Unhelpful error message (Error: unexpected token: "(") when defining method that starts with a capital letter.

[Zopolis4]

Bug Report

The following code fails to compile:

def Foo(bar)
 puts "qux #{bar}"
end

Foo("quux")

with the following error:

In temp.cr:1:8

 1 | def Foo(bar)
            ^
Error: unexpected token: "("

I believe the starting with the capital letter is the issue, because renaming the method to foo or fFoo or _Foo fixed it (successfully printing the output of qux quux), but the error message does not make that clear.

I was additionally unable to find any documentation detailing that method names couldn't start with capital letters, so assuming I am not incorrect about the cause of the error it may help to update the relevant documentation to take note of that in addition to updating the error message.

Tested on:

Crystal 1.14.0 [dacd97bcc] (2024-10-09)

LLVM: 18.1.6
Default target: x86_64-unknown-linux-gnu

[devnote-dev]

You're on the right track — Crystal allows def Foo.bar syntax to define a method on a type without explicitly reopening it, I think in this case it's expecting the remaining part (.bar), when it should rightfully error on "Foo".

[Zopolis4]

Is that also the case for def FOO.bar?

Because I get the same when doing:

def FOO(bar)
 puts "qux #{bar}"
end

FOO("quux")

which then gives

In EEEE.cr:1:8

 1 | def FOO(bar)
            ^
Error: unexpected token: "("

[devnote-dev]

Yes, any identifier that starts with a capital letter is initially parsed as a constant, which usually refers to a type.

[Blacksmoke16]

This is called out in https://crystal-lang.org/reference/1.14/syntax_and_semantics/methods_and_instance_variables.html:

Method names begin with a lowercase letter and, as a convention, only use lowercase letters, underscores and numbers.

[Zopolis4]

Got it, thanks. In that case, its only the error message that needs to be updated to point to the actual problem.