(* $Id: hoare.txt,v 1.2 1995/02/07 16:39:30 regensbu Exp $ *)

Proves about loops and tail-recursive functions
===============================================

Problem A

P = while B1       do S od
Q = while B1 or B2 do S od

Prove P;Q = Q    (provided B1, B2 have no side effects)

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Looking at the denotational semantics of while, we get

Problem B

[|B1|]:State->Bool
[|B2|]:State->Bool
[|S |]:State->State
f     :State->State

p = fix LAM f.LAM x. if [| B1 |] x                  then f([| S |] x) else x fi
q = fix LAM f.LAM x. if [| B1 |] x orelse [|b2 |] x then f([| S |] x) else x fi

Prove q o p = q          rsp.       ALL x.q(p(x))=q(x)

Remark: 1. Bool is the three-valued domain {UU,FF,TT} since tests B1 and B2 may
           not terminate.
        2. orelse is the sequential or like in ML

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If we abstract over the structure of stores we get

Problem C

b1:'a -> Bool
b2:'a -> Bool
g :'a ->'a
h :'a ->'a

p = fix LAM h.LAM x. if b1(x)              then h(g(x)) else x fi
q = fix LAM h.LAM x. if b1(x) orelse b2(x) then h(g(x)) else x fi

where g is an abstraction of [| S |]

Prove q o p = q 

Remark: there are no restrictions wrt. definedness or strictness for any of 
        the involved functions.

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In a functional programming language the problem reads as follows:

p(x) = if b1(x) 
         then p(g(x))
         else x fi

q(x) = if b1(x) orelse b2(x) 
         then q(g(x))
         else x fi


Prove:  q o p = q


-------------

In you like to test the problem in ML (bad guy) you have to introduce 
formal parameters for b1,b2 and g.

fun p b1 g x = if b1(x) 
         then p b1 g (g(x))
         else x;


fun q b1 b2 g x = if b1(x) orelse b2(x) 
         then q b1 b2 g (g(x))
         else x;

Prove: for all b1 b2 g . 
            (q b1 b2 g) o (p b1 g) = (q b1 b2 g)

===========

It took 4 person-days to formulate and prove the problem C in the
Isabelle logic HOLCF. The formalisation was done by conservative extension and
all proof principles where derived from pure HOLCF.


    





