Improved tail recursion and loop example

doc/DRAWL.md

@@ -6,7 +6,7 @@ LISP interpreted on a bytecode VM running on a 1 MHz 6502 is going to be sssllll
 
 - The PROG feature isn't present. Programming is limited to interpreting lambda S-expressions
 - Number values are limited to 32 bit integers, no floating point
-- Deep recursion. The 6502 architecture limits recursion, so don't expect too much here
+- General recursion. The 6502 architecture limits recursion (but see tail recursion below), so don't expect too much here
 - Arrays not implemented
 
 However, the code is partitioned to allow for easy extension so some of these missing features could be implemented.
@@ -14,7 +14,7 @@ However, the code is partitioned to allow for easy extension so some of these mi
 ## Features of DRAWL
 
 - 32 bit integers and basic math operators. Hey, better than you probably expected
-- Recursion handles about nine levels deep. Better than nothing
+- Tail recursion handles handles deep recursion. Check out [loop.lisp](https://github.com/dschmenk/PLASMA/blob/master/src/lisp/loop.lisp)
 - Fully garbage collected behind the scenes
 - Optionally read LISP source file at startup
 - SET and SETQ implemented for setting variables

src/lisp/README.md

@@ -6,6 +6,6 @@ s-expr.pla is the guts of the system. Some features missing from the LISP 1.5 ma
 
 drawl.pla because this is a sslloowwww implementation of LISP, so named DRAWL in keeping with the speech theme. The file reading and REPL functions are contained here. DRAWL is meant to be extensible so adding PROG and floating point is easily possible.
 
-The sample LISP code comes from the LISP 1.5 manual. These are the only LISP programs that have been run on DRAWL, so other LISP programs may uncover bugs or limitations of DRAWL.
+The sample LISP code comes from the LISP 1.5 manual. These are the only LISP programs that have been run on DRAWL, so other LISP programs may uncover bugs or limitations of DRAWL. HOwever, tail recursion is supported - check out loop.lisp
 
 More information and links can be found here: https://github.com/dschmenk/PLASMA/blob/master/doc/DRAWL.md

src/lisp/drawl.pla

@@ -52,9 +52,6 @@ end
 // REPL interface to S-expression evaluator
 //
 
-const GC_RESET  = 2
-byte gc_trigger = GC_RESET
-
 const FILEBUF_SIZE = 128
 var readfn            // read input routine
 var fileref, filebuf  // file read vars
@@ -165,7 +162,7 @@ new_sym("BYE")=>natv = @natv_bye
 new_sym("MEM")=>natv = @natv_memavail
 while not quit
   putln; print_expr(eval_expr(readfn()))
-  gc_trigger--; if gc_trigger == 0; gc; gc_trigger = GC_RESET; fin
+  gc
 loop
 putln
 done

src/lisp/loop.lisp

@@ -0,0 +1,11 @@
+(LABEL LOOP (LAMBDA (I M FN)
+              (COND ((AND (< I M) (FN I)),(LOOP (+ 1 I) M FN))
+                     (T,(EQ I M)))
+            )
+)
+(LABEL LPRINT (LAMBDA (N)
+                (ATOM (PRINT N))
+              )
+)
+
+(LOOP 1 100 LPRINT)

src/lisp/s-expr.pla

@@ -476,44 +476,56 @@ end
 // Evaluate expression
 //
 
-def enter_lambda(expr, params)
+def enter_lambda(curl, expr, params)#2 // curl, expr
   var args, arglist, pairlist, pair
 
   if !expr or expr=>car <> sym_lambda
     puts("Invalid LAMBDA expression: ")
     print_expr(expr)
-    return NULL
+    return NULL, NULL
   fin
-  //
-  // Build arg list before prepending to assoc_list
-  //
-  args    = expr=>cdr=>car
-  arglist = NULL
-  while args
+  args = expr=>cdr=>car
+  if curl == expr
+    //
+    // Update current associations during tail recursion
+    //
+    while args
+      assoc(args=>car)=>cdr = eval_expr(params=>car)
+      args                  = args=>cdr
+      params                = params=>cdr
+    loop
+  else
+    //
+    // Build arg list before prepending to assoc_list
+    //
+    arglist = NULL
+    while args
+      if arglist
+        pairlist=>cdr = new_cons
+        pairlist      = pairlist=>cdr
+      else
+        arglist  = new_cons
+        pairlist = arglist
+      fin
+      pair          = new_cons
+      pair=>car     = args=>car
+      pair=>cdr     = eval_expr(params=>car)
+      pairlist=>car = pair
+      args          = args=>cdr
+      params        = params=>cdr
+    loop
     if arglist
-      pairlist=>cdr = new_cons
-      pairlist      = pairlist=>cdr
-    else
-      arglist  = new_cons
-      pairlist = arglist
+      pairlist=>cdr = assoc_list
+      assoc_list    = arglist
     fin
-    pair          = new_cons
-    pair=>car     = args=>car
-    pair=>cdr     = eval_expr(params=>car)
-    pairlist=>car = pair
-    args          = args=>cdr
-    params        = params=>cdr
-  loop
-  if arglist
-    pairlist=>cdr = assoc_list
-    assoc_list    = arglist
   fin
-  return expr=>cdr=>cdr=>car
+  return expr, expr=>cdr=>cdr=>car
 end
 
 export def eval_expr(expr)#1
-  var alist_enter, expr_car
+  var alist_enter, curl, expr_car
 
+  curl        = NULL // Current lambda
   alist_enter = assoc_list
   while expr
     if expr->type == CONS_TYPE
@@ -526,7 +538,7 @@ export def eval_expr(expr)#1
           expr = expr_car=>natv(expr=>cdr) // Native function
           break
         elsif expr_car=>lambda // DEFINEd lambda S-expression
-          expr = enter_lambda(expr_car=>lambda, expr=>cdr)
+          curl, expr = enter_lambda(curl, expr_car=>lambda, expr=>cdr)
         elsif expr_car == sym_cond // Inline cond() evaluation
           expr = expr=>cdr
 	  while expr
@@ -537,10 +549,10 @@ export def eval_expr(expr)#1
 	    expr = expr=>cdr
 	  loop
         else // Symbol associated with lambda
-          expr = enter_lambda(assoc(expr_car)=>cdr, expr=>cdr)
+          curl, expr = enter_lambda(curl, assoc(expr_car)=>cdr, expr=>cdr)
         fin
       elsif expr_car->type == CONS_TYPE and expr_car=>car == sym_lambda
-          expr = enter_lambda(expr_car, expr=>cdr) // Inline lambda
+          curl, expr = enter_lambda(NULL, expr_car, expr=>cdr) // Inline lambda
       fin
     else
       //

src/mklisp

@@ -23,3 +23,4 @@ cat lisp/list.lisp                 | ./ac.jar -ptx DRAWL.po lisp/LIST.LISP TXT
 cat lisp/maplist.lisp              | ./ac.jar -ptx DRAWL.po lisp/MAPLIST.LISP TXT
 cat lisp/gcd.lisp                  | ./ac.jar -ptx DRAWL.po lisp/GCD.LISP TXT
 cat lisp/fact.lisp                 | ./ac.jar -ptx DRAWL.po lisp/FACT.LISP TXT
+cat lisp/loop.lisp                 | ./ac.jar -ptx DRAWL.po lisp/LOOP.LISP TXT

src/mkrel

@@ -183,6 +183,7 @@ cp lisp/list.lisp         prodos/bld/lisp/LIST.LISP.TXT
 cp lisp/maplist.lisp      prodos/bld/lisp/MAPLIST.LISP.TXT
 cp lisp/gcd.lisp          prodos/bld/lisp/GCD.LISP.TXT
 cp lisp/fact.lisp         prodos/bld/lisp/FACT.LISP.TXT
+cp lisp/loop.lisp         prodos/bld/lisp/LOOP.LISP.TXT
 
 #mkdir prodos/bld/examples
 #cp samplesrc/examples/ex.1.pla  prodos/bld/examples/EX.1.PLA.TXT