3324 - completely redo type abbreviations

The old approach with '&' and '@' modifiers turned out to be a bad idea
because it introduces notions of precedence. Worse, it turns out you
want different precedence rules at different times as the old test
alluded:

  x:@number:3  # we want this to mean (address number 3)
  x:address:@number  # we want this to mean (address array number)

Instead we'll give up and focus on a single extensible mechanism that
allows us to say this instead:

  x:@:number:3
  x:address:@:number

In addition it allows us to shorten other types as well:

  x:&:@:num

  type board = &:@:&:@:char  # for tic-tac-toe

Hmm, that last example reminds me that we don't handle abbreviations
inside type abbreviation definitions so far..

1 files changed, 4 insertions, 0 deletions

diff --git a/017parse_tree.cc b/017parse_tree.cc
index 8efcb83e..83a96d6b 100644
--- a/017parse_tree.cc
+++ b/017parse_tree.cc
@@ -7,6 +7,9 @@
 // the first element of a type tree is always an atom, and left and right
 // pointers of non-atoms are never NULL. All type trees are 'dotted' in lisp
 // parlance.
+//
+// For now you can't use the simpler 'colon-based' representation inside type
+// trees. Once you start typing parens, keep on typing parens.
 
 :(scenarios load)
 :(scenario dilated_reagent_with_nested_brackets)
@@ -23,6 +26,7 @@ type_names = parse_string_tree(type_names);
 :(code)
 string_tree* parse_string_tree(string_tree* s) {
   assert(s->atom);
+  assert(!s->value.empty());
   if (s->value.at(0) != '(') return s;
   string_tree* result = parse_string_tree(s->value);
   delete s;