Unexpected error when checking if function parameter is a string variable type

[alexsch01]

Zig Version

0.13.0

Steps to Reproduce and Observed Behavior

const std = @import("std");

pub fn customPrint(x: anytype) void {
    const typeName = @typeName(@TypeOf(x));
    if(
        std.mem.startsWith(u8, typeName, "*const [") and
        std.mem.endsWith(u8, typeName, ":0]u8")
    ) {
        std.debug.print("{s}\n", .{x});
        return;
    }

    std.debug.print("{any}\n", .{x});
}

pub fn main() void {
    customPrint("Hello World");
    customPrint(123);
}

/usr/lib/zig/std/fmt.zig:471:5: error: invalid format string 's' for type 'comptime_int' (exit status 1)

Expected Behavior

Hello World 123

[rohlem]

What is tripping up your code here is the fact that function calls, like std.mem.startsWith and std.mem.endsWith in your if condition, are by default evaluated at runtime, even if all arguments are comptime-known. You have to force comptime evaluation using the comptime unary operator: if (comptime (std.mem.startsWith(u8, typeName, "*const [") and std.mem.endsWith(u8, typeName, ":0]u8"))) { ... } .