From 526b71bce7facf398b34d9dd8cbdee5c465021f5 Mon Sep 17 00:00:00 2001
From: Simon Brooke <simon@journeyman.cc>
Date: Sat, 17 Aug 2019 13:48:49 +0100
Subject: [PATCH] Considerably improved REPL, with better parse error handling

---
 README.md            | 17 ++++++++++++---
 project.clj          |  2 ++
 src/beowulf/core.clj | 52 ++++++++++++++++++++++++++++++++++++++------
 src/beowulf/read.clj | 12 ++++++----
 4 files changed, 69 insertions(+), 14 deletions(-)

diff --git a/README.md b/README.md
index 31efe93..e335a1c 100644
--- a/README.md
+++ b/README.md
@@ -4,15 +4,22 @@ LISP 1.5 is to all Lisp dialects as Beowulf is to Emglish literature.
 
 ## What this is
 
-A work-in-progress towards an implementation of Lisp 1.5 in Clojure.
+A work-in-progress towards an implementation of Lisp 1.5 in Clojure. The
+objective is to build a complete and accurate implementation of Lisp 1.5
+as described in the manual, with, in so far as is possible, exactly the
+same bahaviour; the only intended deviation is that the parser reads
+'mexprs' (meta language expressions) as well as sexprs.
 
 ## BUT WHY?!!?!
 
 Because.
 
-Because Lisp is the only computer language worth learning, and if a thing is worth learning, it's worth learning properly; which means going back to the beginning and trying to understand that.
+Because Lisp is the only computer language worth learning, and if a thing
+is worth learning, it's worth learning properly; which means going back to
+the beginning and trying to understand that.
 
-Because there is, so far as I know, no working implementation of Lisp 1.5 for modern machines.
+Because there is, so far as I know, no working implementation of Lisp 1.5
+for modern machines.
 
 Because I'm barking mad, and this is therapy.
 
@@ -24,6 +31,10 @@ Download from http://example.com/FIXME.
 
 `java -jar beowulf-0.1.0-standalone.jar`
 
+This will start a Lisp 1.5 read/eval/print loop (REPL).
+
+To end a session, type `quit` at the command prompt.
+
 ## Learning Lisp 1.5
 
 The `Lisp 1.5 Programmer's Manual` is still
diff --git a/project.clj b/project.clj
index 3985efa..7738a79 100644
--- a/project.clj
+++ b/project.clj
@@ -6,7 +6,9 @@
   :dependencies [[org.clojure/clojure "1.10.0"]
                  [org.clojure/math.numeric-tower "0.0.4"]
                  [org.clojure/tools.trace "0.7.10"]
+                 [environ "1.1.0"]
                  [instaparse "1.4.10"]]
   :main ^:skip-aot beowulf.core
+  :plugins [[lein-environ "1.1.0"]]
   :target-path "target/%s"
   :profiles {:uberjar {:aot :all}})
diff --git a/src/beowulf/core.clj b/src/beowulf/core.clj
index 9d9e9ea..49eb105 100644
--- a/src/beowulf/core.clj
+++ b/src/beowulf/core.clj
@@ -1,15 +1,53 @@
 (ns beowulf.core
   (:require [beowulf.eval :refer [EVAL oblist]]
-            [beowulf.read :refer [READ]])
+            [beowulf.read :refer [READ]]
+            [clojure.pprint :refer [pprint]]
+            [environ.core :refer [env]])
   (:gen-class))
 
-(defn -main
+(defn repl
   "Read/eval/print loop."
-  [& args]
-  (println "Béowulf is mín nama")
+  []
   (loop []
     (print ":: ")
     (flush)
-    (let [input (READ)]
-      (println (str ">  " (EVAL input @oblist)))
-      (recur))))
+    (try
+      (let [input (read-line)]
+        (cond
+          (= input "quit") (throw (ex-info "Færwell!" {:cause :quit}))
+          input (println (str ">  " (EVAL (READ input) @oblist)))
+          :else (println)))
+      (catch
+        Exception
+        e
+        (let [data (ex-data e)]
+          (if
+            data
+            (case (:cause data)
+              :parse-failure (println (:failure data))
+              :quit (throw e)
+              ;; default
+              (pprint data))
+            (println (.getMessage e))))))
+    (recur)))
+
+(defn -main
+  [& args]
+  (println
+    (str
+      "Hider wilcuman. Béowulf is mín nama\nSíðe "
+      (System/getProperty "beowulf.version")
+      "\n\n"))
+  (try
+    (repl)
+    (catch
+      Exception
+      e
+      (let [data (ex-data e)]
+        (if
+          data
+          (case (:cause data)
+            :quit (println (.getMessage e))
+            ;; default
+            (pprint data))
+          (println e))))))
diff --git a/src/beowulf/read.clj b/src/beowulf/read.clj
index bac8430..c68cfd9 100644
--- a/src/beowulf/read.clj
+++ b/src/beowulf/read.clj
@@ -64,9 +64,13 @@
       scale-factor := #'[0-9]*'")))
 
 (defn simplify
-  "Simplify this parse tree `p`."
+  "Simplify this parse tree `p`. If `p` is an instaparse failure object, throw
+  an `ex-info`, with `p` as the value of its `:failure` key."
   ([p]
-   (simplify p :sexpr))
+   (if
+     (instance? instaparse.gll.Failure p)
+     (throw (ex-info "Parse error" {:cause :parse-failure :failure p}))
+     (simplify p :sexpr)))
   ([p context]
   (if
     (coll? p)
@@ -274,5 +278,5 @@
   `(generate (simplify (parse ~s))))
 
 (defn READ
-  []
-  (gsp (read-line)))
+  [input]
+  (gsp (or input (read-line))))