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

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

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Copilot Mar 15, 2026
ee6132e
Fix consLazy laziness: use mutable in test and prevent tail forcing o…
Copilot Mar 15, 2026
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2 changes: 1 addition & 1 deletion src/FSharpx.Collections/LazyList.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -115,7 +115,7 @@ module LazyList =
lzy(fun () -> (consc x (lzy(fun () -> (force(l()))))))

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

let uncons(s: LazyList<'T>) = s.Uncons

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12 changes: 11 additions & 1 deletion tests/FSharpx.Collections.Tests/LazyListTests.fs
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Expand Up @@ -329,7 +329,17 @@ module LazyList =

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)
let mutable tailForced = false

let tail =
lazy
(tailForced <- true
LazyList.ofList [ 2; 3 ])

let ll = LazyList.consLazy 1 tail
Expect.isFalse "consLazy divergence: construction should not force the tail" tailForced
let _ = LazyList.head ll
Expect.isFalse "consLazy divergence: head should not force the tail" tailForced
}

test "consLazy infinite" {
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