Re: [PATCH] Fix error messages involving internal definitions

[Mark H Weaver <mhw@netris.org>]

Andy Wingo <wingo@pobox.com> writes:
>> Thirdly, "definition in expression context" is a confusing message for
>> Scheme beginners, who are likely to make this mistake.
>
> The problem is that I'm not sure that the error message you suggest is
> correct.  You show:
>
>>   (let ((x 1))
>>     #f
>>     (define blah 3))
>>
>> Currently, you get a message like this:
>>
>>   unknown location: definition in expression context in subform blah of 3
>>
>> With this patch, you get a message like this:
>>
>>   /home/mhw/guile-modules/foo.scm:5:2: internal definition in a context where definitions are not allowed in form (define blah 3)
>
> And this is much better.  But, it is not the right error message for a
> form like:
>
>   (if 1
>       (define bar 2))
>
> So, that's question 1: can we come up with some other message that's
> more helpful while also being accurate?

How about if we simply remove 'internal' from the error message?

> "Definition in expression context" does have the advantage that it can
> be searched for in the manual (if we put it there), or on the web.  If
> all things were equal, it would have the advantage of being shorter as
> well.

How about this?

  "definition in expression context, where definitions are not allowed,"

> Question 2 is about the implementation.  I'm sure you winced as much as
> I did at adding a seventh return value from syntax-type :)  I was
> reading though and noted in the comment above syntax-type that the "s"
> return value already has the source information for the expression.  So
> a more minimal change like the attached patch yields the error message:
>
>   /tmp/foo.scm:5:2: definition in expression context in form blah
>
> WDYT?  I think I prefer the more minimal approach in that patch, but
> either way is fine.

I agree that it is painful to add another value, but personally I think
it is very important to include the entire from to make the error
message comprehensible, especially for Scheme beginners who are quite
likely to make this mistake.

> Feel free to commit whatever you think is best, here.

   Thanks!
     Mark