AMDG On 02/19/2018 08:38 PM, Zach Laine via Boost wrote:
On Mon, Feb 19, 2018 at 11:13 AM, Steven Watanabe via Boost < boost@lists.boost.org> wrote:
<snip> General Notes:
- There are three primary modes for processing an expression tree, which I will describe as: 1. transform: Takes a tree and produces another tree. 2. evaluate: Takes a tree and evaluates it in some way. 3. for_each: Processes each node and returns nothing. yap::transform can handle all three as long as you explicitly handle all nodes. The default behavior is (1), which makes (2) and (3) somewhat inconvenient. Your solution in the examples seems to be transform/evaluate for (2) and returning a dummy result for (3). Unfortunately, transform/evaluate doesn't work well if evaluation involves any special constructs that affect control-flow. Just try to handle something like this: let(_a=2) [ _a + 1 ] // evaluates to 3
That does not look problematic to me, though I don't know what the intended semantics are.
http://www.boost.org/libs/phoenix/doc/html/phoenix/modules/scope/let.html
If _a is a terminal and refers to something for which "= 2" and "+1" are well-formed, I would expect even evaluate() to do the right thing.
_a is a placeholder. _a = 2 is not an assignment to some external object. It is a variable definition that is scoped within the let expression. It's not really possible to transform this into something that can be evaluated in a single pass. I claim that it is impossible to implement `let` using Yap without duplicating all the work of evaluate. (Keep in mind that `let` can be nested and can be mixed with if_else.)
Returning a dummy result is a bit annoying, but shouldn't cause any real problems, as long as terminals are captured by reference in the result. All in all, I'd like to have some way to change the default behavior of transform, or perhaps have separate functions with different default behavior. Just an idea: switch the default behavior based on the result of transforming the subexpressions: Expression/non-Expression/void.
I don't yet get what you're suggesting here. Right now, transform() returns whatever you tell it to,
The issue is not what I'm telling it to do, but rather what it does on its own when I don't tell it anything.
except that it doesn't special-case void return. You can (I have have extensively in the examples) return an Expression or non-Expression from a transform, or from any subset of overloads your transform object provides. What is the behavior that you're suggesting?
You've chosen to make it impossible to customize the behavior of evaluate. I believe that Brook brought up evaluate_with_context, which is basically what I want.
This has the side effect that you must explicitly wrap terminals when returning, but I think that's actually a good thing, as a transform that returns unwrapped terminals, expecting them to be wrapped by the caller, may have inconsistent behaviour. In addition, there is another possible mode that has better type safety which I will call: 4. manual: No default behavior. If a node is not handled explicitly, it is a hard compile error.
But this isn't transform() at all, because it doesn't recurse. It only matches the top-level expression, or you get a hard error. Why not just write my_func() that takes a certain expression and returns another?
You're right. The only benefit is that tag transforms are a bit more convenient. Also, it allows for a more consistent interface.
Calling it with the wrong type of expression will result your desired hard error. Wait, is the use case that I think my transform matches all subexpressions within the top-level expression, and I want to verify that this is the case? I don't know how often that will come up. I can't think of a single time I've written a Yap transform expecting it to match all nodes, except to evaluate it. It could be useful in those cases now that I think of it.
If you're building a completely new grammar whose meaning has no relationship to the built in meaning of the operators, (e.g. Spirit), then you basically have to handle everything explicitly.
- Combining transforms isn't exactly easy, because of the way transforms recurse. For example, if I have two transforms that process disjoint sets of nodes, I can't really turn them into a single transform.
I'm not necessarily opposed to the idea, but how would combining transforms work? Would each node be matched against multiple transform objects, using whichever one works, or something else?
Probably the behavior is to choose the first one that matches. That would make it easy to write a transform that overrides some behavior of another transform. (Note that I really have no idea how to make something like this actually work.)
- How useful is it to separate the Expression concept from the ExpressionTemplate concept?
Types and templates are not the same kind of entity. I don't know how to even go about combining these. Moreover, I think they should remain separated, because sometimes a function takes an Expression, sometimes not, sometimes it requires an ExpressionTemplate template parameter, and sometimes not. These are orthogonal requirements for the caller, no?
For example, transform requires an ExpressionTemplate for nodes that are not handled explicitly, but that isn't very clear from the documentation, which just says that it takes an Expression.
I don't understand what you mean. transform() does not require an extrinsic ExpressionTemplate template parameter, and does not use one. It just recycles the ExpressionTemplate that was originally used to instantiate whatever it needs to copy.
Let me rephrase the question: Is it useful to allow Expressions that are not instantiations of an ExpressionTemplate, given that transform can choke on such classes.
- You say that it's fine to mix and match expressions that are instantiated from different templates. Why would I want to do this? The first thing that comes to mind is combining two independent libraries that both use YAP, but I suspect that that won't work out very well. It seems a bit too easy for a transform to inadvertently recurse into an unrelated expression tree in this case.
I have two types, M and S (m and s are two objects of those respective types). M is matrix-like, and has m*m in its set of well-formed expressions; m[x] is ill-formed for any x. S is string-like, and has s[int(2)] in its set of well-formed expressions; s*s is ill-formed. M and S are in the same library, one that I maintain.
If I want to use these in Yap expressions, I probably want to be able to write m*m and s[5], but not m[2] or s*s. So I write two expression templates with the right operations defined:
template <...> struct m_expr { // ... BOOST_YAP_USER_BINARY_OPERATOR_MEMBER(times, ::m_expr) };
template <...> struct s_expr { // ... BOOST_YAP_USER_BINARY_OPERATOR_MEMBER(aubscript, ::s_expr) };
Now I can write a Yap expression like:
lookup_matrix(S("my_matrix")) * some_matrix
and transform() it how ever I like. Requiring the two *_expr templates to be unified would be weird.
It seems a bit odd to have matrices and strings in one library, rather than matrices and vectors, but I get your point. Please consider adding something like this to the documentation.
- The value of a terminal cannot be another expression.
Is this a necessary restriction?
Not really; I probably put that restriction there due to lack of imagination. Terminals' values should be treated as simple types, but I'm pretty sure there's no real reason those types can't happen to model Expression.
(This is related to mixing different libraries using YAP, as the most common form is for library A to treat the expressions from library B as terminals. Since a terminal can't hold an Expression, we would need to somehow un-YAP them first.)
As I outlined above, I don't think that's going to be the most common case of mixing ExpressionTemplates in a single Yap expression.
That's why I said mixing different *libraries*, not just mixing different templates from a single library (which are most likely intended to work together). The basic idea is that library A creates expressions that are models of some concept C, while library B expects a terminal that models the same concept C. library A and library B know nothing about each other, and are only connected because they both know about C.
- Unwrapping terminals redux: Unwrapping terminals is convenient when you have terminals of the form: struct xxx_tag {}; auto xxx = make_terminal(xxx_tag{}); AND you are explicitly matching this terminal with auto operator()(call_tag, xxx_tag, T...) or the like. I expect that matching other terminals like double or int is somewhat more rare in real code. If you are matching any subexpression, then unwrapping terminals is usually wrong.
Why do you say that? The only way it is the way it is now is convenience, as you might have guessed. When I know I want to match a particular terminal of a particular type, I'd rather write:
auto operator(some_unary_tag, some_type && x); auto operator(some_unary_tag, some_type const & x);
versus:
auto operator(some_unary_tag, my_expr<yap::expr_kind::terminal, some_type &&> const & x); auto operator(some_unary_tag, my_expr<yap::expr_kind::terminal, some_type const &> const & x);
This latter form is what I started with, and I very quickly grew tired of writing all that.
You can always use a typedef or alias. i.e. my_term<some_type&&>.
If I match a generic expression that may or may not be a terminal, I do have to use yap::as_expr(), which is definitely an inconvenience:
template <typename Expr> auto operator(some_unary_tag, Expr const & x) { return some_function_of(yap::as_expr(x)); }
My intuition is that this is the most common pattern, and should be what we optimize for. Also, changing terminals to yap::expression may cause random weirdness if the transform returns an Expression. In addition, I feel that needing to call as_expr is more surprising than needing to write out the the terminal as an expression. Actually... I have a much better idea. Why don't you allow transform for a non-expr to match a terminal tag transform? <snip> In Christ, Steven Watanabe