Ichthyostega
89a9202c6c
we don't need them and they even tend to increase build times due to unnecessary compound-includes at some core headers
121 lines
3.2 KiB
C++
121 lines
3.2 KiB
C++
/*
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MAYBE.hpp - dealing with optional values
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Copyright (C) Lumiera.org
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2011, Hermann Vosseler <Ichthyostega@web.de>
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This program is free software; you can redistribute it and/or
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modify it under the terms of the GNU General Public License as
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published by the Free Software Foundation; either version 2 of
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the License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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/** @file maybe.hpp
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** Support for representation of optional values.
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** This implements a concept ("option monad") known from functional programming,
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** allowing to express the fact of some value possibly be unavailable. Using this
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** approach allows to avoid the dangerous technique of (ab)using NULL pointers to
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** represent missing values.
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**
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** While a NULL pointer carries this special meaning just by convention, marking a
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** parameter or return value as optional states this fact first class, and enforces
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** the necessary "is available" check through the type system. Surprisingly, this
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** leads not only to more secure, but also much more compact code, as we're now
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** able to substitute a fallback just by a "or else use this" clause.
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** Basically, there are different ways to access the actual value
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** - access through implicit conversion raises an exception for missing values
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** - evaluation as boolean allows to check, if the value is available
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** - an alternative or fallback value may be attached.
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**
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** @todo WIP and rather brainstorming as of 2/10
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**
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** @see backend::ThreadJob usage example
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*/
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#ifndef LIB_MAYBE_H
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#define LIB_MAYBE_H
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#include "lib/error.hpp"
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//#include "lib/wrapper.hpp"
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#include "lib/util.hpp"
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#include <string>
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namespace lib {
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using util::isnil;
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using std::string;
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namespace error = lumiera::error;
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namespace maybe {
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}
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/**
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* A value, which might be unavailable
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* @throw error::State on any attempt to access a missing value
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* without prior checking the availability
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*/
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template<typename VAL>
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class Maybe
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{
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VAL value_;
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public:
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/** mark an invalid/failed result */
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Maybe ()
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{ }
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/** standard case: valid result */
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Maybe (VAL const& value)
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: value_(value)
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{ }
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bool
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isValid() const
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{
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UNIMPLEMENTED ("check if optional value is available");
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}
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void
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maybeThrow(Literal explanation =0) const
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{
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if (!isValid())
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throw error::State (explanation.empty()? "optional value not available" : string(explanation),
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error::LUMIERA_ERROR_BOTTOM_VALUE);
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}
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VAL
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get() const
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{
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maybeThrow();
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return value_;
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}
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};
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} // namespace lib
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#endif
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