const HexConversionError = error { NotHex };

fn fromHexChar(c : u8) HexConversionError!u4 {
    return switch(c) {
        'A' ... 'F' => c - 'A' + 0x10,
        'a' ... 'f' => c - 'a' + 0x10,
        '0' ... '9' => c - '0',
        else => HexConversionError.NotHex
    };
}

/home/felix/projects/zig-tracer/tracer.zig:17:12: error: expected type 'HexConversionError!u4', found 'HexConversionError!u8'
    return switch(c) {
           ^
/home/felix/projects/zig-tracer/tracer.zig:17:12: note: error union payload 'u8' cannot cast into error union payload 'u4'
    return switch(c) {
           ^
/home/felix/projects/zig-tracer/tracer.zig:16:42: note: return type declared here
fn fromHexChar(c : u8) HexConversionError!u4 {