From 52d4c08f36cfbc8f34dcbd7a00f6204b5a7c8e91 Mon Sep 17 00:00:00 2001
From: David Schmenk <dschmenk@gmail.com>
Date: Thu, 11 Jul 2024 11:23:37 -0700
Subject: [PATCH] Improve tail recursion by inlining cond in eval_expr

---
 src/lisp/s-expr.pla | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/src/lisp/s-expr.pla b/src/lisp/s-expr.pla
index 0a87506..7d4bc1f 100644
--- a/src/lisp/s-expr.pla
+++ b/src/lisp/s-expr.pla
@@ -35,7 +35,7 @@ end
 
 predef eval_expr(expr)
 
-var sym_quote, sym_lambda
+var sym_quote, sym_lambda, sym_cond
 res[t_elem] pred_true  = 0, 0, BOOL_TRUE
 res[t_elem] pred_false = 0, 0, BOOL_FALSE
 
@@ -527,6 +527,15 @@ export def eval_expr(expr)#1
           break
         elsif expr_car=>lambda // DEFINEd lambda S-expression
           expr = enter_lambda(expr_car=>lambda, expr=>cdr)
+        elsif expr_car == sym_cond
+          expr = expr=>cdr
+	  while expr
+	    if eval_expr(expr=>car=>car) == @pred_true
+	      expr = expr=>car=>cdr=>car
+	      break
+	    fin
+	    expr = expr=>cdr
+	  loop
         else // Symbol associated with lambda
           expr = enter_lambda(assoc(expr_car)=>cdr, expr=>cdr)
         fin
@@ -779,6 +788,7 @@ new_assoc(new_sym("T"),   @pred_true)
 new_assoc(new_sym("F"),   @pred_false)
 sym_lambda = new_sym("LAMBDA")
 sym_quote  = new_sym("QUOTE")
+sym_cond   = new_sym("COND")
 sym_quote=>natv         = @natv_quote
 new_sym("CAR")=>natv    = @natv_car
 new_sym("CDR")=>natv    = @natv_cdr
@@ -788,7 +798,6 @@ new_sym("EQ")=>natv     = @natv_eq
 new_sym("NOT")=>natv    = @natv_not
 new_sym("AND")=>natv    = @natv_and
 new_sym("OR")=>natv     = @natv_or
-new_sym("COND")=>natv   = @natv_cond
 new_sym("SET")=>natv    = @natv_set
 new_sym("SETQ")=>natv   = @natv_setq
 new_sym("NULL")=>natv   = @natv_null