From b4091f28dcff4bd75bd27bb26b5d1d01995aacba Mon Sep 17 00:00:00 2001
From: Simon Brooke <simon@journeyman.cc>
Date: Thu, 15 Aug 2019 09:47:48 +0100
Subject: [PATCH] Fixed seqability of cons cell, added quote read macro

---
 src/beowulf/cons_cell.clj | 25 +++++++++++----
 src/beowulf/read.clj      | 65 +++++++++++++++++++++++----------------
 2 files changed, 58 insertions(+), 32 deletions(-)

diff --git a/src/beowulf/cons_cell.clj b/src/beowulf/cons_cell.clj
index 8139bc4..2bbb5e4 100644
--- a/src/beowulf/cons_cell.clj
+++ b/src/beowulf/cons_cell.clj
@@ -3,22 +3,35 @@
 
 (def NIL (symbol "NIL"))
 
+(def NIL (symbol "NIL"))
+
 (deftype ConsCell [CAR CDR]
   clojure.lang.ISeq
   (cons [this x] (ConsCell. x this))
-  (count [this] (if
-                 (= (.CDR this) NIL)
-                  0
-                  (inc (count (.CDR this)))))
   (first [this] (.CAR this))
+  ;; next and more must return ISeq:
+  ;; https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/ISeq.java
   (more [this] (if
                  (= (.CDR this) NIL)
                  clojure.lang.PersistentList/EMPTY
                  (.CDR this)))
-  (next [this] (.CDR this))
-  (seq [this] this) ;; doesn't work - `Method beowulf/cons_cell/ConsCell.seq()Lclojure/lang/ISeq; is abstract`
+  (next [this] (if
+                 (= (.CDR this) NIL)
+                 nil ;; next returns nil when empty
+                 (.CDR this)))
+
+  clojure.lang.Seqable
+  (seq [this] this)
+
+  ;; for some reason this marker protocol is needed otherwise compiler complains
+  ;; that `nth not supported on ConsCell`
+  clojure.lang.Sequential
 
   clojure.lang.IPersistentCollection
+  (count [this] (if
+                 (= (.CDR this) NIL)
+                  0
+                  (inc (count (.CDR this)))))
   (empty [this] false)
   (equiv [this other] false))
 
diff --git a/src/beowulf/read.clj b/src/beowulf/read.clj
index 49ce3fa..216b18f 100644
--- a/src/beowulf/read.clj
+++ b/src/beowulf/read.clj
@@ -4,36 +4,47 @@
 ;;  (:import [beowulf.cons-cell ConsCell])
   )
 
+(declare generate)
+
 (def parse
   "Parse a string presented as argument into a parse tree which can then
   be operated upon further."
   (i/parser
-    "expr := mexpr | sexpr;
-    mexpr := fncall | defn | cond;
-    fncall := fn-name lsqb args rsqb;
-    lsqb := '[';
-    rsqb := ']';
-    defn := mexpr opt-space '=' opt-space mexpr;
-    cond := lsqb (cond-clause semi-colon opt-space)* cond-clause rsqb;
-    cond-clause := expr opt-space arrow opt-space expr;
-    arrow := '->';
-    args := (arg semi-colon opt-space)* arg;
-    arg := mexpr | sexpr;
-    fn-name := #'[a-z]*';
-    semi-colon := ';';
+    (str
+      ;; top level: we accept mexprs as well as sexprs.
+      "expr := mexpr | sexpr;"
 
-    list := lpar sexpr rpar | lpar (sexpr sep)* rpar | lpar (sexpr sep)* dot-terminal;
-    dotted-pair := lpar dot-terminal ;
-    dot := '.';
-    lpar := '(';
-    rpar := ')';
-    sexpr := atom | number | dotted-pair | list;
-    dot-terminal := sexpr space dot space sexpr rpar;
-    space := #'\\p{javaWhitespace}+';
-    opt-space := #'\\p{javaWhitespace}*';
-    sep := ',' | opt-space;
-    atom := #'[A-Z][A-Z0-9]*';
-    number := #'-?[0-9][0-9.]*';"))
+      ;; mexprs. I'm pretty clear that Lisp 1.5 could never read these,
+      ;; but it's a convenience.
+      "mexpr := fncall | defn | cond;
+      fncall := fn-name lsqb args rsqb;
+      lsqb := '[';
+      rsqb := ']';
+      defn := mexpr opt-space '=' opt-space mexpr;
+      cond := lsqb (cond-clause semi-colon opt-space)* cond-clause rsqb;
+      cond-clause := expr opt-space arrow opt-space expr;
+      arrow := '->';
+      args := (arg semi-colon opt-space)* arg;
+      arg := mexpr | sexpr;
+      fn-name := #'[a-z]*';
+      semi-colon := ';';"
+
+      ;; sexprs. Note it's not clear to me whether Lisp 1.5 had the quote macro,
+      ;; but I've included it on the basis that it can do little harm.
+      "list := lpar sexpr rpar | lpar (sexpr sep)* rpar | lpar (sexpr sep)* dot-terminal;
+      dotted-pair := lpar dot-terminal ;
+      dot := '.';
+      lpar := '(';
+      rpar := ')';
+      sexpr := atom | number | dotted-pair | list | quoted-expr;
+      quoted-expr := quote sexpr;
+      quote := '\\'';
+      dot-terminal := sexpr space dot space sexpr rpar;
+      space := #'\\p{javaWhitespace}+';
+      opt-space := #'\\p{javaWhitespace}*';
+      sep := ',' | opt-space;
+      atom := #'[A-Z][A-Z0-9]*';
+      number := #'-?[0-9][0-9.]*';")))
 
 (defn simplify
   "Simplify this parse tree `p`."
@@ -45,9 +56,11 @@
       (remove
         #(if (coll? %) (empty? %))
         (case (first p)
-          (:arrow :dot :lpar :lsqb :opt-space :rpar :rsqb :semi-colon :sep :space) nil
+          (:arrow :dot :lpar :lsqb :opt-space :quote :rpar :rsqb :semi-colon :sep :space) nil
           (:arg :expr :mexpr :sexpr) (simplify (second p))
           (:args :cond :cond-clause :dot-terminal :dotted-pair :fncall :list) (map simplify p)
+          ;; the quote read macro (which probably didn't exist in Lisp 1.5, but...)
+          :quoted-expr [:fncall [:fn-name "quote"] [:args (simplify (nth p 2))]]
           ;;default
           p)))
     p))