# How to force runtime evaluation? **URL:** \ **Category:** Help\ **Created:** [June 27, 2023, 10:54pm UTC](https://ziggit.dev/t/how-to-force-runtime-evaluation/943 "2023-06-27T22:54:09Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![matklad](https://ziggit.dev/user_avatar/ziggit.dev/matklad/32/2462_2.png) [@matklad](https://ziggit.dev/u/matklad)\ **Post date:** [June 27, 2023, 10:54pm UTC](https://ziggit.dev/t/how-to-force-runtime-evaluation/943/1 "2023-06-27T22:54:09Z")
For exploratory purposes, I want to write some code, and make sure it isn’t evaluated at compile time. What’s the best way to achieve that? Right now, I am trying ```zig pub fn main() { f(92) } noinline fn f(arg: i32) void {} ``` where I think arg will be a runtime value. Does this work? Is there a better way to do it? ---
**Author:** ![dude\_the\_builder](https://ziggit.dev/user_avatar/ziggit.dev/dude_the_builder/32/557_2.png) [@dude\_the\_builder](https://ziggit.dev/u/dude_the_builder)\ **Post date:** [June 27, 2023, 11:20pm UTC](https://ziggit.dev/t/how-to-force-runtime-evaluation/943/2 "2023-06-27T23:20:28Z")
I’m not 100% sure, but I think that if the arg is a `var`, it can’t be evaluated at compile time. So ```zig var n: i32 = 92; f(n); ``` should do the trick? I arrive at this conclusion given that if you try to use a `var i` instead of a `comptime var i` with an `inline while` you get an error stating the `i` can’t be evaluated at compile time. ---
**Author:** ![AndrewCodeDev](https://ziggit.dev/letter_avatar_proxy/v4/letter/a/278dde/32.png) [@AndrewCodeDev](https://ziggit.dev/u/AndrewCodeDev)\ **Post date:** [June 28, 2023, 4:03am UTC](https://ziggit.dev/t/how-to-force-runtime-evaluation/943/3 "2023-06-28T04:03:33Z")
Have you seen this thread? There was an @isComptime built in proposed a while ago: [Add `@inComptime` builtin · mlugg/zig@35d82d3 · GitHub](https://github.com/mlugg/zig/commit/35d82d31be3d2f2611049f41dc2616f898d70871) I’m reading through the history of it right now and if I find out what happened to it I’ll post more here. Here you can see something actually went in on this a while ago: [Add `@inComptime` builtin · mlugg/zig@35d82d3 · GitHub](https://github.com/mlugg/zig/commit/35d82d31be3d2f2611049f41dc2616f898d70871#diff-9674f03a76728eff712dfbd966487142bf4103b4a58049ee9696028e890f86bc) ---
**Author:** ![AndrewCodeDev](https://ziggit.dev/letter_avatar_proxy/v4/letter/a/278dde/32.png) [@AndrewCodeDev](https://ziggit.dev/u/AndrewCodeDev)\ **Post date:** [June 28, 2023, 4:33am UTC](https://ziggit.dev/t/how-to-force-runtime-evaluation/943/4 "2023-06-28T04:33:23Z")
Yeah, so this works for me - maybe it’ll help you? zig version 0.11.0-dev.3867+ff37ccd29 ```zig fn foo() void { if (@inComptime()) { @compileError("Woops"); } } pub fn main() !void { // no-compile error: foo(); // compile error: comptime foo(); } ``` ---
**Author:** ![kristoff](https://ziggit.dev/user_avatar/ziggit.dev/kristoff/32/9_2.png) [@kristoff](https://ziggit.dev/u/kristoff)\ **Post date:** [June 28, 2023, 10:49am UTC](https://ziggit.dev/t/how-to-force-runtime-evaluation/943/5 "2023-06-28T10:49:11Z")
The rule right now is that a function is called at comptime implicitly only when you’re already in a comptime context, otherwise it will need to be prefixed with `comptime`. There has been discussion about eventually have the compiler “try” to resolve function calls at comptime without explicitly asking for it, but none of that exists today. So a function call in the top scope of a file is always comptime, while a function call inside main is always at runtime unless prefixed with `comptime`. Not sure how that interacts with LLVM optimizations though, so you might get a non-comptime compile time resolution by LLVM when the function is simple enough. ---
**Author:** ![Damjan94](https://ziggit.dev/user_avatar/ziggit.dev/damjan94/32/580_2.png) [@Damjan94](https://ziggit.dev/u/Damjan94)\ **Post date:** [June 28, 2023, 1:06pm UTC](https://ziggit.dev/t/how-to-force-runtime-evaluation/943/6 "2023-06-28T13:06:55Z")
Thanks for the links, there seems to be a [warning](https://github.com/mlugg/zig/commit/35d82d31be3d2f2611049f41dc2616f898d70871#diff-9674f03a76728eff712dfbd966487142bf4103b4a58049ee9696028e890f86bcR8596) > This can be used to provide alternative, comptime-friendly implementations of functions. It **should not** be used, for instance, to **exclude** certain functions from being evaluated at comptime. ---
**Author:** ![AndrewCodeDev](https://ziggit.dev/letter_avatar_proxy/v4/letter/a/278dde/32.png) [@AndrewCodeDev](https://ziggit.dev/u/AndrewCodeDev)\ **Post date:** [June 28, 2023, 3:25pm UTC](https://ziggit.dev/t/how-to-force-runtime-evaluation/943/7 "2023-06-28T15:25:30Z")
Yes, and thank you for pointing out that those do have warnings attached to them (probably should have added that). @matklad seems like the kind of programmer that will dig into implementations to find out what the rules are, but for the general case it’s good to follow the warnings. ---
**Author:** ![mlugg](https://ziggit.dev/user_avatar/ziggit.dev/mlugg/32/437_2.png) [@mlugg](https://ziggit.dev/u/mlugg)\ **Post date:** [July 1, 2023, 4:33pm UTC](https://ziggit.dev/t/how-to-force-runtime-evaluation/943/8 "2023-07-01T16:33:55Z")
Hi, just thought I’d quickly note: that note in `@inComptime`’s documentation isn’t any kind of technical restriction, it’s more of a recommendation. One of the main concerns with implementing that builtin was that people would start needlessly (and potentially wrongly) writing things like this: ```zig if (@inComptime()) @compileError("This function must be run at runtime"); ``` I chose to add this note simply to make it clear that this isn’t recommended. There’s no weird behavior here; the builtin does indeed just return whether it was evaluated in a `comptime` scope or not.