From 52d8cf7782b1eb4ce07b5e7daa5bd4ca88b17f0e Mon Sep 17 00:00:00 2001
From: Jacques Comeaux <jacquesrcomeaux@gmail.com>
Date: Sat, 10 Sep 2022 17:54:41 -0500
Subject: Begin writing evaluator

---
 eval.s | 117 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 117 insertions(+)
 create mode 100644 eval.s

diff --git a/eval.s b/eval.s
new file mode 100644
index 0000000..b4c539a
--- /dev/null
+++ b/eval.s
@@ -0,0 +1,117 @@
+R0 has e (s-exp to be evaluated)
+a (the environment / symbol table) at a known location
+
+; e to be evaluated in R0
+; result of evaluation in R0
+eval: (e) =
+  BIT #1, R0            ; test if e is cons or atom
+  BEQ not_atom          ; branch if not atom(e)
+  JSR PC, #assoc        ; return assoc(e)
+  ; TODO assumes assoc puts result in R0
+  RTS PC
+not_atom:
+  MOV @R0, R2           ; hd <- car(e) ; R2
+  MOV 2(R0), R3         ; tl <- cdr(e) ; R3
+  BIT #1, R1            ; test if hd is cons or atom
+  BNE head_is_atom      ; branch if atom(hd)
+  ;otherwise:
+  MOV @R2, R0           ; arg1 <- car(hd)     ; R0 (first)
+  MOV 2(R2), R4         ; rest <- cdr(hd)     ; R4
+  MOV 2(R4), R5         ; rest' <- cdr(rest)  ; R5
+  MOV @(R4), R4         ; second <- car(rest) ; R4
+  MOV @(R5), R5         ; third <- car(rest') ; R5
+
+  MOV "LABEL", R1       ; arg2 <- at"LABEL"
+  JSR PC, #eq           ; test eq(first, at"LABEL")
+                        ; result is in R1
+  BEQ not_label         ; branch if not label
+  ;if is label
+  ; push entry l[second, hd] onto symbol table a
+  ; a <- cons(l[second, hd], a)
+  MOV R2, (#symbol_table); push hd onto symbol table column 1
+  MOV R4, (#symbol_table); push second onto symbol table column 0
+
+  ; r0 <- cons(third, tl)
+  MOV R5, R0        ; arg1 <- third
+  MOV R3, R1        ; arg2 <- tl
+  JSR PC, #cons     ; result in R0
+  JSR PC, #eval     ; result <- eval(cons(third, tl))
+  pop off of a      ; a <- cdr a
+  RTS PC            ; return result in R0
+
+not_label:
+                        ; first still in R0
+  MOV "LAMBDA", R1      ; arg2 <- at"LAMBDA"
+  JSR PC, #eq           ; test eq(first, at"LAMBDA")
+                        ; result is in R1
+  BEQ error             ; branch if not lambda
+
+  ;if is lambda
+  save old a
+
+  ; push onto a ; a <- append(pair(second, evlis(tl)), a)
+  evlis: (symbols, args)
+  BIT #1, R0    ; test if args is cons or atom
+  if its atom jump to done
+  ; if its cons:
+  BIT #1, R1    ; test if symbols is cons or atom
+  if its atom jump to done
+  ; if both are cons:
+  symbol <- car(symbols)
+  symbols <- cdr(symbols)
+  arg1 <- car(args)
+  args <- cdr(args)
+  JSR PC, #eval               ; eval (arg1) ; result in R0
+  ; push (symbol, eval(arg))
+  MOV R0, (#symbol_table)     ; push R0 to column 1
+  MOV   , (#symbol_table)     ; push symbol to column 0
+  jump to evlis(symbols, args)
+  done:
+
+  MOV R5, R0                  ; arg1 <- third
+  JSR PC, #eval               ; result <- eval(third)
+
+  pop off of a ; a <- old a
+
+  return result
+
+error:
+  this should never happen. ill-formed s-exp
+  return bad ; maybe just return the original s-exp
+head_is_atom: (hd, tl)
+  return cases
+    [ (eq(hd, at"QUOTE"), car(tl))
+    , (eq(hd, at"ATOM"), atom(eval(car(tl))))
+    , (eq(hd, at"EQ"), eq(eval(car(tl)), eval(cadr(tl))))
+    , (eq(hd, at"COND"), evcon(car(tl)))
+    , (eq(hd, at"CAR"), car(eval(car(tl))))
+    , (eq(hd, at"CDR"), cdr(eval(car(tl))))
+    , (eq(hd, at"CONS"), cons(eval(car(tl)), eval(cadr(tl))))
+    , (at"T", eval(cons(assoc(hd), tl)))
+    ]
+
+evcon(c) =
+  cases
+    [ (eval(caar(c)), eval(cadar(c)))
+    , (at"T", evcon(cdr(c)))
+    ]
+
+evlis(m) =
+  cases
+    [ (null(m), at"NIL")
+    , (at"T", cons(eval(car(m)), evlis(cdr(m))))
+    ]
+
+eq
+assoc
+cons
+
+at"QUOTE"
+at"ATOM"
+at"EQ"
+at"COND"
+at"CAR"
+at"CDR"
+at"CONS"
+at"LABEL"
+at"LAMBDA"
-- 
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