[plt-scheme] Student parsing problem

This is an interesting error, and as a beginner I ran into this  
myself (1984, probably february or march :-)

I have not seen this kind of error since then, however. Before you  
invest time, let's see how many people have had students who stumbled  
over this.

-- Matthias








On Jun 2, 2009, at 5:36 PM, Jay McCarthy wrote:

> Yes, that would work for Beginner. But for the syntax highlighter, I
> don't think there's a nice way. My final sentence's "but" should only
> apply to the right-hand side of the "and".
>
> I'd change Beginner to show ya'll, but from what I can tell it use
> parameters to communicate the different syntax to the real C-reader.
> So I'd need to muck with that and add a new parameter... very messy
> and not sure its what the Powers That Be would prefer.
>
> Jay
>
> On Tue, Jun 2, 2009 at 3:33 PM, Carl Eastlund  
> <carl.eastlund at gmail.com> wrote:
>> On Tue, Jun 2, 2009 at 5:27 PM, Jay McCarthy  
>> <jay.mccarthy at gmail.com> wrote:
>>> I just helped a student debug their program. It looked like this:
>>>
>>> (define (function1 arg)
>>>  ...
>>>  )
>>> `;;(html (body "Text"))
>>>
>>> (define (function2 arg)
>>>  ...)
>>>
>>> (function2 1)
>>>
>>> Naturally it is easy to tell that the problem is that ` finds the  
>>> next
>>> s-expression, even if there are comments and whitespace between  
>>> it and
>>> the next one; so function2 isn't really defined at all.
>>>
>>> Now, it seems reasonable for this to be a problem that the user  
>>> should
>>> deal with in the Module and other higher languages. But you can get
>>> the same problem in Beginner (w/ List Abbrevs):
>>>
>>> (define (foo bar)
>>>  1)
>>> `;; Example
>>>
>>> (define (zog zag)
>>>  2)
>>>
>>> (zog 1)
>>>
>>> It would be nice, IMHO, if Beginner restricted ` and ' to be  
>>> adjacent
>>> and the standard syntax Scheme highlighter in other languages made
>>> quoted expression visually distinct, but it doesn't appear that that
>>> is easy to do without trying to do a full parse, plus there is the
>>> obvious macro problem.
>>>
>>> Jay
>>
>> This sounds like a custom reader problem to me, which avoids all
>> issues of parsing in the editor and "macro problems".  "Just" make it
>> a read error to have whitespace, comments, etc. between ` or ' and
>> their subsequent, implicitly-wrapped s-expression.
>>
>> --Carl
>>
>
>
>
> -- 
> Jay McCarthy <jay at cs.byu.edu>
> Assistant Professor / Brigham Young University
> http://teammccarthy.org/jay
>
> "The glory of God is Intelligence" - D&C 93
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