Bug #7539: Misleading error message "can't convert nil into string"

• Status: Closed
• Assignee: marcandre (Marc-Andre Lafortune)
• Target version: 2.6
• ruby -v: ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-linux]

[connec (Chris Connelly)]

When trying to call String#+ with nil as an argument, you get the error "can't convert nil into String", which does not make sense (in fact seeming blatantly false) as nil.to_s, String(nil) etc. all return '' without errors.

Ideally, this method should use to_s to convert the argument, or else report an error along the lines of "can't append nil to string".

Minimal test case:

Actual:
> '' + nil
TypeError: can't convert nil into String

Expected:
> '' + nil
''

[kosaki (Motohiro KOSAKI)]

I support "can't append nil to string" error.

[marcandre (Marc-Andre Lafortune)]

You'll get this error message for every failed conversion; the use the corresponding implicit conversion methods (to_str, to_int, ...)

"" + 42
[].concat(1..2)

I'm surprised this leads to confusion, but we could change the message to something like:

"no implicit conversion of nil into String"

[rosenfeld (Rodrigo Rosenfeld Rosas)]

kosaki (Motohiro KOSAKI) wrote:

I support "can't append nil to string" error.

I'd prefer a more descriptive message like:

"can't append nil to string (object doesn't respond to to_str)"

This way someone would know that this would work:

nil.class.module_eval{alias to_str to_s}; '' + nil

[phluid61 (Matthew Kerwin)]

I support adding the 'implicit' word to the error message.

"no implicit conversion of nil into String"

I remember when I first encountered it, knowing to search for "implicit
conversion" would have helped me understand the difference between #to_s
and #to_str earlier.

[note: it's more than just implementing #to_str , there are other
implications for implicit casting]

[duerst (Martin Dürst)]

• File patch#7539.txt patch#7539.txt added
• Target version changed from 1.9.3 to 2.6

rosenfeld (Rodrigo Rosenfeld Rosas) wrote:

kosaki (Motohiro KOSAKI) wrote:

I support "can't append nil to string" error.

I'd prefer a more descriptive message like:

"can't append nil to string (object doesn't respond to to_str)"

Looking at the implementation, a "can't append" would need some special code. A message such as ""can't convert nil into String (using to_str)" would be relatively easy to generate. I have attached a patch (not compiled or tested, sorry). This change would show up in many other places that use the same conversion logic, so let's make sure we really want it.

[drbrain (Eric Hodel)]

I am worried about implying that the user should add to_str, to_ary, etc. to fix this problem. Rarely is implementing these methods appropriate since they are used for objects that are intended to behave exactly as a String, Array, etc.

An error message that omitted mention of these methods can help prevent users from accidentally shooting themselves in the foot.

[marcandre (Marc-Andre Lafortune)]

• Assignee set to marcandre (Marc-Andre Lafortune)

Looking at the implementation, a "can't append" would need some special code

Indeed. It would be redundant anyways, since the error message starts with "in `+':", or whichever method was called last.

I am worried about implying that the user should add to_str, to_ary, etc. to fix this problem

Agreed, which is why I proposed making explicit the fact that implicit conversion failed.

So, I'll commit "no implicit conversion of into " unless there's a better wording proposed

[marcandre (Marc-Andre Lafortune)]

• Status changed from Open to Closed
• % Done changed from 0 to 100

This issue was solved with changeset r38979.
Chris, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.

---

• object.c: Improve error for failed implicit conversions [Bug #7539]

• error.c: Adapt rdoc

• test/ruby/test_object.rb: Test for above