126 lines
4 KiB
C++
126 lines
4 KiB
C++
/*
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VERB-TOKEN.hpp - double dispatch based on DSL tokens
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Copyright (C)
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2014, Hermann Vosseler <Ichthyostega@web.de>
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**Lumiera** is free software; you can redistribute it and/or modify it
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under the terms of the GNU General Public License as published by the
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Free Software Foundation; either version 2 of the License, or (at your
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option) any later version. See the file COPYING for further details.
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*/
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/** @file verb-token.hpp
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** Building blocks for a simple DSL using double dispatch to a handler function.
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** Actually this is a specialised variation of the visitor pattern, where the
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** "verb tokens" of the language are the domain objects accepting a "receiver"
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** (visitor) to provide the concrete implementation function for each "verb".
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**
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** The intended usage is to set up a language comprised of several abstract actions ("verbs"),
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** but to defer the concrete implementation to a specific set of handler functions, which
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** is provided late, at application time. This way, we can send a sequence of verbs towards
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** an unknown receiver, which supplies the actual meaning within the target context. In
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** the end, there is a double-dispatch based both on the individual verb given and the
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** concrete receiver, which needs to implement the interface used in the definition
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** of the verb tokens. The handler functions defined within this interface may
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** take additional arguments, which are passed through on application to
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** the concrete receiver, e.g. `VERB_doit (receiver, arg1, arg2)`
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** results in the invocation of \c receiver.doit(arg1,arg2)
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**
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** @see [prominent usage: the Diff system](\ref lib::diff::DiffLanguage)
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** @see VerbFunctionDispatch_test
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**
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*/
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#ifndef LIB_VERB_TOKEN_H
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#define LIB_VERB_TOKEN_H
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#include "lib/symbol.hpp"
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#include "lib/util.hpp"
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#include <utility>
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#include <string>
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namespace lib {
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using std::string;
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/**
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* Action token implemented by double dispatch to a handler function,
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* as defined in the "receiver" interface (parameter `REC`).
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* The token is typically part of a DSL and can be applied
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* to a concrete receiver subclass.
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* @tparam REC the type receiving the verb invocations
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* @tparam SIG signature of the actual verb function, expected
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* to exist on the receiver (REC) interface
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* @remarks while the included ID Literal is mostly for diagnostics,
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* it also serves as identity for comparisons. Conceptually what
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* we want is to compare the function "offset", but this leads
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* into relying on implementation defined behaviour.
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* @note the #VERB macro simplifies definition of actual tokens
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*/
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template<class REC, class SIG>
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class VerbToken;
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template<class REC, class RET, typename... ARGS>
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class VerbToken<REC, RET(ARGS...)>
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{
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public:
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typedef RET (REC::*Handler) (ARGS...);
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protected:
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Handler handler_;
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Literal token_;
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public:
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RET
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applyTo (REC& receiver, ARGS&& ...args)
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{
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REQUIRE ("NIL" != token_);
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return (receiver.*handler_)(std::forward<ARGS>(args)...);
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}
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VerbToken(Handler handlerFunction, Literal token)
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: handler_(handlerFunction)
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, token_(token)
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{ }
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VerbToken()
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: handler_{}
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, token_("NIL")
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{ }
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/* default copyable */
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operator string() const
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{
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return string(token_);
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}
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Literal const&
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getID() const
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{
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return token_;
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}
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/** equality of VerbToken, based on equality of the #token_ Literal
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* @remarks member pointers to virtual functions aren't comparable, for good reason
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*/
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bool operator== (VerbToken const& o) const { return token_ == o.token_; }
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bool operator!= (VerbToken const& o) const { return token_ != o.token_; }
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};
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#define VERB(RECEIVER, FUN) VERB_##FUN (&RECEIVER::FUN, STRINGIFY(FUN))
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} // namespace lib
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#endif /*LIB_VERB_TOKEN_H*/
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