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66 lines
2.2 KiB
Mathematica
66 lines
2.2 KiB
Mathematica
%---------------------------------------------------------------------------%
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% vim: ft=mercury ts=4 sw=4 et
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%---------------------------------------------------------------------------%
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% Copyright (C) 2000, 2005-2006 The University of Melbourne.
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% Copyright (C) 2014-2015, 2017-2018 The Mercury team.
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% This file is distributed under the terms specified in COPYING.LIB.
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%---------------------------------------------------------------------------%
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%
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% File: enum.m.
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% Author: stayl.
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% Stability: medium
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%
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% This module provides the typeclass `enum', which describes types
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% which can be converted to and from integers without loss of information.
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%
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%---------------------------------------------------------------------------%
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%---------------------------------------------------------------------------%
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:- module enum.
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:- interface.
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% A type T can be declared to be a member of the enum typeclass if
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%
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% - all values X of type T can be converted to an int N using
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% the instance's to_int member function, with each distinct X
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% being translated by to_int to a distinct N; and
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%
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% - for all values N of type int that are equal to to_int(X) for some X,
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% from_int(N) = X.
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%
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% - for all values N of type int that are not equal to to_int(X) for any X,
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% from_int(N) should fail.
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%
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% In mathematical notation, the following must hold:
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%
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% all [X] (X = from_int(to_int(X)))
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% all [X, Y] (to_int(X) = to_int(Y)) => X = Y)
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% all [N] (some [X] N = to_int(X) => from_int(N) = X)
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% all [N] (not (some [X] N = to_int(X))) => from_int(N) fails
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%
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:- typeclass enum(T) where [
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func to_int(T) = int,
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func from_int(int) = T is semidet
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].
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% `det_from_int(I)' returns the result of `from_int(I)', but throws an
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% exception if `from_int' fails.
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%
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:- func det_from_int(int) = T <= enum(T).
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%---------------------------------------------------------------------------%
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%---------------------------------------------------------------------------%
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:- implementation.
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:- import_module require.
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det_from_int(I) = X :-
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( if X0 = from_int(I) then
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X = X0
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else
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unexpected($pred, "from_int failed")
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).
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%---------------------------------------------------------------------------%
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