pereira@sri-unix.UUCP (08/15/83)
% File: RdTok Author: R.A.O'Keefe Updated: 18 November 82
/* This file is an interface to the Prolog system routine $read_tokens.
For some reason which I agreed to at the time but now forget, the
tokeniser is only accessible from compiled code. Now that the plsys(_)
mask exists for nasty internal things, it could really go there.
However, the structures returned by the tokeniser are unpleasant enough
that an extra interface is worth having.
read_tokens(TokenList, VarList)
returns a list of Tokens and a list of Variables, where the elements
of the VarList have the form Atom=Variable. For example, reading
the term above might result in ['TokenList'=_1, 'VarList'=_2]. The
elements of the TokenList take the form
var(Var,Name) -- the name is retained for error messages
integer(Int) -- 18-bit integers or xwd(_,_) pairs
atom(Name) -- most atoms
string(ChList) -- for string constants "..."
Punct -- for special punctuation ( ) [ ] { } , |
*/
:- public
read_tokens/2.
:- mode
% clean_neg(+, +, -),
clean_pos(+, +, -),
clean_tok_list(+, -),
clean_var_list(+, -),
read_tokens(-, -),
sixbit_to_ascii(+, -).
read_tokens(Toks, Vars) :-
'$read_tokens'(RawToks, RawVars),
clean_var_list(RawVars, Vars), !,
clean_tok_list(RawToks, Toks).
clean_var_list([var(Name,Var,_)|Rest], [Atom=Var|More]) :-
sixbit_to_ascii(Name, VarName),
name(Atom, VarName), !,
clean_var_list(Rest, More).
clean_var_list([], []).
sixbit_to_ascii([Three|Rest], Ascii) :-
A is (Three>>12)/\63,
B is (Three>> 6)/\63,
C is (Three )/\63,
one_sixbit_char(A, Ascii, A1),
one_sixbit_char(B, A1, A2),
one_sixbit_char(C, A2, More), !,
sixbit_to_ascii(Rest, More).
sixbit_to_ascii([], []).
one_sixbit_char(0, X, X) :- !.
one_sixbit_char(A, [H|T], T) :- A < 27, !, H is 96+A.
one_sixbit_char(63,[95|T], T) :- !.
one_sixbit_char(Z, [H|T], T) :- Z > 36, !, H is 28+Z.
one_sixbit_char(D, [H|T], T) :- H is 21+D.
%clean_tok_list([atom(-),xwd(Hi,Lo)|Rest], [integer(Int)|More]) :-
% clean_neg(Hi, Lo, Int), !,
% clean_tok_list(Rest, More).
clean_tok_list([xwd(Hi,Lo)|Rest], [integer(Int)|More]) :-
clean_pos(Hi, Lo, Int), !,
clean_tok_list(Rest, More).
clean_tok_list([var(Var,Name)|Rest], [var(Var,Atom)|More]) :-
sixbit_to_ascii(Name, VarName),
name(Atom, VarName), !,
clean_tok_list(Rest, More).
clean_tok_list([Other|Rest], [Other|More]) :- !,
clean_tok_list(Rest, More).
clean_tok_list([], []).
% reading a negative number is complicated by the fact that the token
% reader will accept 36-bit numbers. Unfortunately, the interpreter
% will not (X is xwd(_,_) truncates). On the whole, the simplest
% thing to do seems to be to represent the result as an integer where
% we can, otherwise to represent it as xwd(-,-) or -xwd(-,-). Then at
% least eval(-,-) in LONG will do the right thing.
clean_pos(0, N, N) :- !(N) =< 8'377777, !.
clean_pos(-1, N, N) :- !(N) > 8'377777, !.
clean_pos(Hi,Lo, xwd(Hi,Lo)).
%clean_neg(0, N, M) :- !(N) =< 8'377777+1, !, M is -N.
%clean_neg(-1, N, M) :- !(N) >= 8'377777, !, M is -N.
%clean_neg(Hi,Lo, -xwd(Hi,Lo)).