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- [section The YAP Way]
- There are certain idioms that _yap_ is written to support. Before getting
- into the nuts and bolts of how _yap_ operates, let's define these idioms.
- [heading _eval_xform_]
- This is the main idiom you'll see reinforced in the examples. The idea is
- that you capture an expression:
- auto expr_0 = /* ... */ ;
- then transform it one or more times:
- auto expr_1 = boost::yap::transform(expr_0, my_transform_1);
- auto expr_2 = boost::yap::transform(expr_1, my_transform_2);
- // ...
- auto expr_n = boost::yap::transform(expr_n_minus_1, my_transform_n);
- and then finally you evaluate it:
- auto const result = boost::yap::evaluate(expr_n);
- Each call to _xform_ here produces a new _Expr_ that can subsequently be
- transformed. This is conceptually similar to what happens inside many
- compilers. Capturing the expression is analogous to the compiler's parse; the
- transformations are analogous to optimization passes; and the evaluation is
- analogous to code generation.
- This keeps the meaning of your code quite clear and easy to follow. For this
- reason, I think you should try to use _yap_ in this way when you can.
- [heading _xform_as_eval_]
- This is a variant of _eval_xform_, where the _eval_ call at the end is
- unnecessary, because the final (or perhaps only) transform does all the
- evaluation we need.
- For instance, here is the `get_arity` transform object used in the _calc3_
- example (don't worry too much about the implementation _emdash_ we'll return
- to this later in the docs in much greater detail):
- [calc3_get_arity_xform]
- Here is how this might be used:
- auto expr = 1_p * 2_p;
- auto const arity = boost::yap::transform(expr, get_arity{});
- static_assert(arity.value == 2, "Called with wrong number of args.");
- In this case, _xform_ produces a non-_Expr_ value, all by itself. We got our
- result without ever needing to call _eval_.
- [note Whether _xform_ returns an _Expr_ or non-_Expr_ is entirely up to the
- caller. The transform object passed as the second argument to _xform_ defines
- what _xform_'s return type will be.]
- [endsect]
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