125 lines
3.7 KiB
Plaintext
125 lines
3.7 KiB
Plaintext
[/==============================================================================
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Copyright (C) 2001-2010 Joel de Guzman
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Copyright (C) 2001-2005 Dan Marsden
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Copyright (C) 2001-2010 Thomas Heller
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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===============================================================================/]
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[section:actions More on Actions]
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As you know from the [link phoenix.inside.actor Actors in Detail] section,
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Actions are what brings life to a Phoenix expression tree.
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When dealing with a Phoenix expression tree, it gets evaluated top-down.
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Example:
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_1 + 3 * _2
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Can be visualized as an AST in the following way:
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[$images/simple_ast.png]
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In terms of actions this means:
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* `rule::plus` is matched
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* evaluate left:
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* `rule::placeholder` is matched
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* evaluate right:
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* `rule::multiplies` is matched
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* evaluate left:
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* `rule::value` is matched
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* evaluate right:
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* `rule::placeholder` is matched
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Every time a rule is matched, an action will be called. The action determines
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how the Phoenix AST will be traversed.
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[heading Writing an Action]
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As mentioned in [link phoenix.inside.actor Actors in Detail] actions are
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__proto_primitive_transforms__ for convenience Phoenix provides an abstraction
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to this:
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template <typename Fun>
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struct call;
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This is similar to __proto_call__ but does more. It calls the `Fun` function
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object passed as template parameter with the `Context` and the children of the
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expression associated with the rule.
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Lets have an (simplified) example on how to write an evaluation action for
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`rule::plus`:
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struct plus_eval
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{
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typedef int result_type;
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template <typename Lhs, typename Rhs, typename Context>
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result_type operator()(Lhs const& lhs, Rhs const &rhs, Context & ctx)
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{
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return eval(lhs, ctx) + eval(rhs, ctx);
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}
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};
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template <>
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struct default_actions::when<rule::plus>
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: call<plus_eval>
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{};
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That's it. When evaluating a `plus` expression, the `plus_eval` callable gets
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called with the left hand side and right hand side expression and the associated
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Context.
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[*But there is more:] As Actions /can/ be full fletched __proto_transforms__, you can
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in fact use any proto expression you can imagine as the action. Phoenix predifines a
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set of callables and transform to deal with the Context information passed along and
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of course every Phoenix expression can be used as a Phoenix grammar or
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__proto_pass_through_transform__.
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[variablelist
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[
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[`functional::context(Env, Actions)`]
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[A __proto_callable__ that creates a new context out of the `Env` and `Actions` parameter]
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]
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[
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[`functional::env(Context)`]
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[A __proto_callable__ that returns the environment out of the `Context` parameter]
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]
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[
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[`functional::actions(Context)`]
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[A __proto_callable__ that returns the actions out of the `Context` parameter]
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]
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[
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[`_context`]
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[A __proto_primitive_transform__ that returns the current context]
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]
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[
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[`_env`]
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[A __proto_primitive_transform__ that returns the current environment]
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]
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[
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[`_actions`]
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[A __proto_primitive_transform__ that returns the current actions]
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]
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[
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[`context(env, actions)`]
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[A regular function that creates a context]
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]
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[
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[`env(ctx)`]
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[A regular function that returns the environment from the given context]
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]
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[
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[`actions(ctx)`]
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[A regular function that returns the actions from the given context]
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]
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]
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Phoenix is equipped with a predefined set of expressions, rules and actions to
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make all the stuff work you learned in the __phoenix_starter_kit__ and __phoenix_modules__
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sections. See the [link phoenix.inside.rules next section] for more details!
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[endsect]
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