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Groups > comp.lang.postscript > #3764

New parser rewrite appears to be working nicely

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I just spent the evening and morning writing a parser for BNF syntax. 
Pretty meager stuff, I know. But the debugging effort went rather
smoothly thanks to the introduction of error message support.
Even the bare-bones error messages told me which production was
failing.

(pc12.ps)run
/to-string{ dup type /stringtype ne {
            dup length string exch 0 exch {3 copy putinterval length add} forall pop } if }
          def

/space  ( \n) anyof def
/space? //space many def
/alpha (a)(z) range (A)(Z) range alt def
/identifier (-) char //alpha alt some
            {to-string} using
            def

/terminal (") char  (") noneof many
                    xthen (") char thenx def
/non-terminal (<) char  //identifier
                        xthen (>) char thenx def
/Symbol //non-terminal def
/Expression //terminal //non-terminal alt  //space? thenx some def
/BNF //space?  //Symbol    xthen //space? thenx
               (::=) str then    //space? thenx
	       //Expression then //space? thenx
               def

/BNF-parse {
    0 0 3 2 roll string-input BNF
    dup first /OK eq { second first } if
} def

(
  <postal-address> ::= <name-part> <street-address> <zip-part>
) BNF-parse
pq


[Terminal transcript. 'Norah' is my laptop's name.]

]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$ gsq pc12bnf.ps
Error: /undefined in alpha
Operand stack:
   identifier
Execution stack:
   %interp_exit   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %stopped_push   --nostringval--   --nostringval--   --nostringval--   false   1   %stopped_push   1990   1   3   %oparray_pop   1989   1   3   %oparray_pop   1977   1   3   %oparray_pop   1833   1   3   %oparray_pop   --nostringval--   %errorexec_pop   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %stopped_push
Dictionary stack:
   --dict:737/1123(ro)(G)--   --dict:0/20(G)--   --dict:75/200(L)--   --dict:37/58(L)--   --dict:2/2(L)--   --dict:62/120(L)--
Current allocation mode is local
GPL Ghostscript 9.54.0: Unrecoverable error, exit code 1
]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$ gsq pc12bnf.ps
Error: /undefined in BNF
Operand stack:
   BNF   --nostringval--   --nostringval--
Execution stack:
   %interp_exit   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %stopped_push   --nostringval--   --nostringval--   --nostringval--   false   1   %stopped_push   1990   1   3   %oparray_pop   1989   1   3   %oparray_pop   1977   1   3   %oparray_pop   1833   1   3   %oparray_pop   --nostringval--   %errorexec_pop   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %stopped_push   --nostringval--   --nostringval--
Dictionary stack:
   --dict:737/1123(ro)(G)--   --dict:0/20(G)--   --dict:75/200(L)--   --dict:37/58(L)--   --dict:2/2(L)--   --dict:69/120(L)--
Current allocation mode is local
Current file position is 611
GPL Ghostscript 9.54.0: Unrecoverable error, exit code 1
]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$ gsq pc12bnf.ps
Fail
[[{(<) eq} (not satisfied)] [[(\n) [0 0]] {0 0 (  <postal-address> ::= <name-part> <street-address> <zip-part>\n) string-input}]]
GS>]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$ gsq pc12bnf.ps
Fail
[[( after ) (\n)] [[{(<) eq} (not satisfied)] [[( ) [0 0]] {0 1 ( <postal-address> ::= <name-part> <street-address> <zip-part>\n) string-input}]]]
GS>]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$ gsq pc12bnf.ps
Fail
[[( after ) [(<) (p)]] [[{(>) eq} (not satisfied)] [[(o) [0 4]] {0 5 (stal-address> ::= <name-part> <street-address> <zip-part>\n) string-input}]]]
GS>]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$ gsq pc12bnf.ps
Fail
[[( after ) [(<) (p) (o) (s) (t) (a) (l) (-) (a) (d) (d) (r) (e) (s) (s) (>) (:) (:) (=)]] [[{(-) eq} (not satisfied)] [[(<) [0 23]] {0 24 (name-part> <street-address> <zip-part>\n) string-input}]]]
GS>]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$ gsq pc12bnf.ps
Fail
[[( after ) [(<) (p) (o) (s) (t) (a) (l) (-) (a) (d) (d) (r) (e) (s) (s) (>) (:) (:) (=)]] [[{dup (A) ge exch (Z) le and} (not satisfied)] [[(<) [0 23]] {0 24 (name-part> <street-address> <zip-part>\n) string-input}]]]
GS>]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$ gsq pc12bnf.ps
p
(o)
GS>]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$ gsq pc12bnf.ps
stack:
[(p) (o) (s) (t) (a) (l) (-) (a) (d) (d) (r) (e) (s) (s) (:) (:) (=) (n) (a) (m) (e) (-) (p) (a) (r) (t)]
]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$ gsq pc12bnf.ps
stack:
[(p) (o) (s) (t) (a) (l) (-) (a) (d) (d) (r) (e) (s) (s) (:) (:) (=) (n) (a) (m) (e) (-) (p) (a) (r) (t) (s) (t) (r) (e) (e) (t) (-) (a) (d) (d) (r) (e) (s) (s) (z) (i) (p) (-) (p) (a) (r) (t)]
]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$ gsq pc12bnf.ps
Error: /undefined in to-string
Operand stack:
   --nostringval--   (<)   --nostringval--   --nostringval--
Execution stack:
   %interp_exit   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %stopped_push   --nostringval--   --nostringval--   --nostringval--   false   1   %stopped_push   1990   1   3   %oparray_pop   1989   1   3   %oparray_pop   1977   1   3   %oparray_pop   1833   1   3   %oparray_pop   --nostringval--   %errorexec_pop   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %stopped_push   --nostringval--   --nostringval--   --nostringval--   --nostringval--   --nostringval--   --nostringval--   --nostringval--   --nostringval--   --nostringval--   --nostringval--   --nostringval--
Dictionary stack:
   --dict:737/1123(ro)(G)--   --dict:0/20(G)--   --dict:75/200(L)--   --dict:37/58(L)--   --dict:2/2(L)--   --dict:70/120(L)--
Current allocation mode is local
Current file position is 779
GPL Ghostscript 9.54.0: Unrecoverable error, exit code 1
]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$ gsq pc12bnf.ps
stack:
[(postal-address) (:) (:) (=) (name-part) (street-address) (zip-part)]
]0;~/pcomb/ps
Norah@laptop ~/pcomb/ps
$

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New parser rewrite appears to be working nicely luser droog <luser.droog@gmail.com> - 2022-05-30 10:43 -0700
  Re: New parser rewrite appears to be working nicely luser droog <luser.droog@gmail.com> - 2022-06-03 19:11 -0700

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