Returning procedures with different noSideEffect pragmas

[adelq]

I am trying to write a procedure that returns different procedures conditionally, and one variant of the procedure returned happens to have no side effects and the other does not. This is unexpected behavior since both returned procedure variants adhere to the procedure signature ((int) -> void) but because one of them gets annotated with the {.noSideEffects.} pragma, the other variant is rejected as well. The following code should reproduce the error:

Example

import sugar

proc makeAdder(a: int): (int) -> void =
  proc discard_adder(x: int) {.closure.} =
    discard a + x

  proc echo_adder(x: int) {.closure.} =
    echo("printing from adder")

  if a > 0:
    discard_adder
  else:
    echo_adder

let newAdder = makeAdder(0)
newAdder(5)

Current Output

Error: type mismatch: got <proc (x: int){.closure, gcsafe, locks: 0.}> but expected 'proc (x: int){.closure, noSideEffect, gcsafe, locks: 0.}'

Expected Output

printing from adder

Possible Solution

• Adding a pragma that is the inverse of the {.noSideEffects.} pragma such that a function can be marked as potentially having side effects.
• Using the loosest annotation rather than the strictest annotation for the {.noSideEffects.} pragma when determining procedure return pragmas.

Additional Information

$ nim -v
Nim Compiler Version 1.2.0

[cooldome]

This works:

import sugar

proc makeAdder(a: int): (int) -> void =
  proc discard_adder(x: int) {.closure.} =
    discard a + x

  proc echo_adder(x: int) {.closure.} =
    echo("printing from adder")

  if a > 0:
    result = discard_adder
  else:
    result = echo_adder

let newAdder = makeAdder(0)
newAdder(5)

The issue is that if a > 0: x else: y needs to return one type. Nim struggles a bit.

[adelq]

Thanks, that worked perfectly.

[Clyybber]

The original snippet should still work though :)

[adelq]

I think this is actually the same bug as #12642, because re-ordering the branches of the if statement makes it compile correctly. This bug has been present since v0.15.0 and is currently present in v1.2.0.