wrong type mismatch when using `?`

[Anton-4]

app [main!] { pf: platform "./platform/main.roc" }

import pf.Stdout

main! = || {
    # fails
    #Stdout.line!(Str.inspect(question_fail(["1", "not a number", "100"])))

    # works
    Stdout.line!(Str.inspect(this_works(["1", "not a number", "100"])))
}

# question_fail : List(Str) -> Try(Str, _)
# question_fail = |strings| {
#     first_str = strings.first()?

#     Ok(first_str)
# }

this_works : List(Str) -> Try(Str, _)
this_works = |strings| {
    first_str_res = strings.first()

    first_str_res
}

Uncommenting yields:

❯ ./zig-out/bin/roc test/fx/hello_world.roc --no-cache
-- TYPE MISMATCH ---------------------------------

This expression is used in an unexpected way:
   ┌─ test/fx/hello_world.roc:15:17
   │
15 │     first_str = strings.first()?
   │                 ^^^^^^^

It has the type:
    List(Str)

But I expected it to be:
    { unknown: _field }

Found 1 error(s) and 0 warning(s) for test/fx/hello_world.roc.

[matthewmcgarvey]

I looked into this a little bit. I don't know that I got to the source of this error, but for sure there's a problem with the desugaring of .first()?.

Here is where the desugaring turns the error case into Err(#err) => return Err(#err)

roc/src/canonicalize/Can.zig

Line 5256 in 7ae2aa1

// === Branch 2: Err(#err) => return Err(#err) ===

The current parsing does not allow return to be used in a match expression like this without curly braces because it parses it as an expression rather than as a statement which would handle it.

Here is a desugared, slightly tweaked version of your example

app [main!] { pf: platform "./platform/main.roc" }

import pf.Stdout

main! = || {
    dbg question_fail(["1", "not a number", "100"])
}

question_fail : List(Str) -> Try(Str, _)
question_fail = |strings| {
  first_str = match strings.first() {
    Ok(ok) => ok
    #Err(err) => return Err(err) # fails
    Err(err) => { return Err(err) } # works
  }

  Ok(first_str)
}

❯ zig-out/bin/roc test/fx/app.roc
-- RETURN OUTSIDE FUNCTION -----------------------

The return keyword can only be used inside a function body.

Use `return` to exit early from a function and provide a value. For example:
    foo = |x| { if x < 0 { return Err(NegativeInput) }; Ok(x) }

   ┌─ test/fx/app.roc:13:17
   │
13 │     Err(err) => return Err(err)
   │                 ^^^^^^

[Anton-4]

Thanks @matthewmcgarvey, that's very useful info! This issue is quite high on my TODO list, I should be able to get to it "soon".

[Anton-4]

I first tried changing the desugaring but that broke other stuff. The desugaring did not turn out to be a problem. I have a full description of the fix in #8718 (comment)