Typo's in instance variable names can give confusing error messages

A May Of WTFs
1

When you mistype an instance variable you get an error like:

undefined method `some_method' for nil:NilClass

which can be very confusing for a newcomer. I’m not saying we should reintroduce WhinyNils, but some DidYouMean like functionality would be really helpful.

2

That unfortunately is impossible, or close to impossible to implement.

However you can turn on Ruby’s verbose mode:

>> $VERBOSE = true
=> true
>> @foo.bar
(irb):5: warning: instance variable @foo not initialized
NoMethodError (undefined method `bar' for nil:NilClass)
3

I realise this is hard to do, but this confuses newbies all the time. It’s definitely something we’ve taken for granted as being unfixable.

1 Like
4

I like the idea of hooking into DidYouMean. Maybe there’s an opportunity for a PR there that monkeypatches nil? Or some other deeply ridiculous, deeply Rubyish thing that Just Might Work?

5

Maybe have actionview_precompiler look at the AST to find referenced ivars? https://github.com/jhawthorn/actionview_precompiler

1 Like
6

Looks like DYM has experimental support for ivars. We might be able to add a tracepoint for uninitialized ivars, but I know people commonly leave ivars uninitialized and disable warnings so it might be a perf hit for that code.

2 Likes
7

I made a PR for tracepoints on uninitialized ivars here. I will be very honest and say I haven’t thought of the consequences. It may break patterns like:

def foo
  @foo ||= "neat"
end
3 Likes
8

The OP example wasn’t clear to me, I guess it’s from something like @mistyped_ivar.some_method. I think using ruby -w or $VERBOSE = true or ruby -d is a good way to catch those.

9

I frequently see this as a problem in templates, as that’s where Rails prefers to use instance variables. I think it’s best to avoid them in the first place and always use a 2-line method to make accessor methods available from the controller to views: Alternative to Rails' sharing of instance variables between controller and views. · GitHub

Any typo will result in a more comprehensible “method not found” error.