Stream: ideas

Topic: remove NonEmptyList opaque example

Brendan Hansknecht (Dec 15 2023 at 17:08):

In the tutoral, we say that a NonEmptyList is a good example of an opaque type. I think it is only a kinda ok example and confuses people. I think we should find a different example.

The reason it is only kinda ok is due to what a good implementation would look like. A good implementation of NonEmptyList would be written in roc as:

NonEmptyList a : { head: a, rest: List a}

This type contains enough information by itself that it doesn't strictly need to be opaque. You could also write NonEmptyList as:

NonEmptyList a := List a

But then a function like first is written as:

first = \@NonEmptyList list ->
    when List.first list is
        Ok x -> x
        Err _ -> crash "this shouldn't be possible"

Just doesn't feel like a good example.

Brendan Hansknecht (Dec 15 2023 at 17:09):

I think. we should change this to something simpler that needs verification. My top thoughts are Email, Url, and Path. All of which could be seen as just a wrapper around a Str or List U8, but you would add verification when creating one. That way any @Email is guaranteed to be in the right format.

Brendan Hansknecht (Dec 15 2023 at 17:10):

It also means we could write up a super simple in tutorial example to help people grok it better.

Brendan Hansknecht (Dec 15 2023 at 17:10):

Thoughts? Better type suggestions?

Anton (Dec 15 2023 at 17:33):

I agree! email sounds good

Brendan Hansknecht (Dec 15 2023 at 18:10):

Maybe we could show an identical example to username, but with fromStr returning a Result Email [InvalidEmail].

Hannes Nevalainen (Dec 15 2023 at 18:52):

Except emails is actually suoer complicated to validate (if you wanna follow the spec) :sweat_smile:

Brendan Hansknecht (Dec 15 2023 at 18:54):

Yeah, might want to just call to a fake validation function or do some basic form of validation with a comment that it is just for show

Hannes Nevalainen (Dec 15 2023 at 18:59):

The complete implementation has been left as an exercise for the reader :big_smile:

Sky Rose (Dec 15 2023 at 23:14):

String.contains s "@" oughta do.

Brendan Hansknecht (Dec 15 2023 at 23:18):

Roughly this sounds easy enough: https://www.mailboxvalidator.com/resources/articles/acceptable-email-address-syntax-rfc/

Brendan Hansknecht (Dec 15 2023 at 23:19):

Maybe just split at @ and then check the contained characters as a simple proxy

Brendan Hansknecht (Dec 15 2023 at 23:19):

Just for example purposes

Last updated: Jun 16 2026 at 16:19 UTC