--Copyright (c) 1991-2002, The Numerical ALgorithms Group Ltd.
--All rights reserved.
--
--Redistribution and use in source and binary forms, with or without
--modification, are permitted provided that the following conditions are
--met:
--
--    - Redistributions of source code must retain the above copyright
--      notice, this list of conditions and the following disclaimer.
--
--    - Redistributions in binary form must reproduce the above copyright
--      notice, this list of conditions and the following disclaimer in
--      the documentation and/or other materials provided with the
--      distribution.
--
--    - Neither the name of The Numerical ALgorithms Group Ltd. nor the
--      names of its contributors may be used to endorse or promote products
--      derived from this software without specific prior written permission.
--
--THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
--IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
--TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
--PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER
--OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
--EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
--PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
--PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
--LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
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--SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
)abbrev domain VOID Void
++ Author: Stephen M. Watt, Gabriel Dos Reis
++ Date Created: 1986
++ Date Last Updated: October 5, 2009
++ Basic Operations: 
++ Related Domains: ErrorFunctions, ResolveLatticeCompletion, Exit
++ Also See:
++ AMS Classifications:
++ Keywords: type, mode, coerce, no value
++ Examples:
++ References:
++ Description:
++   This type is used when no value is needed, e.g., in the \spad{then}
++   part of a one armed \spad{if}.
++   All values can be coerced to type Void.  Once a value has been coerced
++   to Void, it cannot be recovered.

Void: CoercibleTo OutputForm with
        void: () -> %            ++ void() produces a void object.
    == add
        void()      == voidValue()$Foreign(Builtin)
        coerce(v:%) == empty()$OutputForm

import SetCategory
)abbrev domain EXIT Exit
++ Author: Stephen M. Watt
++ Date Created: 1986
++ Date Last Updated: May 30, 1991
++ Basic Operations: 
++ Related Domains: ErrorFunctions, ResolveLatticeCompletion, Void
++ Also See:
++ AMS Classifications:
++ Keywords: exit, throw, error, non-local return
++ Examples:
++ References:
++ Description:
++   A function which does not return directly to its caller should
++   have Exit as its return type.
++
++   Note: It is convenient to have a formal \spad{coerce} into each type from
++   type Exit. This allows, for example, errors to be raised in 
++   one half of a type-balanced \spad{if}.
Exit: SetCategory == add
        coerce(n:%) == error "Cannot use an Exit value."
        n1 = n2     == error "Cannot use an Exit value."

import Void
import Exit
)abbrev package RESLATC ResolveLatticeCompletion
++ Author: Stephen M. Watt
++ Date Created: 1986
++ Date Last Updated: May 30, 1991
++ Basic Operations: 
++ Related Domains: ErrorFunctions, Exit, Void
++ Also See:
++ AMS Classifications:
++ Keywords: mode, resolve, type lattice
++ Examples:
++ References:
++ Description:
++   This package provides coercions for the special types \spadtype{Exit}
++   and \spadtype{Void}.
ResolveLatticeCompletion(S: Type): with
        coerce: S -> Void 
             ++ coerce(s) throws all information about s away.
             ++ This coercion allows values of any type to appear
             ++ in contexts where they will not be used.
             ++ For example, it allows the resolution of different types in
             ++ the \spad{then} and \spad{else} branches when an \spad{if}
             ++ is in a context where the resulting value is not used.
        coerce: Exit -> S
             ++ coerce(e) is never really evaluated.  This coercion is 
             ++ used for formal type correctness when a function will not
             ++ return directly to its caller.
    == add
        coerce(s: S): Void == void()
        coerce(e: Exit): S ==
            error "Bug: Should not be able to obtain value of type Exit"

