Newbie parsing a simple bespoke language questions

"Chris Morley" <>
Mon, 10 Dec 2007 22:34:47 -0000

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Newbie parsing a simple bespoke language questions (Chris Morley) (2007-12-10)
Re: Newbie parsing a simple bespoke language questions (Chris Morley) (2007-12-12)
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From: "Chris Morley" <>
Newsgroups: comp.compilers
Date: Mon, 10 Dec 2007 22:34:47 -0000
Organization: Zen Internet
Keywords: parse, question
Posted-Date: 10 Dec 2007 20:35:33 EST


I am building a parser for a simple bespoke script like interpreter
which has very basic syntax & structure and a very limited
context/state held during parsing. My parser is in C++ using bison
2.3. Most commands are of the type:

block destination {
mycommand par1, par2,par3 /flag1 /flag2
mycommand2 ...
block destination {...}

I'm sure you get the drift!

I've done quite a bit of reading on the internet and can see two ways to
check the semantics of each statement (ensure pars are of correct type &
flags are appropriate).

1) produce a tree (probably just a stack is enough as it is so simple) and
check the semantics in some C++ code after generating the tree.
command: COMMAND params flags { somecommand($1,$2,$3) };
params: /* nothing */
      | params ',' param { push($3) };

etc. where params and flags are a stack or vector &
somecommand(ID,pars,flags) checks the types. And have one line in the
grammar for all commands.

2) put the semantics in the Bison source & have lines in the grammar for
each command
commands: c_one {}
                                | c_two {}
                                | etc...
c_one: "command1" PAR_INTEGER ',' PAR_FLOAT flags { command1($2, $4,$5);}
c_two: "command2" PAR_INTEGER ',' PAR_INTEGER flags { command2($2, $4,$5);}

I can't see a clincher either way in the pros and cons! I'd appreciate some
input before I head off doing it all one way and finding a huge D'OH! moment
and have to change it all!


[My advice is invariably to have the parser accept a larger language
and do the type checking in your semantic code. That lets you produce
useful error messages like "foo command requires two integers" rather
than a generic syntax error. -John]

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