Improve syntax error messages

These will only do much good when we have better printing,
but I figured this kind of thing should go in.

Author: promovicz

command-interface/parser.dylan

@@ -10,6 +10,8 @@ define class <command-parse-error> (<simple-error>)
     init-keyword: parser:;
   slot error-token :: <command-token>,
     init-keyword: token:;
+  slot error-options :: <list>,
+    init-keyword: options:;
 end class;
 
 define class <command-ambiguous-error> (<command-parse-error>)
@@ -158,14 +160,16 @@ define method parser-advance (parser :: <command-parser>, token :: <command-toke
                   format-string: "Unrecognized token \"%s\"",
                   format-arguments: vector(token-string(token)),
                   parser: parser,
-                  token: token));
+                  token: token,
+                  options: acceptable));
     // more than one: ambiguous token
     otherwise =>
       signal(make(<command-ambiguous-error>,
                   format-string: "Ambiguous token \"%s\"",
                   format-arguments: vector(token-string(token)),
                   parser: parser,
-                  token: token));
+                  token: token,
+                  options: matches));
   end select;
 end method;
 

command-interface/tty-cli.dylan

@@ -41,6 +41,18 @@ define method editor-execute (editor :: <tty-command-shell>)
                n-spaces(size(editor-prompt(editor))),
                command-annotate(src, token-srcloc(pe.error-token)),
                condition-to-string(pe));
+    case
+      instance?(pe, <command-ambiguous-error>) =>
+        format-out("\nCan be interpreted as:\n");
+        for(option in error-options(pe))
+          format-out("  %=\n", option);
+        end;
+      instance?(pe, <command-unknown-error>) =>
+        format-out("\nPossible options:\n");
+        for(option in error-options(pe))
+          format-out("  %=\n", option);
+        end;
+    end;
   exception (e :: <error>)
     // print condition and clear
     format-out("Error: %=\n", e);