Make "required type for cb: Callback" error point me toward fixing the problem

[iffy]

Summary

Compiling this program produces the following error:

import asyncdispatch
let event = newAsyncEvent()
event.addEvent(proc() =
  echo "Event signalled"
)
event.trigger()
event.trigger()

Compiler error:

$ nim c --hints:off /tmp/events.nim 
/private/tmp/events.nim(4, 6) Error: type mismatch: got <AsyncEvent, proc (){.gcsafe, locks: 0.}>
but expected one of: 
proc addEvent(ev: AsyncEvent; cb: Callback)
  first type mismatch at position: 2
  required type for cb: Callback
  but expression 'proc () = echo ["Event signalled"]' is of type: proc (){.gcsafe, locks: 0.}

expression: addEvent(event, proc () = echo ["Event signalled"])

As an inexperienced Nim user, though, I don't know in what way my proc doesn't meet the requirements to be a Callback.

It would be nice if the definition of Callback was included in the error output. Or it would be nice if the documentation for addEvent included a link to the definition of Callback

I should have tried on a newer Nim version, but it's bedtime:

$ nim --version
Nim Compiler Version 1.0.2 [MacOSX: amd64]
Compiled at 2019-10-22
Copyright (c) 2006-2019 by Andreas Rumpf

git hash: 193b3c66bbeffafaebff166d24b9866f1eaaac0e
active boot switches: -d:release

[metagn]

Error message is now:

Error: type mismatch
Expression: addEvent(event, proc () = echo ["Event signalled"])
  [1] event: AsyncEvent
  [2] proc () = echo ["Event signalled"]: proc (){.gcsafe.}

Expected one of (first mismatch at [position]):
[2] proc addEvent(ev: AsyncEvent; cb: Callback)
  expression 'proc () = echo ["Event signalled"]' is of type: proc (){.gcsafe.}
  Calling convention mismatch: got '{.nimcall.}', but expected '{.closure.}'.

The reason is shown at the bottom, the calling convention is different.