support todo in constants

[giacomocavalieri]

I'd like to discuss what supporting todo in constants would look like. Right now it's a syntax error, but allowing them could have its use, for example enabling the "Fill labels" code action in constant records: #4730

What would its behaviour be? Rendering the exception throwing code where the constant is used sounds reasonable to me

[seafoamteal]

I'd expect that as well, it seems the most intuitive way to go about it to me.

This seems quite straightforward on the Erlang target. The following

pub const wibble = Thing(field: todo)

would just become

{thing, erlang:error(#{gleam_error => todo,
                message => <<"`todo` expression evaluated. This code has not yet been implemented."/utf8>>,
                file => <<?FILEPATH/utf8>>,
                module => <<"main"/utf8>>,
                function => <<"main"/utf8>>,
                line => 9})}

wherever the constant is used.

But on JavaScript, where top-level module constants are produced, the current strategy of a lambda that's immediately called to throw an exception means we'd throw when the constant is evaluated, not when it's used.

For the JS target, we could replace the entire constant with the throw lambda at the site where it's used, but I'm not sure how to make the constant definition work properly.

[giacomocavalieri]

Thinking a bit more about this, I can think of a couple of things that can be tricky and we should consider.

Since constants are handled slightly differently on JS and Erlang, introducing a side-effecting expression like todo in them would end up in different behaviours on each target:

• Since we inline constants in Erlang, the crash would only happen if the constant is used somewhere, might not happen at all!
• On JS the crash would always happen as the program starts since constants are regular top level js consts

So far this hasn't been an issue since we limit constants to be pure, but this would introduce some strange differences I'm afraid. I'm not sure that would be acceptable

[lpil]

If we ignore the use of constants from FFI we could have the compiler inline the todo at any point it is used. In JavaScript files I'm not sure what we could do though.

[seafoamteal]

Oh haha, we've thought of the same thing at the same time!

[gungun974]

Do we want to have gleam program with todo in constant to even be runnable ? Adding todo in constant make only sense for writing in LSP with code actions but contrary to our current todo, this one can’t really be a warning since gleam constant are supposed to be compile time know (literal)

What I want to say with this is having a todo inside a constant should not emit a warning but emit an error because that’s something that need to be fixed ASAP however it should be a recognized keyword for formater and LSP to work (specially for the code action of #4730). So it’s not really necessary to think what a JS or Erlang output could be since we would just forbid the build. Like when we use a non literal value in a case.

[gungun974]

@lpil I’m not sure to have understood. What are the reasons to transform this github issue into a github conversation.
Does that mean the constant todo is thrown in the idea box and another solution for #4730 need to be found ?

[lpil]

Issues are actionable work and for problems, while this is an incomplete feature proposal.