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Issue 4490: Added thread name to IO.debug #4525

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Issue 4490: Added thread name to IO.debug #4525

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Added thread name to IO.debug
alstepan Nov 19, 2025
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alstepan Nov 20, 2025
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alstepan Nov 20, 2025
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7 changes: 4 additions & 3 deletions core/shared/src/main/scala/cats/effect/IO.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -758,13 +758,14 @@ sealed abstract class IO[+A] private () extends IOPlatform[A] {
implicit S: Show[B] = Show.fromToString[B]): IO[A] =
guaranteeCase {
case Outcome.Succeeded(ioa) =>
ioa.flatMap(a => IO.println(s"${prefix}: Succeeded: ${S.show(a)}"))
ioa.flatMap(a =>
IO.println(s"[${Thread.currentThread().getName}] ${prefix}: Succeeded: ${S.show(a)}"))
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@djspiewak djspiewak Dec 28, 2025

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Thread.currentThread() is a side effect and should be wrapped in IO. The reason for this becomes much more apparent when you look at the output for the Canceled case in particular, which will frequently be inconsistent with the thread on which the IO is actually executing!

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@asarkar asarkar Dec 28, 2025

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It’d be helpful for the same reason prefix exists, debugging.


case Outcome.Errored(ex) =>
IO.println(s"${prefix}: Errored: ${ex}")
IO.println(s"[${Thread.currentThread().getName}] ${prefix}: Errored: ${ex}")

case Outcome.Canceled() =>
IO.println(s"${prefix}: Canceled")
IO.println(s"[${Thread.currentThread().getName}] ${prefix}: Canceled")
}

/**
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