How to define the length of the bytes type?

[btrepp]

Is there any way of constraining the bytes type?, it seems mainly a stand-in for string, but there's no clear way to restrict or constrain it easily.

At the moment I have found I can hack it into strings, then use the strings definitions to work with it.

Eg https://cuelang.org/play/?id=ijXnA8UOWSm#w=function&i=cue&f=eval&o=cue

import "strings"

input: bytes
_str:"\(input)"
_str: strings.MaxRunes(512)
_str: strings.MinRunes(512)

input: 'a'

And even having to do similar things to work with [..bytes] to 'join' it into a normal 'bytes' type, eg concatenating the list.

It's workable, but it feels very awkward. I also think runes, might be interpreted differently from a raw length, but I want to work with just bytes or ideally [...byte] by that doesn't seem to exist

So my two questions

1. How to constrain data:bytes, to have a specific length?
2. How to concatenate [...bytes] into a single bytes instance?
3. I found that there is a uint8 constraint, so [...uint8] would mean list.Concat works, but can we create bytes from [...uint8]?

Update 1: Ah uint8, is non ideal, it's a bigger int, constant to a uint8, but converting that adds extra stuff

https://cuelang.org/play/?id=EGkAFUwlJKI#w=function&i=cue&f=eval&o=cue

import "strings"

#data: [...bytes]
let strs = [for d in #data { "\(d)"}]
let str= strings.Join(strs,"")
let output = strings.ByteSlice(str,0,len(str))

data: output
data: bytes

#data: ['\x48','\x65','\x6C\x6C\x6F']

Is a bit of the way, for 2, though it feels weird to go 'via' strings to 'Join', lists of bytes

Update: 2

Okay so 1 and 2 is maybe workable here
https://cuelang.org/play/?id=kn8MP-PGOyQ#w=function&i=cue&f=eval&o=cue

import "strings"

#data: [...bytes]
let strs = [for d in #data { "\(d)"}]
let str= strings.Join(strs,"")
let output = strings.ByteSlice(str,0,len(str))

data: output
data: bytes

if len(data) != 6  {
  data: "data length mismatch \(len(data)) != 10"
}

#data: ['\x48','\x65','\x6C\x6C\x6F']

but probably suffers from

1. I am abusing the string conversions still,
2. I feel like using string as an error message is awkward. I did try to define 'bottom' | if Len is unexpected, however you can't provide any user context to that, its just 'error', it would be nice to specify hey the input is incorrect, especially in more complex data structures. Is it possible to add context/messages to a bottom type?