From c0be663f9b4c98e7126112a4fcc882901bff05a0 Mon Sep 17 00:00:00 2001
From: Jacques Comeaux <jacquesrcomeaux@gmail.com>
Date: Sat, 20 Aug 2022 19:40:07 -0500
Subject: Add LISP reference

---
 McCarthy.hs | 111 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 111 insertions(+)
 create mode 100644 McCarthy.hs

diff --git a/McCarthy.hs b/McCarthy.hs
new file mode 100644
index 0000000..9f1cb5c
--- /dev/null
+++ b/McCarthy.hs
@@ -0,0 +1,111 @@
+module McCarthy where
+
+import Prelude hiding ((^), (+), null)
+
+data Sexp = Atom String | Pair Sexp Sexp deriving Show
+
+at = Atom
+
+l []       = at"NIL"
+l (x : xs) = Pair x (l xs)
+
+t = l[l[at"AB",at"C"],at"D"]
+
+atom (Atom _ )  = at"T"
+atom (Pair _ _) = at"F"
+
+eq (Atom x, Atom y)
+    | x == y    = at"T"
+    | otherwise = at"F"
+eq _ = error "undefined"
+
+car (Pair e1 _) = e1
+car _ = error "undefined"
+
+cdr (Pair _ e2) = e2
+cdr _ = error "undefined"
+
+cons (e1, e2) = Pair e1 e2
+
+cases ((Atom x, y) : ps)
+  | x == "T" = y
+  | x == "F" = cases ps
+cases _ = error "undefined"
+
+(^) p q = cases[(p, q), (at"T",at"F")]
+(+) p q = cases[(p, at"T"), (at"T", q)]
+n p = cases[(p, at"F"), (at"T", at"T")]
+(-->) p q = cases[(p,q), (at"T",at"T")]
+
+ff(x) = cases[(atom(x), x), (at"T",ff(car(x)))]
+
+subst(x, y, z) = cases
+    [ (atom(z), cases[(eq(z, y), x), (at"T", z)])
+    , (at"T", cons(subst(x, y, car(z)), subst(x, y, cdr(z))))
+    ]
+
+equal(x, y) =
+    (atom(x) ^ atom(y) ^ eq(x,y)) +
+    (n (atom(x)) ^ n(atom(y)) ^ equal(car(x), car(y)) ^ equal(cdr(x), cdr(y)))
+
+null(x) = atom(x) ^ eq(x, at"NIL")
+
+append(x, y) = cases
+    [ (null(x), y)
+    , (at"T", cons(car(x), append(cdr(x),y)))
+    ]
+
+pair(x, y) = cases
+    [ (null(x) ^ null(y), at"NIL")
+    , (n(atom(x)) ^ n(atom(y)), cons(l[car(x), car(y)], pair(cdr(x), cdr(y))))
+    ]
+
+caar(x)   = car(car(x))
+cadar(x)  = car(cdr(car(x)))
+cadr(x)   = car(cdr(x))
+caddr(x)  = car(cdr(cdr(x)))
+caddar(x) = car(cdr(cdr(car(x))))
+
+assoc(x, y) = cases
+    [ (eq(caar(y), x), cadar(y))
+    , (at"T", assoc(x, cdr(y)))
+    ]
+
+sub2(x, z) = cases
+    [ (null(x), z)
+    , (eq(caar(x), z), cadar(x))
+    , (at"T", sub2(cdr(x), z))
+    ]
+
+sublis(x, y) = cases
+    [ (atom(y), sub2(x, y))
+    , (at"T", cons(sublis(x, car(y)),sublis(x, cdr(y))))
+    ]
+
+apply(f, args) = eval(cons(f, appq(args)), at"NIL")
+
+appq(m) = cases
+    [ (null(m), at"NIL")
+    , (at"T", cons(l[at"QUOTE", car(m)], appq(cdr(m))))
+    ]
+
+eval(e, a) = cases
+    [ (atom(e), assoc(e, a))
+    , (atom(car(e)), cases
+          [ (eq(car(e), at"QUOTE"), cadr(e))
+          , (eq(car(e), at"ATOM"), atom(eval(cadr(e), a)))
+          , (eq(car(e), at"EQ"), eq(eval(cadr(e), a), eval(caddr(e), a)))
+          , (eq(car(e), at"COND"), evcon(cadr(e), a))
+          , (eq(car(e), at"CAR"), car(eval(cadr(e), a)))
+          , (eq(car(e), at"CDR"), cdr(eval(cadr(e), a)))
+          , (eq(car(e), at"CONS"), cons(eval(cadr(e), a), eval(caddr(e), a)))
+          , (at"T", eval(cons(assoc(car(e), a), cdr(e)), a))
+          -- , (at"T", eval(cons(assoc(car(e), a), evlis(cdr(e), a)), a))
+          ]
+      )
+    , (eq(caar(e), at"LABEL"), eval(cons(caddar(e), cdr(e)), cons(l[cadar(e), car(e)], a)))
+    , (eq(caar(e), at"LAMBDA"), eval(caddar(e), append(pair(cadar(e), evlis(cdr(e), a)), a)))
+    ]
+  where
+    evcon(c, a) = cases[(eval(caar(c), a), eval(cadar(c), a)), (at"T", evcon(cdr(c),a))]
+    evlis(m, a) = cases[(null(m), at"NIL"), (at"T", cons(eval(car(m), a), evlis(cdr(m), a)))]
-- 
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