On 2017-02-24 15:00, Bernd wrote:
Hello,
Eight years ago someone asked whether there is a parser combinator
library for free pascal, nothing like that existed at that time and
also does not seem to exist up to the present day.
While I was reading about parser combinators in functional programming
languages (during my 42nd attempt to learn Haskell) I thought to
myself why not try to implement something like that in Object Pascal,
just so see how far we can push the boundaries of this imperative
object oriented language.
This is what I have come up with so far:
https://github.com/prof7bit/fpc_parser_combinators
Since we don't have lambdas I choose the next closest approach to
emulate them with object instances instead. This leads to a lot of
boiler plate in the definition of the elementary parsers and
combinators but fortunately it can all be hidden away in the library
and the usage of the combinators looks quite neat:
// define the grammar
EXPR := Num or _PARENS;
MULFUNC := Sym('mul') and EXPR and EXPR;
ADDFUNC := Sym('add') and EXPR and EXPR;
INNER := MULFUNC or ADDFUNC or Num;
PARENS := Sym('(') and INNER and Sym(')');
Please also note the unorthodox usage of and/or operators to invoke
the combinators :-)
Cool! Parsing is my favourite area of computing science..
And my 42 attempt at learning haskell I gave up because it seemed not to
be able to do a simple char by char parser like any procedural language
can do...
But, now that you've mentioned this, my interest has peaked in both
Object Pascal and Haskell.
Please post improvements or variations of this theme, especially I am
interested in how to properly build up a syntax tree in the most
generic and reusable manner, this is something I have not yet
completely understood (because I am myself still quite unfamiliar with
this whole parsing business) currently all my parsers only return
arrays of strings that I can optionally post-process with an optional
hook function after a parser has completed.
I think parsing is still something computing science has to study more.
There are many undiscovered techniques. The biggest issue I have seen is
large long case statements where you start duplicating code in the case
statement. Rob Pike has done some work on this, but I've had little time
to research his solution proposed.
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