CS 342 Lecture -*- Outline -*-

* Type checking in the REAL CLU

** structural (for equates)
                        ai1 = array[int]
                        ai2 = array[int]
                        x: ai1 := ai2$[0: 4]

** By name for abstract types
	names of clusters distinct
                 name of cluster is NOT equal to rep type of cluster
                        true even inside the cluster
                        so name equiv. for clusters

** security of data abstraction (langauge enforcement of data abtraction)
        goals:
                make sure modules outside cluster cannot "discover" rep
                help preserve invariant when objects are sent out from cluster
		    inductive proof:
                	establish rep invariant
				when create an object of rep type.
                	when pass out, want it to be protected,
                        	so change type using "up"
                        	(wrap object in shield)

                	when get object as argument,
				melt the shield using "down"
				can assume rep invariant holds

			when mutate an object,
				re-establish invariant before passing out again
				finally re-wrap using up before passing out
			
*** up
	wraps object in "shield"
                wrapping can happen at several levels of abstraction
                        sorted list { list { array } }
*** down
	melts shield
*** cvt
         shorthand (down on the way in, up on way out)
*** rep type not equal to abstract type
	even inside the cluster!
	ensures information hiding
		cannot change type of an expression (no coercions)
			except in cluster that implements that type

** Across module boundaries (library, type-safe linkage)
	advert: important for large projects to split work
			to compile only what's changed
	problem: how to type check program if compiler doesn't
			see it all at once?

	answer: use "library" to store information about module interfaces
			when check against the library
				assume that impl will have that interface
				record assumptions in object files
			when link, validate those assumptions