HELP PROTECT                                    Robert Duncan, July 1990

Adding secure entry and exit code to functions.


    CONTENTS - (Use <ENTER> g to access required sections)

 -- The Protect Module
 -- The Protect Function


-- The Protect Module -------------------------------------------------

signature Protect
structure Protect : Protect
    The structure -Protect- is an autoloadable library module  described
    by the following signature:

        signature Protect = sig
            val protect : (unit -> 'a) * ('a -> unit) -> ('b -> 'c)
                                                       -> 'b -> 'c
            val guard   : 'a ref -> ('b -> 'c) -> 'b -> 'c
        end


-- The Protect Function -----------------------------------------------

val protect (entry:unit -> 'a, exit:'a -> unit) (f:'b -> 'c) : 'b -> 'c
    Adds the  given  entry  and  exit code  to  the  function  -f-.  The
    resulting function behaves like  -f-, except that  on each call  the
    function -entry- is applied first and  the result saved away, to  be
    given as argument to the function  -exit- after the call to -f-  has
    returned. Once  the  entry  code has  completed,  execution  of  the
    corresponding exit code is guaranteed regardless of whether the call
    to -f- terminates normally or abnormally.

    A possible definition for -protect- would be:

        fun protect (entry,exit) f x =
            let val s = entry()
            in  fst(f x, exit s:unit)
                handle e =>
                    (exit s; raise e)
            end;

    The true definition  is even more  secure than this  however, as  it
    will recover from errors  which cannot normally  be handled from  ML
    (such as ``out of memory'' errors).


val guard (r : 'a ref) (f : 'b -> 'c) : 'b -> 'c
    A specialisation of the function -protect- which guards the value of
    a reference for the duration of the call to -f-.

    Definition:

        fun guard r = protect (fn() => !r, fn x => r := x);


--- C.all/pml/help/protect
--- Copyright University of Sussex 1991. All rights reserved. ----------