unable to evaluate constant expression | Zig Programming Language | Page 1

vast herald Dec 18, 2022, 7:12 PM

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const global = "from us";

fn bar(a: []const u8, b: []const u8) []const u8{
  
 return a ++ b;
}

fn hello(a: []const u8, b: []const u8) []const u8 {
  var barred = bar(a, b);
  std.debug.print("THE BARRED {}", .{barred});
  return "Hello" ++ barred ++ " " ++ global;
}

pub fn main() !void {
    // Prints to stderr (it's a shortcut based on `std.io.getStdErr()`)
    var result = hello("friends", "family");
    std.debug.print("SOME STUFF{}", .{result});
}

this code is giving me the following error

./src/main.zig:14:21: error: unable to evaluate constant expression
  return "Hello" ++ barred ++ " " ++ global;
                    ^
./src/main.zig:7:9: error: unable to evaluate constant expression
 return a ++ b;

vast pulsar Dec 18, 2022, 7:32 PM

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The ++ concatenation operator only works on comptime-known values, and barred is a runtime value. If you need to concatenate strings that are runtime values, you can use sdlib functions to join them

vast herald Dec 18, 2022, 7:36 PM

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or could i make my args comptime?

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and i guess if not, im not sure if you can rec any specific stdlib functions... was just looking around github and not seeing any for concatenation

vast pulsar Dec 18, 2022, 7:44 PM

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vast herald or could i make my args comptime?

Ah yes, the error is there too. You could make the args to bar() comptime. You might also need to call it with the comptime keyword

vast herald Dec 18, 2022, 7:48 PM

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got it to work comptime... but I'd love to be able to print it out runtime too

vast pulsar Dec 18, 2022, 7:48 PM

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If you want to use comptime values, here is a fixed version of your snippet

const std = @import("std");
const global = "from us";

fn bar(comptime a: []const u8, comptime b: []const u8) []const u8 {
    return a ++ b;
}

fn hello(comptime a: []const u8, comptime b: []const u8) []const u8 {
    const barred = comptime bar(a, b);
    std.debug.print("THE BARRED {s}", .{barred});
    return "Hello" ++ barred ++ " " ++ global;
}

pub fn main() !void {
    var result = hello("friends", "family");
    std.debug.print("SOME STUFF {s}", .{result});
}

vast herald Dec 18, 2022, 7:48 PM

#

const global = "from us";

fn bar(comptime a: []const u8, comptime b: []const u8) []const u8{
  
 return a ++ b;
}

fn hello(comptime a: []const u8, comptime b: []const u8) []const u8 {
  var barred = comptime bar(a, b);
  // std.debug.print("THE BARRED {}", .{barred});
  return "Hello" ++ barred ++ " " ++ global;
}

pub fn main() !void {
    // var foo: []const u8 = "something"
    comptime {
      var result = hello("friends", "family");
      @compileLog(result);
    }
    std.debug.print(result);
    // Prints to stderr (it's a shortcut based on `std.io.getStdErr()`)
}

vast pulsar Dec 18, 2022, 7:48 PM

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haha, same time

vast herald Dec 18, 2022, 7:48 PM

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lol

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ah but you're able to still use std.debug because you're just making the vars comptime w/o using a comptime block?

vast pulsar Dec 18, 2022, 7:49 PM

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vast herald and i guess if not, im not sure if you can rec any specific stdlib functions... ...

https://github.com/ziglang/zig/blob/master/lib/std/mem.zig#L2503-L2505
std.mem.join

vast herald Dec 18, 2022, 7:51 PM

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yeah I seem to be struggling to print the output at runtime

vast pulsar Dec 18, 2022, 7:51 PM

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vast herald ah but you're able to still use std.debug because you're just making the vars co...

Might be some confusion here, std.debug.print only needs the the format string to be comptime, not the extra args. Here it's being passed a string literal SOME STUFF {s}, and string lits are comptime known. The passed arg result doesn't need to be comptime

vast herald Dec 18, 2022, 7:53 PM

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const global = "from us";

fn bar(comptime a: []const u8, comptime b: []const u8) []const u8{
 return a ++ b;
}

fn hello(comptime a: []const u8, comptime b: []const u8) []const u8 {
  var barred = comptime bar(a, b);
  return "Hello" ++ barred ++ " " ++ global;
}

pub fn main() !void {
    comptime {
     var result = hello("friends", "family");
     std.debug.print("THE RESULT {}", .{result});
    }
}```

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It yells at me when I do that

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/Users/interpretations/Downloads/zig-macos-aarch64-0.11.0-dev.514+4be1bb4aa/lib/std/Thread/Mutex.zig:111:18: error: unable to evaluate constant expression
        os.darwin.os_unfair_lock_lock(&self.oul);
                 ^
/Users/interpretations/Downloads/zig-macos-aarch64-0.11.0-dev.514+4be1bb4aa/lib/std/Thread/Mutex.zig:45:19: note: called from here
    self.impl.lock();
                  ^
/Users/interpretations/Downloads/zig-macos-aarch64-0.11.0-dev.514+4be1bb4aa/lib/std/debug.zig:90:22: note: called from here
    stderr_mutex.lock();
                     ^
./src/main.zig:20:21: note: called from here
     std.debug.print("THE RESULT {}", .{result});
                    ^
./src/main.zig:16:21: note: called from here
pub fn main() !void {
                    ^

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even if i remove the comptime block in main

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go tit

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got it*

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const global = "from us";

fn bar(comptime a: []const u8, comptime b: []const u8) []const u8{
 return a ++ b;
}

fn hello(comptime a: []const u8, comptime b: []const u8) []const u8 {
  var barred = comptime bar(a, b);
  return "Hello" ++ barred ++ " " ++ global;
}

pub fn main() !void {
     var result = comptime hello("friends", "family");
     std.debug.print("THE RESULT {s}", .{result});
}

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seemed the trick was comptime before the final func execution

vast pulsar Dec 18, 2022, 7:57 PM

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Did the version I pasted not work for you? What version of zig are you using?

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Oh, I also made barred a const instead of var. Anyway

vast herald Dec 18, 2022, 8:02 PM

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yours works

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I'm trying to recall what the difference was in mine that felt significant to me

severe fiber Dec 20, 2022, 7:33 PM

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To be clear: ++ is std.mem.concat, but with comptime-known-sized values, or slices that are comptime-known.

#unable to evaluate constant expression