Improving Compiler Error Messages

[Walter Bright]

bearophile wrote:
> It's hard to answer similar questions in an objective way.
> In Python 2.x the line continuation is implicit if you are inside a parenthesis:
> print (x +
>  5)

The trouble with that, from a language perspective, is what does the grammar 
look like? Since the newline is significant to parsing, it should be a token. 
But inside ( ), there's a problem with it being a token, because now the grammar 
is messed up by needing to accept an optional newline token in between any other 
two tokens - but only if inside ( ).

The obvious technical solution is to turn off recognition of newline tokens 
inside of ( ). This is characterized as "smart" parsing, but I have a less kind 
word for it - "hack" - because it means there's not a clean way to separate the 
lexer from the parser, and not a clean way to express the grammar.

 From a language user's perspective this probably doesn't matter, but these 
kinds of things have a way of biting back in unanticipated nasty ways in the 
future. For example, C++'s use of < > for templates is a similar hack in the 
grammar. The C++ committee was warned at the time it was a very bad idea that 
would cause all kinds of problems, but they went ahead anyway because they 
thought it wouldn't affect users.

Turns out it did affect them, and it has caused a lot of grief over the years. 
There's another hack in C++0x in an attempt to paper over the problem.