# Attempto Parser APE

1. Go to [GitHub](https://github.com/Attempto/APE) and clone the repo.
2. In your terminal run

    make install` in your terminal.
    `./ape.exe -text "John waits." -cdrsxml

Also, following the other instruction of the readme all worked out of the box (I had `swipl` already installed).

## Running the Parser in Prolog

We can also run the uncompiled version of ape.pl:

swipl ape.pl -text "John waits." -cdrsxml
?- ape.

To get a better idea of how `ape.pl` works, we can follow the execution in [trace mode](https://www.swi-prolog.org/pldoc/man?section=debugger). [^debug]

swipl ape.pl -text "John waits." -cdrsxml
?- trace.
?- ape.

This allows us to enter a computation using the `return`-key or to skip a computation using the `s`-key.

If we only want to see when a predicates is called (`call`) and succeeds (`exit`) but prefer ignore the backtracking (`fail` and `redo`), we run (still in trace mode)

?- visible(-all).
?- visible(+call).
?- visible(+exit).
?- ape.

While this gives us some idea of how `ape.pl` works, it does not yet give us easy access to the points in the computation where the rules of the Attempto grammar are applied.

## The Grammar

The Attempto grammar is in [`grammar.fit`](https://github.com/Attempto/APE/blob/master/prolog/parser/grammar.fit) and was translated into a prolog file `grammar.plp` when we called `make install`. It may be instructive to compare the two files (see Appendix A). For now, we just note that, while the grammar is specified in `grammar.fit`, the prolog file that is actually executed is `grammar.plp`.

In [line 47 of `grammar.fit`](https://github.com/Attempto/APE/blob/61c9b264ddc813eb60a0f624de5645b4d202780f/prolog/parser/grammar.fit#L45-L53) we find the Prolog predicate `acetext`. We can define it as a spy point and use the `l`-key to leap to the next call of that predicate.

?- spy(acetext).
?- ape.

Since we are not interested in actually debugging the parser, but rather in using its trace to understand the grammar, it is helpful to output the trace into a file [^output] and then filter the lines that are of interest to us:

swipl ape.pl -text "John waits." -cdrsxml
?- set_prolog_flag(color_term,false).
?- leash(-all).
?- visible(-all).
?- visible(+call).
?- visible(+exit).
?- protocol("./trace.txt").
?- trace.
?- ape.
?- nodebug.
?- noprotocol.
?- halt.
less trace.txt

## Appendix A: The Meaning of the Grammar Rules

For example rule 23 in `grammar.fit`

%-23---------------------------------------------------------------------------
% ExistentialQuestionTopic -->
%	NP[+NOM,+Q] ExistentialGlobalQuestionQuantor
%
% Example: Which code is there [?]
%------------------------------------------------------------------------------

existential_question_topic(
		display!tree![topic,NP,Quantor] &
		drs!in!DrsIn &
		drs!out!DrsOut &
		drs!id!ID
	) -->
	np(
		display!tree!NP &
		syn!head!case!<nom &
		syn!nonlocal!wh!in!<no_wh &
		syn!nonlocal!wh!out!<q &
		syn!nonlocal!gap!in![] &
		syn!nonlocal!gap!out![] &
		syn!nonlocal!subj!ref!nosubj &
		syn!head!agr!AGR &
		drs!in!DrsIn &
		drs!out!DrsOut &
		drs!scope!in!Scope &
		drs!scope!out!Scope &
		drs!id!ID
	),
	grammar_functionwords:existential_global_question_quantor(
		display!tree!Quantor &
		syn!head!agr!AGR
	).

turns into Prolog

existential_question_topic('$sign'(A,'$display'(B,[topic,C,D],E,F),G,H,I,'$drs'(J,K,L,M,N,O,P,Q),R))
-->
np('$sign'(S,'$display'(T,C,U,V),W,'$syn'(X,'$head'(Y,'$agr'(Z,A1,B1,C1),D1,'$case'(E1,'$nom'),F1,G1,H1,I1,J1,K1,L1),'$nonlocal'(M1,N1,O1,'$inout'(P1,[],[]),Q1,R1,S1,T1,U1,V1,'$whinout'(W1,'$wh'(X1,'$not_r'('$no_wh')),'$wh'(Y1,'$not_r'('$q'))),'$refagr'(Z1,nosubj,A2),B2)),C2,'$drs'(D2,K,E2,M,N,F2,G2,'$inout'(H2,I2,I2)),J2)),
grammar_functionwords:existential_global_question_quantor('$sign'(K2,'$display'(L2,D,M2,N2),O2,'$syn'(P2,'$head'(Q2,'$agr'(Z,A1,B1,C1),R2,S2,T2,U2,V2,W2,X2,Y2,Z2),A3),B3,C3,D3)).

The meaning of `-->` is similar to Prolog's `:-` and is explained in the article [Definite Clause Grammar](https://en.wikipedia.org/wiki/Definite_clause_grammar) which also has generally useful background for example on context-free and context-sensitive languages. See also this [tutorial](https://github.com/Anniepoo/swipldcgtut/blob/master/dcgcourse.adoc).

[^debug]: [Overview of the Debugger](https://www.swi-prolog.org/pldoc/man?section=debugoverview).

[^output]: I got this from [stackexchange](https://stackoverflow.com/a/43046698/4600290).