cannot create a FlowVar[ tuple[a: string] ]

[rayman22201]

Attempting to create a FlowVar[ tuple[ a: string ] ] fails with:
test.nim(6, 15) Error: cannot create a flowVar of type: tuple[a: string]

Creating a FlowVar[string] directly works.
Creating a FlowVar of a tuple with fields of any other type works. Only tuples containing strings seem to fail.

This was discovered by attempting to use execCmdEx() from the OsProc module with spawn.

Example

import threadpool, osproc

proc test(): tuple[a: string] =
  return (a: "flowvar string tuple")

let a = spawn test()

sync()

echo ^a

$ nim -v
Nim Compiler Version 0.20.99 [Linux: amd64]
Compiled at 2019-07-11
Copyright (c) 2006-2019 by Andreas Rumpf

active boot switches: -d:release

[alaviss]

it's worth noting that this works with https://github.com/yglukhov/threadpools :)

[Chris3606]

This still shows up in Nim v1.4.2; is there an intended workaround for this, short of using the linked version of threadpools?

[alaviss]

Nope, official threadpool is sorta unmaintained by now. Alternatives includes https://github.com/mratsim/weave and https://github.com/yglukhov/threadpools

[Chris3606]

Hm. I can definitely work around it or use an alternative for my use case... But for future reference (both for me and those who encounter this issue in the future), given that, are there future plans to remove/replace the current Nim standard library threadpool implementation? Particularly since one of the linked implementations appears to not support gc:arc...

[alaviss]

AFAIK weave will be the future. ARC supports will come after we got Isolated[T] in place I think, since it would be much easier to move memory between threads. See nim-lang/RFCs#160 and nim-lang/RFCs#244 and nim-lang/RFCs#295 for the future of Nim's multithreading/async.

[ringabout]

It seems to work with --gc:arc?

import threadpool

proc test(): tuple[a: string] =
  return (a: "flowvar string tuple")

let a = spawn test()

sync()

echo typeof(a)
echo ^a

FlowVar[tuple[a: string]]
(a: "flowvar string tuple")