[Haskell-cafe] An irritating Parsec problem

[Andrew Coppin]

Andrew Coppin wrote:
> Philippa Cowderoy wrote:
>> On Wed, 15 Oct 2008, Andrew Coppin wrote:
>>
>>  
>>> Philippa Cowderoy wrote:
>>>    
>>>> expressions = do es <- many1 expression
>>>>                  eof
>>>>                  return es
>>>>         
>>> Ah - so "eof" fails if it isn't the end of the input?
>>>
>>>     
>>
>> eof = notFollowedBy anyChar
>>
>> (assuming I've got the identifiers right, that's the actual 
>> definition too)
>>   
>
> OK, well that'll make it fail alright. Now I just gotta figure out how 
> to get a sane error message out of it! ;-)
>
> (The example I showed is very simple; real parsers generally aren't.)

Actually, I added this to my real parser, and it actually seems to do 
exactly what I want. Give it an invalid expression and it immediately 
pinpoints exactly where the problem is, why it's a problem, and what you 
should be doing instead. Neat!