Com S 641 meeting -*- Outline -*-

* the qualification principle

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       THE QUALIFICATION PRINCIPLE
               (Schmidt 4)
                    or
             BLOCK STRUCTURE

Any semantically meaningful syntactic
category should be allowed to have



U-blocks :

        U ::= ... | 


Typing rule:




Semantic analogy:



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        ... local declarations

        ... | begin D in U end

        U is called the *body* of the block
        Explain the semantics, that the D are only visible to the body

        This gives *block structure*, as in Algol-60, Pascal, or Scheme


        pi1 |- D: pi2 dec,  pi3 |- U: t
        _______________________________   where pi3 = pi1 `unionMinus` pi2
            pi1 |- begin D in U end : t
        
        
        ...
               define I1(I2)=U in I1(V)
        is-like
               begin define I2 = V in U end

        The trouble with this analogy is that the two things have different
        syntactic categories.  The first is a program, and the U-block is a U

        Q: What are expression-blocks in Haskell or ML?
                let expressions

        Q: How is the qualification principle like the abstracion principle?
                (discuss this, see 4.9)