Minor progress on Lisp functions

TEST.lsp

@@ -0,0 +1,35 @@
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Beowulf  Sysout file generated at 2023-03-29T12:34:39.278
+;; generated by simon
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+((NULL
+    LAMBDA (X) (COND ((EQUAL X (QUOTE NIL)) (QUOTE T)) ((QUOTE T) (QUOTE F))))
+  (GCD
+    LAMBDA
+    (X Y)
+    (COND
+      ((GREATERP X Y) (GCD Y X))
+      ((EQUAL (REMAINDER Y X) 0) X) ((QUOTE T) (GCD (REMAINDER Y X) X))))
+  (NIL)
+  (T . T)
+  (F)
+  (ADD1)
+  (APPEND)
+  (APPLY)
+  (ATOM)
+  (CAR)
+  (CDR)
+  (CONS)
+  (DEFINE)
+  (DIFFERENCE)
+  (EQ)
+  (EQUAL)
+  (EVAL)
+  (FIXP)
+  (INTEROP)
+  (NUMBERP)
+  (OBLIST)
+  (PLUS)
+  (PRETTY)
+  (QUOTIENT) (REMAINDER) (RPLACA) (RPLACD) (SET) (SYSIN) (SYSOUT) (TIMES))

doc/mexpr.md

@@ -0,0 +1,62 @@
+# M-Expressions
+
+M-Expressions ('mexprs') are the grammar which John McCarthy origininally used to write Lisp, and the grammar in which many of the function definitions in the [Lisp 1.5 Programmer's Manual](https://www.softwarepreservation.org/projects/LISP/book/LISP%201.5%20Programmers%20Manual.pdf) are stated. However, I have not seen anywhere a claim that Lisp 1.5 could *read* M-Expressions, and it is not clear to me whether it was even planned that it should do so.
+
+Rather, it seems to me probably that M-Expressions were only ever a grammar intended to be written on paper, like [Backus Naur Form](https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form), to describe and to reason about algorithms.
+
+I set out to make Beowulf read M-Expressions essentially out of curiousity, to see whether it could be done. I had this idea that if it could be done, I could implement most of Lisp 1.5 simply by copying in the M-Expression definitions out of the manual.
+
+Consequently, the Beowulf parser can parse the M-Expression grammar as stated in the manual, and generate S-Expressions from it according to the table specified on page 10 of the manual.
+
+There are two problems with this.
+
+## Problems with interpreting M-Expressions
+
+### Generating idiomatic Lisp
+
+In the M-Expression notation, a lower case character or sequence of characters represents a variable; an upper case character represents a constant. As the manual says,
+
+> 2 . The obvious translation of letting a constant translate into itself will not work.
+Since the translation of `x` is `X`, the translation of `X` must be something else to avoid
+ambiguity. The solution is to quote it. Thus `X` is translated into `(QUOTE X)`.
+
+Thus, necessarily, the translation of a constant must always be quoted. In practice, key constants in Lisp such as `T` are bound to themselves, so it is idiomatic in Lisp, certainly in the way we have learned to use it, to write, for example,
+
+```
+(SET (QUOTE NULL) 
+    (QUOTE (LAMBDA (X) 
+        (COND 
+            ((EQUAL X NIL) T) (T F)))))
+```
+
+However, the literal translation of
+
+```
+null[x] = [x = NIL -> T; T -> F]
+```
+
+is
+
+```
+(SET (QUOTE NULL) 
+    (QUOTE (LAMBDA (X) 
+        (COND 
+            ((EQUAL X (QUOTE NIL)) (QUOTE T))
+            ((QUOTE T) (QUOTE F))))))
+```
+
+This is certainly more prolix and more awkward, but it also risks being flat wrong.
+
+Is the value of `NIL` the atom `NIL`, or is it the empty list `()`? If the former, then the translation from the M-Expression above is correct. However, that means that recursive functions which recurse down a list seeking the end will fail. So the latter must be the case.
+
+`NULL` is described thus (Ibid, p11):
+
+> This is a predicate useful for deciding when a list is exhausted. It is true if and only if its argument is `NIL`.
+
+I think there is an ambiguity in referencing constants which are not bound to themselves in the M-Expression notation as given in the manual. This is particularly problematic with regards to `NIL`, but there may be others instances.
+
+### Curly braces
+
+The use of curly braces is not defined in the grammar as stated on page 10. They are not used in the initial definition of `APPLY` on page 13, but they are used in the more developed restatement on page 70. I believe they are to be read as indicating a `DO` statement -- a list of function calls to be made sequentially but without strict functional dependence on one another -- but I don't find the exposition here particularly clear and I'm not sure of this.
+
+Consequently, the M-Expression interpreter in Beowulf does not interpret curly braces.

resources/count.lsp

@@ -1 +0,0 @@
-(DEFUN COUNT (L) (COND ((EQ '() L) 0) (T (PLUS 1 (COUNT (CDR L))))))

resources/gcd.mexpr.lsp

@@ -1,3 +1,5 @@
 gcd[x;y] = [x>y -> gcd[y;x];
             rem[y;x] = 0 -> x;
             T -> gcd[rem[y;x];x]]
+
+;; gcd[x;y] = [x>y -> gcd[y;x]; rem[y;x] = 0 -> x; T -> gcd[rem[y;x];x]]

resources/length.lsp

@@ -0,0 +1 @@
+(DEFUN LENGTH (L) (COND ((EQ NIL L) 0) (T (ADD1 (LENGTH (CDR L))))))

resources/lisp1.5.lsp

@@ -7,6 +7,7 @@
 ;; Binding all system functions to NIL so that you can see on the OBLIST that
 ;; they exist.
 (ADD1 . NIL)
+(AND . NIL)
 (APPEND . NIL)
 (APPLY . NIL)
 (ATOM . NIL)

src/beowulf/bootstrap.clj

@@ -13,7 +13,7 @@
             [clojure.tools.trace :refer [deftrace]]
             [beowulf.cons-cell :refer [CAR CDR CONS LIST make-beowulf-list make-cons-cell
                                        pretty-print T F]]
-            [beowulf.host :refer [ADD1 DIFFERENCE FIXP NUMBERP PLUS QUOTIENT
+            [beowulf.host :refer [AND ADD1 DIFFERENCE FIXP NUMBERP PLUS QUOTIENT
                                   REMAINDER RPLACA RPLACD SUB1 TIMES]]
             [beowulf.io :refer [SYSIN SYSOUT]]
             [beowulf.oblist :refer [*options* oblist NIL]]
@@ -404,6 +404,7 @@
       (APPLY fn args environment)
       (case function-symbol ;; there must be a better way of doing this!
         ADD1 (apply ADD1 args)
+        AND (apply AND args)
         APPEND (apply APPEND args)
         APPLY (apply APPLY args)
         ATOM (ATOM? (CAR args))
@@ -417,6 +418,7 @@
         ;; think about EVAL. Getting the environment right is subtle
         FIXP (apply FIXP args)
         INTEROP (when (lax? INTEROP) (apply INTEROP args))
+        LIST (apply LIST args)
         NUMBERP (apply NUMBERP args)
         OBLIST (OBLIST)
         PLUS (apply PLUS args)

src/beowulf/host.clj

@@ -13,6 +13,17 @@
 ;; those which can be implemented in Lisp should be, since that aids
 ;; portability.
 
+(defn AND 
+  "True if and only if none of my `args` evaluate to either `F` or `NIL`,
+   else `F`.
+   
+   In `beowulf.host` principally because I don't yet feel confident to define
+   varargs functions in Lisp."
+  [& args]
+  (if (empty? (filter #(or (= 'F %) (empty? %)) args))
+    'T
+    'F))
+
 (defn RPLACA
   "Replace the CAR pointer of this `cell` with this `value`. Dangerous, should
   really not exist, but does in Lisp 1.5 (and was important for some