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https://github.com/triqs/dft_tools
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61 lines
1.8 KiB
ReStructuredText
61 lines
1.8 KiB
ReStructuredText
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.. highlight:: c
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Evaluating clef expressions
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===============================
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Forming expressions is nice, but completely useless unless one can *evaluate* them
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by affecting some values to the placeholder and actually *do* the computation.
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Expressions are evaluated with the eval function ::
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eval ( expression , placeholder_1 = value_1, placeholder_2 = value_2, ...)
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The evaluation can be :
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* *complete* : when a value has been affected to every placeholder, so the result is not
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an expression any more, but the result of the computation....
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* *partial* : when some placeholder are still free (no value has been affected to them).
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The result is another expression where the parts that could be computed have been computed.
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Complete evaluation
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--------------------
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Example ::
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eval (x_ + 2*y_ , x_=1, y_ = 2)
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auto e = x_ + 2*y_;
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auto r = eval( e , x_=1, y_ = 2);
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Note that :
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* The order of placeholder does not matter in calling eval.
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* It is an error to put the same placeholder twice.
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* The correct version of eval is found by ADL (Argument Dependent Lookup).
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(It if the clef::eval since arguments are defined in the clef namespace)
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Partial evaluation
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--------------------
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The evaluation can also be partial, in which case the compiler replaces the placeholder whose value is provided
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and rebuild a new expression tree.
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Example ::
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auto e1 = x_ + 2*y_;
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// return type ---> value
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auto r = eval( e1 , x_ =1); // expression ---> 1 + (2 * y_)
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auto e2 = eval( e1 , x_ =x_ + y_); // expression ---> ((x_ + y_) + (2 * y_))
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auto r = eval( e2, x_ = 1, y_ = 2); // int ---> 7
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During the partial evaluation, all subtrees that can evaluated are evaluated.
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For example ::
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eval(x_ + f(y_) , y_=1); // --> x + f(1)
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In this case, the function f is called by eval.
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