Some integration with Type Checking.

Now the UI highlights commands and numbers as you move the mouse, numbers
are blue, commands that type-check are green, commands that fail to
type-check are orange and will not be interpreted, and if there is no
stack effect information available for a command it is grey but you can
still attempt to execute it.

You can still evaluate whole expressions by selceting them and
right-inter-clicking before you release the left button, or by putting
the cursor on a line and typing ctrl-enter, which will run the whole
line.  These expressions are NOT (yet) type-checked.

joy/utils/polytypes.py

@@ -9,6 +9,7 @@ and we can introduce a kind of Kleene Star or sequence type that can stand for
 an unbounded sequence of other types.
 
 '''
+import sys
 from inspect import stack as inspect_stack
 from itertools import chain, product
 from logging import getLogger
@@ -343,6 +344,31 @@ def infer(*expression):
     return sorted(set(_infer(list_to_stack(expression))))
 
 
+def type_check(name, stack):
+    '''
+    Trinary predicate.  True if named function type-checks, False if it
+    fails, None if it's indeterminate (because I haven't entered it into
+    the FUNCTIONS dict yet.)
+    '''
+    try:
+        func = FUNCTIONS[name]
+    except KeyError:
+        return # None, indicating unknown
+
+    for fi, fo in infer(func):
+        try:
+          U = unify(fi, stack)
+        except JoyTypeError, e:
+            #print e
+            continue
+        except ValueError, e:
+            #print >> sys.stderr, name, e, stack
+            continue
+        #print U
+        return True
+    return False
+
+
 a0, a1, a2, a3, a4, a5, a6, a7, a8, a9 = A
 b0, b1, b2, b3, b4, b5, b6, b7, b8, b9 = B
 n0, n1, n2, n3, n4, n5, n6, n7, n8, n9 = N