I tried a lot but this didn't work\: https://play.nim-lang.org/#pasty=SmFvxfDs,
with error\:

\`\`\`
/usercode/in.nim(17, 3) Error\: type mismatch
Expression\: ff f
  [1] f\: proc (A\: int)\: char{.noSideEffect, gcsafe.}

Expected one of (first mismatch at [position])\:
[1] proc ff(cb\: Callable[[int], char])
/usercode/in.nim(11, 3) Callable\: type mismatch\: got \<\>
but expected one of\:
Alias
/usercode/in.nim(11, 3) Callable\: expression '' has no type (or is ambiguous)
/usercode/in.nim(11, 3) Callable\: concept predicate failed
\`\`\`