114 lines
3.8 KiB
C
114 lines
3.8 KiB
C
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/**
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* @brief evaluate a single cond clause; if the test part succeeds return a
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* pair whose car is t and whose cdr is the value of the action part
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*/
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#include "debug.h"
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#include "memory/node.h"
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#include "memory/pointer.h"
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#include "memory/pso2.h"
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#include "memory/tags.h"
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#include "ops/progn.h"
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#include "ops/stack_ops.h"
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#include "ops/truth.h"
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#include "payloads/cons.h"
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#include "payloads/exception.h"
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/**
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* if the car of a consp evaluates to non-nil, the clause succeeded and the
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* cond expression must conclude, even if the result of the clause is nil.
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*
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* Therefore this funtion will
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* @return nil if the test failed;
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* @return a pair `(t . <value>)` if the test succeeded.
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*/
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struct pso_pointer eval_cond_clause( struct pso_pointer clause,
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struct pso4 *frame,
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struct pso_pointer frame_pointer,
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struct pso_pointer env ) {
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struct pso_pointer result = nil;
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#ifdef DEBUG
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debug_print( L"\n\tCond clause: ", DEBUG_EVAL, 0 );
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debug_print_object( clause, DEBUG_EVAL, 0 );
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debug_println( DEBUG_EVAL);
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#endif
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if ( consp( clause ) ) {
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struct pso_pointer val =
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eval_form( frame, frame_pointer, c_car( clause ),
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env );
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if ( !c_nilp( val ) ) {
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result =
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cons( t,
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c_progn( frame, frame_pointer, c_cdr( clause ), env ) );
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#ifdef DEBUG
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debug_print( L"\n\t\tCond clause ", DEBUG_EVAL, 0 );
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debug_print_object( clause, DEBUG_EVAL, 0 );
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debug_print( L" succeeded; returning: ", DEBUG_EVAL, 0 );
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debug_print_object( result, DEBUG_EVAL, 0 );
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debug_println( DEBUG_EVAL);
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} else {
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debug_print( L"\n\t\tCond clause ", DEBUG_EVAL, 0 );
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debug_print_object( clause, DEBUG_EVAL, 0 );
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debug_print( L" failed.\n", DEBUG_EVAL, 0 );
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#endif
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}
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} else {
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result = throw_exception( c_string_to_lisp_symbol( frame_pointer, L"cond" ),
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c_string_to_lisp_string
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(frame_pointer, L"Arguments to `cond` must be lists" ),
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frame_pointer );
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}
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return result;
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}
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/**
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* Special form: conditional. Each `clause` is expected to be a list; if the first
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* item in such a list evaluates to non-nil, the remaining items in that list
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* are evaluated in turn and the value of the last returned. If no arg `clause`
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* has a first element which evaluates to non nil, then nil is returned.
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*
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* * (cond clauses...)
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*
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* @param frame my stack frame.
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* @param frame_pointer a pointer to my pso4.
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* @param env the environment in which arguments will be evaluated.
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* @return the value of the last expression of the first successful `clause`.
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*/
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struct pso_pointer
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lisp_cond( struct pso4 *frame, struct pso_pointer frame_pointer,
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struct pso_pointer env ) {
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struct pso_pointer result = nil;
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bool done = false;
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for ( int i = 0; ( i < frame->payload.stack_frame.args ) && !done; i++ ) {
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struct pso_pointer clause_pointer = fetch_arg( frame, i );
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// TODO: WHOOPS! This isn't right. If the test of a cond clause
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// evaluates to non-nil, but the last form of the clause evaluates
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// to nil, the form still succeeded and we should still exit `cond`.
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//
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result = eval_cond_clause( clause_pointer, frame, frame_pointer, env );
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if ( !c_nilp( result ) && c_truep( c_car( result ) ) ) {
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result = c_cdr( result );
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done = true;
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break;
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}
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}
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#ifdef DEBUG
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debug_print( L"\tCond returning: ", DEBUG_EVAL, 0 );
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debug_print_object( result, DEBUG_EVAL, 0 );
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debug_println( DEBUG_EVAL);
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#endif
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return result;
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}
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