Re^2: Perl not BNF-able??

Why do you say that snippet is not BNF-able? Parts of Perl cannot be expressed by BNF, but your snippet is not one of them as far as I can tell.

statement : sub_proto
statement : sub_def

term      : anon_sub

sub_proto : ws? "sub" ws? sym_name proto? ws? ";"
sub_def   : ws? "sub" ws? sym_name proto? ws? block

anon_sub  : ws? "sub" block

sym_name  : ident "::" sym_name
sym_name  : ident
[download]

Comment on Re^2: Perl not BNF-able?? Download Code

Replies are listed 'Best First'.
Re^3: Perl not BNF-able?? by Anonymous Monk on Jul 05, 2005 at 09:00 UTC
So, where's the requirement that both the proto and the definition have the same signature? It's trivial to write BNF for a proto, it's trivial BNF for the definition (given the BNF for the block). It's impossible to use BNF for the requirement that the signatures (that is, name and prototype) are the same. Your BNF is ok if you're allowed to write: `sub hello($); sub hello() {print("Hello, world");}` [download] But you aren't. The signatures should be the same - and that's unexpressable with BNF.	[reply] [d/l]
Re^4: Perl not BNF-able?? by ikegami (Patriarch) on Jul 05, 2005 at 15:02 UTC
BNF dictates syntax, and the above snippet is syntactically correct. `perl -c` agrees by saying "syntax OK". The following is syntactically correct C++: `Type1 p; Type2 q; p = q;` [download] Are `Type1` and `Type2` assignment compatible? It doesn't matter as far as BNF is concerned because it only specifies the syntax. The error in your Perl snippet and the error in the above C++ snippet (if any) are semantic errors. Semantic analysis is usually done independant of parsing. Semantic analysis checks things such as types, and whether forward declerations have been resolved. These are things that don't affect the compiler's understanding of the code, but affect whether the code is consistent (valid) when looking at the bigger picture. BNF does not have a role in semantic analysis. BNF is therefore sufficient to handle the Perl in your post.	[reply] [d/l] [select]