9, 2026
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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.consLazyimplementation inLazyList.fs. - Expose
consLazyin the public signature fileLazyList.fsiwith documentation. - Add new unit tests covering basic
consLazybehavior 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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| ///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 | ||
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| ///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 | ||
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| ///O(n). Return the length of the list | ||
| member Length : unit -> int | ||
| member Length: unit -> int | ||
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| val (|Cons|Nil|) : LazyList<'T> -> Choice<('T * LazyList<'T>),unit> | ||
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| val (|Cons|Nil|): LazyList<'T> -> Choice<('T * LazyList<'T>), unit> |
| lzy(fun () -> (consc x (lzy(fun () -> (force(l())))))) | ||
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| let consLazy x (l: Lazy<LazyList<'T>>) = | ||
| lzy(fun () -> consc x l.Value) |
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| // 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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🤖 This PR was created by Repo Assist, an automated AI assistant.
Implements the enhancement requested in #134.
Summary
LazyList.consDelayedacceptsunit -> LazyList<'T>(a thunk). When aLazy(LazyList<'T)>value is already available, wrapping it infun () -> tail.Valueworks but is unnecessarily wasteful — it allocates an extra closure and forgoes .NET's built-inLazycaching.consLazyprovides the direct construction:Usage example (infinite list of 1s, as shown in #134):
Implementation
The outer
lzykeeps the head cell lazy until the list is consumed.l.Valueis the standard .NETLazy(T)evaluation, which caches the result and is evaluated at most once.Changes
src/FSharpx.Collections/LazyList.fs— addconsLazysrc/FSharpx.Collections/LazyList.fsi— add signature + doc commenttests/FSharpx.Collections.Tests/LazyListTests.fs— 4 new tests: head access,toList, lazy divergence check (tail not evaluated on construction), and infinite listTest 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