Smarter decoding of terms

This update includes an update to `decodeResult` that makes string
decoding far less aggressive. This also replaces the `!decode` REPL
command with `!output` to allow users to switch output format on the
fly. New tests are included for verifying decoding behavior; this group
needs to be fleshed out further.

src/Research.hs

@@ -160,8 +160,18 @@ toAscii tree = go tree "" True
         ++ go right (prefix ++ (if isLast then "    " else "|   ")) True
 
 decodeResult :: T -> String
-decodeResult tc
-  | Right num  <- toNumber tc = show num
-  | Right str  <- toString tc = "\"" ++ str ++ "\""
-  | Right list <- toList tc   = "[" ++ intercalate ", " (map decodeResult list) ++ "]"
-  | otherwise                 = formatResult TreeCalculus tc
+decodeResult Leaf = "t"
+decodeResult tc =
+  case (toString tc, toList tc, toNumber tc) of
+    (Right s, _, _) | all isCommonChar s -> "\"" ++ s ++ "\""
+    (_, _, Right n) -> show n
+    (_, Right xs@(_:_), _) -> "[" ++ intercalate ", " (map decodeResult xs) ++ "]"
+    (_, Right [], _) -> "[]"
+    _ -> formatResult TreeCalculus tc
+  where
+    isCommonChar c =
+      let n = fromEnum c
+      in (n >= 32 && n <= 126)
+         || n == 9
+         || n == 10
+         || n == 13