Warn if someone attempts to the the array index syntax of C family languages

[vyacheslavhere]

The standard operator for retrieving an element from a list (list[index]) is not supported in Gleam. Using it produces confusing behavior for new developers:

pub fn main() {
  let list = [1,2,3]
  let element = list[0]
  echo element
}

Output:

src/main.gleam:6
[1, 2, 3]
warning: Unused literal
  ┌─ /src/main.gleam:4:21
  │
5 │   let element = list[0]
  │                     ^^^ This value is never used

Hint: You can safely remove it.

Effectively, the code behaves as if it were written like this:

import gleam/io

pub fn main() {
  let list = [1,2,3]
  let element = list
  [0]
  echo element
}

Instead of giving a clear warning about unsupported list indexing, Gleam generates a warning about an "unused literal," which is misleading. I believe there should be a more informative warning or hint indicating that list indexing with [] is not supported.

[lpil]

Hello! Thanks for the report.

Effectively, the code behaves as if it were written like this:

You have! Gleam doesn't have semicolons or significant whitespace, so there is no difference between the two code examples you have given there. In whitespace insensitive languages (which is most languages!) you can place any amount of whitespace anywhere in the program and there will be no difference in behaviour.

This makes adding a warning very challenging. For example, this is a valid program that return [1] and does not emit any warnings.

import gleam/io

pub fn run() -> List(Int) {
  some_function()[0]
}

The standard way that the non-FP programmer is alerted to this problem in their code is through use of the auto-formatter, which will format the code in a way that makes it clear. This is built into the language server so will typically run when files are saved. Are you looking to not use the formatter?

Do you have other ideas for how we could help people understand that indexing isn't a thing in FP?

[vyacheslavhere]

Hi! Thanks for the explanation, that makes sense. I understand now that adding a compiler warning for [] indexing would be tricky.

I think a good way to help newcomers would be to add a note in the language tour stating clearly that Gleam does not support list indexing with []. That way, people encountering this for the first time can quickly understand the idiomatic FP approach without confusion.

[lpil]

The language tour is concise and explains the language itself only. It would be very challenging to keep it this way if we were to start including all the things that the language does not have, there are a lot of those things!

You personally are surprised by it not having the C-style array indexing operator, but it's not a universal expectation. Depending on what people are more familiar with they will have different expectations, and this isn't one of the common confusions that gets reported on github and discord.

[vyacheslavhere]

Thanks for clarifying! I understand that the language tour is meant to stay concise and that it would be impractical to document everything Gleam does not have.

Since the absence of C-style array indexing can be surprising to developers coming from imperative languages like Python, perhaps a note about this could be added in the Gleam for Python users section. That way, newcomers from Python can be made aware of the idiomatic FP approach without affecting the main language tour.