[vset VERSION 1.2]
[comment {-*- text -*- doctools manpage}]
[manpage_begin pt::rde n [vset VERSION]]
[include include/module.inc]
[titledesc {Parsing Runtime Support, PARAM based}]
[require pt::rde [opt [vset VERSION]]]
[require snit]
[require struct::stack 1.6]
[require pt::ast 1.2]
[description]
[include include/ref_intro.inc]

This package provides a class whose instances provide the runtime
support for recursive descent parsers with backtracking, as is needed
for the execution of, for example, parsing expression grammars. It
implements the [manpage {PackRat Machine Specification}], as such that
document is [emph required] reading to understand both this manpage,
and the package itself. The description below does make numerous
shorthand references to the PARAM's instructions and the various parts
of its architectural state.

[para]

The package resides in the Execution section of the Core Layer of
Parser Tools.
[para][image arch_core_transform][para]

[para]

Note: This package not only has the standard Tcl implementation, but
also an accelerator, i.e. a C implementation, based on Critcl.

[para]

[subsection {Class API}]

The package exports the API described here.

[list_begin definitions]

[call [cmd ::pt::rde] [arg objectName]]

The command creates a new runtime object for a recursive descent
parser with backtracking and returns the fully qualified name of the
object command as its result. The API of this object command is
described in the section [sectref {Object API}]. It may be used to
invoke various operations on the object.

[list_end]

[subsection {Object API}]

All objects created by this package provide the following 63 methods
for the manipulation and querying of their state, which is, in essence
the architectural state of a PARAM.

[para]

First some general methods and the state accessors.

[list_begin definitions]

[call [arg objectName] [method destroy]]

This method destroys the object, releasing all claimed memory, and
deleting the associated object command.

[call [arg objectName] [method reset] [arg chan]]

This method resets the state of the runtme to its defaults, preparing
it for the parsing of the character in the channel [arg chan], which
becomes IN.

[include include/channel_notes.inc]

[call [arg objectName] [method complete]]

This method completes parsing, either returning the AST made from the
elements of ARS, or throwing an error containing the current ER.

[call [arg objectName] [method chan]]

This method returns the handle of the channel which is IN.

[call [arg objectName] [method line]]

This method returns the line number for the position IN is currently
at. Note that this may not match with the line number for CL, due to
backtracking.

[call [arg objectName] [method column]]

This method returns the column for the position IN is currently
at. Note that this may not match with the column for CL, due to
backtracking.

[call [arg objectName] [method current]]

This method returns CC.

[call [arg objectName] [method location]]

This method returns CL.

[call [arg objectName] [method locations]]

This method returns the LS. The topmost entry of the stack will be the
first element of the returned list.

[call [arg objectName] [method ok]]

This method returns ST.

[call [arg objectName] [method value]]

This method returns SV.

[call [arg objectName] [method error]]

This method returns ER. This is either the empty string for an empty
ER, or a list of 2 elements, the location the error is for, and a set
of messages which specify which symbols were expected at the
location. The messages are encoded as one of the possible atomic
parsing expressions (special operators, terminal, range, and
nonterminal operator).

[call [arg objectName] [method errors]]

This method returns ES. The topmost entry of the stack will be the
first element of the returned list. Each entry is encoded as described
for [method error].

[call [arg objectName] [method tokens] [opt "[arg from] [opt [arg to]]"]]

This method returns the part of TC for the range of locations of IN
starting at [arg from] and ending at [arg to]. If [arg to] is not
specified it is taken as identical to [arg from]. If neither argument
is specified the whole of TC is returned.

[para]

Each token in the returned list is a list of three elements itself,
containing the character at the location, and the associated line and
column numbers, in this order.

[call [arg objectName] [method symbols]]

This method returns a dictionary containing NC. Keys are two-element
lists containing nonterminal symbol and location, in this order. The
values are 4-tuples containing CL, ST, ER, and SV, in this order. ER
is encoded as specified for the method [method error].

[call [arg objectName] [method known]]

This method returns a list containing the keys of SC. They are
encoded in the same manner as is done by method [method symbols].

[call [arg objectName] [method reducible]]

This method returns ARS. The topmost entry of the stack will be the
first element of the returned list

[call [arg objectName] [method asts]]

This method returns AS. The topmost entry of the stack will be the
first element of the returned list

[call [arg objectName] [method ast]]

This is a convenience method returning the topmost element of ARS.

[call [arg objectName] [method position] [arg loc]]

This method returns the line and column numbers for the specified
location of IN, assuming that this location has already been reached
during the parsing process.

[list_end]

The following methods implement all PARAM instructions. They all have
the prefix "i_".

[para]

The control flow is mainly provided by Tcl's builtin commands, like
[cmd if], [cmd while], etc., plus a few guarded variants of PARAM
instructions and Tcl commands.. That means that these instruction
variants will do nothing if their guard condition is not
fulfilled. They can be recognized by the prefix "i:ok_" and "i:fail_",
which denote the value ST has to have for the instruction to execute.

[para]

The instructions are listed in the same order they occur in the
[manpage {PackRat Machine Specification}], with the guard variants
listed after their regular implementation, if any, or in their place.

[list_begin definitions]

[vset INS input_next][vset IA0 msg][include include/rde_1ins.inc]

[vset INS test_alnum][include include/rde_0ins.inc]
[vset INS test_alpha][include include/rde_0ins.inc]
[vset INS test_ascii][include include/rde_0ins.inc]
[vset INS test_char][vset IA0 char][include include/rde_1ins.inc]
[vset INS test_ddigit][include include/rde_0ins.inc]
[vset INS test_digit][include include/rde_0ins.inc]
[vset INS test_graph][include include/rde_0ins.inc]
[vset INS test_lower][include include/rde_0ins.inc]
[vset INS test_print][include include/rde_0ins.inc]
[vset INS test_punct][include include/rde_0ins.inc]
[vset INS test_range][vset IA0 chars][vset IA1 chare][include include/rde_2ins.inc]
[vset INS test_space][include include/rde_0ins.inc]
[vset INS test_upper][include include/rde_0ins.inc]
[vset INS test_wordchar][include include/rde_0ins.inc]
[vset INS test_xdigit][include include/rde_0ins.inc]

[vset INS error_clear][include include/rde_0ins.inc]
[vset INS error_push][include include/rde_0ins.inc]
[vset INS error_pop_merge][include include/rde_0ins.inc]
[vset INS error_nonterminal][vset IA0 symbol][include include/rde_1ins.inc]

[vset INS status_ok][include include/rde_0ins.inc]
[vset INS status_fail][include include/rde_0ins.inc]
[vset INS status_negate][include include/rde_0ins.inc]

[vset INS loc_push][include include/rde_0ins.inc]
[vset INS loc_pop_discard][include include/rde_0ins.inc]
[vset INS loc_pop_rewind][include include/rde_0ins.inc]
[vset G ok][vset INS loc_pop_rewind][include include/rde_0gins.inc]
[vset IFAIL loc_pop_rewind]
[vset IOK   loc_pop_discard]
[vset IOKX  discard][include include/rde_0cins.inc]

[vset INS symbol_restore][vset IA0 symbol][include include/rde_1ins.inc]

[para] The boolean result of the check is returned as the result of
the method and can be used with standard Tcl control flow commands.

[vset INS symbol_save][vset IA0 symbol][include include/rde_1ins.inc]

[vset INS value_clear][include include/rde_0ins.inc]
[vset IFAIL value_clear]
[vset IOK   value_leaf]
[vset IOKX  leaf][include include/rde_0cins.inc]
[vset IFAIL value_clear]
[vset IOK   value_reduce]
[vset IOKX  reduce][include include/rde_0cins.inc]

[vset G ok][vset INS ast_value_push][include include/rde_0ginsb.inc]
[vset INS ast_push][include include/rde_0ins.inc]
[vset INS ast_pop_rewind][include include/rde_0ins.inc]
[vset G fail][vset INS ast_pop_rewind][include include/rde_0gins.inc]
[vset IFAIL ast_pop_rewind]
[vset IOK   ast_pop_discard]
[vset IOKX  discard][include include/rde_0cins.inc]
[vset INS ast_pop_discard][include include/rde_0ins.inc]
[vset IFAIL ast_pop_discard]
[vset IOK   ast_pop_rewind]
[vset IOKX  rewind][include include/rde_0cins.inc]

[call [arg objectName] [method i:ok_continue]]

This guarded method executes only for "ST == ok". Then it aborts the
current iteration of the innermost loop in the calling Tcl procedure.

[call [arg objectName] [method i:fail_continue]]

This guarded method executes only for "ST == fail". Then it aborts the
current iteration of the innermost loop in the calling Tcl procedure.

[call [arg objectName] [method i:fail_return]]

This guarded method executes only for "ST == fail". Then it aborts the
calling Tcl procedure.

[call [arg objectName] [method i:ok_return]]

This guarded method executes only for "ST == ok". Then it aborts the
calling Tcl procedure.

[list_end]
[para]

The next set of methods are [term {super instructions}], meaning that
each implements a longer sequence of instructions commonly used in
parsers. The combinated instructions of the previous set, i.e. those
with names matching the pattern "i_*/*", are actually super
instructions as well, albeit with limited scope, handling 2
instructions with their control flow. The upcoming set is much broader
in scope, folding as much as six or more PARAM instructions into a
single method call.

[para]

In this we can see the reasoning behind their use well:

[list_begin enumerated]
[enum]
By using less instructions the generated parsers become smaller, as
the common parts are now truly part of the common runtime, and not
explicitly written in the parser's code over and over again.

[enum]
Using less instructions additionally reduces the overhead associated
with calls into the runtime, i.e. the cost of method dispatch and of
setting up the variable context.

[enum]
Another effect of the super instructions is that their internals can
be optimized as well, especially regarding control flow, and stack
use, as the runtime internals are accessible to all instructions
folded into the sequence.
[list_end]

[para]

[list_begin definitions]
[call [arg objectName] [method si:void_state_push]]

This method combines [example {
i_loc_push
i_error_clear
i_error_push
}]

Parsers use it at the beginning of [term void] sequences and choices
with a [term void] initial branch.

[call [arg objectName] [method si:void2_state_push]]

This method combines [example {
i_loc_push
i_error_clear
i_error_push
}]

Parsers use it at the beginning of optional and repeated expressions.

[call [arg objectName] [method si:value_state_push]]

This method combines [example {
i_ast_push
i_loc_push
i_error_clear
i_error_push
}]

Parsers use it at the beginning of sequences generating an AST and
choices with an initial branch generating an AST.

[call [arg objectName] [method si:void_state_merge]]

This method combines [example {
i_error_pop_merge
i_loc_pop_rewind/discard
}]

Parsers use it at the end of void sequences and choices whose last
branch is void.

[call [arg objectName] [method si:void_state_merge_ok]]

This method combines [example {
i_error_pop_merge
i_loc_pop_rewind/discard
i_status_ok
}]

Parsers use it at the end of optional expressions

[call [arg objectName] [method si:value_state_merge]]

This method combines [example {
i_error_pop_merge
i_ast_pop_rewind/discard
i_loc_pop_rewind/discard
}]

Parsers use it at the end of sequences generating ASTs and choices
whose last branch generates an AST

[call [arg objectName] [method si:value_notahead_start]]

This method combines [example {
i_loc_push
i_ast_push
}]

Parsers use it at the beginning of negative lookahead predicates which
generate ASTs.

[call [arg objectName] [method si:void_notahead_exit]]

This method combines [example {
i_loc_pop_rewind
i_status_negate
}]

Parsers use it at the end of void negative lookahead predicates.

[call [arg objectName] [method si:value_notahead_exit]]

This method combines [example {
i_ast_pop_discard/rewind
i_loc_pop_rewind
i_status_negate
}]

Parsers use it at the end of negative lookahead predicates which
generate ASTs.

[call [arg objectName] [method si:kleene_abort]]

This method combines [example {
i_loc_pop_rewind/discard
i:fail_return
}]

Parsers use it to stop a positive repetition when its first, required, expression fails.

[call [arg objectName] [method si:kleene_close]]

This method combines [example {
i_error_pop_merge
i_loc_pop_rewind/discard
i:fail_status_ok
i:fail_return
}]

Parsers use it at the end of repetitions.

[call [arg objectName] [method si:voidvoid_branch]]

This method combines [example {
i_error_pop_merge
i:ok_loc_pop_discard
i:ok_return
i_loc_rewind
i_error_push
}]

Parsers use it when transiting between branches of a choice when both are void.

[call [arg objectName] [method si:voidvalue_branch]]

This method combines [example {
i_error_pop_merge
i:ok_loc_pop_discard
i:ok_return
i_ast_push
i_loc_rewind
i_error_push
}]

Parsers use it when transiting between branches of a choice when the
failing branch is void, and the next to test generates an AST.

[call [arg objectName] [method si:valuevoid_branch]]

This method combines [example {
i_error_pop_merge
i_ast_pop_rewind/discard
i:ok_loc_pop_discard
i:ok_return
i_loc_rewind
i_error_push
}]

Parsers use it when transiting between branches of a choice when the
failing branch generates an AST, and the next to test is void.

[call [arg objectName] [method si:valuevalue_branch]]

This method combines [example {
i_error_pop_merge
i_ast_pop_discard
i:ok_loc_pop_discard
i:ok_return
i_ast_rewind
i_loc_rewind
i_error_push
}]

Parsers use it when transiting between branches of a choice when both
generate ASTs.

[call [arg objectName] [method si:voidvoid_part]]

This method combines [example {
i_error_pop_merge
i:fail_loc_pop_rewind
i:fail_return
i_error_push
}]

Parsers use it when transiting between parts of a sequence and both
are void.

[call [arg objectName] [method si:voidvalue_part]]

This method combines [example {
i_error_pop_merge
i:fail_loc_pop_rewind
i:fail_return
i_ast_push
i_error_push
}]

Parsers use it when transiting between parts of a sequence and the
sucessfully matched part is void, and after it an AST is generated.

[call [arg objectName] [method si:valuevalue_part]]

This method combines [example {
i_error_pop_merge
i:fail_ast_pop_rewind
i:fail_loc_pop_rewind
i:fail_return
i_error_push
}]

Parsers use it when transiting between parts of a sequence and both
parts generate ASTs.

[call [arg objectName] [method si:value_symbol_start] [arg symbol]]

This method combines [example {
if/found? i_symbol_restore $symbol
i:found:ok_ast_value_push
i:found_return
i_loc_push
i_ast_push
}]

Parsers use it at the beginning of a nonterminal symbol generating an
AST, whose right-hand side may have generated an AST as well.

[call [arg objectName] [method si:value_void_symbol_start] [arg symbol]]

This method combines [example {
if/found? i_symbol_restore $symbol
i:found:ok_ast_value_push
i:found_return
i_loc_push
i_ast_push
}]

Parsers use it at the beginning of a void nonterminal symbol whose
right-hand side may generate an AST.

[call [arg objectName] [method si:void_symbol_start] [arg symbol]]

This method combines [example {
if/found? i_symbol_restore $symbol
i:found_return
i_loc_push
i_ast_push
}]

Parsers use it at the beginning of a nonterminal symbol generating an
AST whose right-hand side is void.

[call [arg objectName] [method si:void_void_symbol_start] [arg symbol]]

This method combines [example {
if/found? i_symbol_restore $symbol
i:found_return
i_loc_push
}]

Parsers use it at  the beginning of a void nonterminal symbol whose
right-hand side is void as well.

[call [arg objectName] [method si:reduce_symbol_end] [arg symbol]]

This method combines [example {
i_value_clear/reduce $symbol
i_symbol_save        $symbol
i_error_nonterminal  $symbol
i_ast_pop_rewind
i_loc_pop_discard
i:ok_ast_value_push
}]

Parsers use it at the end of a non-terminal symbol generating an AST
using the AST generated by the right-hand side as child.

[call [arg objectName] [method si:void_leaf_symbol_end] [arg symbol]]

This method combines [example {
i_value_clear/leaf  $symbol
i_symbol_save       $symbol
i_error_nonterminal $symbol
i_loc_pop_discard
i:ok_ast_value_push
}]

Parsers use it at the end of a non-terminal symbol generating an AST
whose right-hand side is void.

[call [arg objectName] [method si:value_leaf_symbol_end] [arg symbol]]

This method combines [example {
i_value_clear/leaf  $symbol
i_symbol_save       $symbol
i_error_nonterminal $symbol
i_loc_pop_discard
i_ast_pop_rewind
i:ok_ast_value_push
}]

Parsers use it at the end of a non-terminal symbol generating an AST
discarding the AST generated by the right-hand side.

[call [arg objectName] [method si:value_clear_symbol_end] [arg symbol]]

This method combines [example {
i_value_clear
i_symbol_save       $symbol
i_error_nonterminal $symbol
i_loc_pop_discard
i_ast_pop_rewind
}]

Parsers use it at the end of a void non-terminal symbol, discarding
the AST generated by the right-hand side.

[call [arg objectName] [method si:void_clear_symbol_end] [arg symbol]]

This method combines [example {
i_value_clear
i_symbol_save       $symbol
i_error_nonterminal $symbol
i_loc_pop_discard
}]

Parsers use it at the end of a void non-terminal symbol with a void
right-hand side.

[call [arg objectName] [method si:next_char] [arg tok]]
[call [arg objectName] [method si:next_range] [arg toks] [arg toke]]
[call [arg objectName] [method si:next_alnum]]
[call [arg objectName] [method si:next_alpha]]
[call [arg objectName] [method si:next_ascii]]
[call [arg objectName] [method si:next_ddigit]]
[call [arg objectName] [method si:next_digit]]
[call [arg objectName] [method si:next_graph]]
[call [arg objectName] [method si:next_lower]]
[call [arg objectName] [method si:next_print]]
[call [arg objectName] [method si:next_punct]]
[call [arg objectName] [method si:next_space]]
[call [arg objectName] [method si:next_upper]]
[call [arg objectName] [method si:next_wordchar]]
[call [arg objectName] [method si:next_xdigit]]

These methods all combine [example {
i_input_next $msg
i:fail_return
}]

with the appropriate [cmd i_test_xxx] instruction. Parsers use them for
handling atomic expressions.

[list_end]
[para]

[include include/feedback.inc]
[manpage_end]
