Sorry for the long post, but I want to avoid any misunderstanding about what I'm looking for :)
I am currently discovering graph databases, and experimenting a bit with bulbflow/neo4j.
Thus, I am trying to use gremlin for most of my requests, but I do not know if the request I want is even feasible or not. I may even be wrong about trying to use a graph db for such a use case, so don't mind telling me whether you think I'm on the right path or not.
First, let me provide a bit of context:
I work on an early-stage open-source project, which is a compiler for a DSL language generating C code. We are currently planning to re-write the whole thing in python for many many reasons (the language, re-designing, opening to a community and such...). The compiler includes what I'll call a cache of the compiled interfaces and templates. The interfaces describe the templates, and each template is associated to a configuration (a list of typed values associated to variables described by the interfaces).
The aim of the request I'm wishing to build is to select a single template implementation depending on an input configuration (actually used in the generation mechanism of the compiler). In the end, I want to be able to request directly through gremlin (if possible at all) a single element I'm looking for in order to provide unicity for the elements that can be found within this "cache". Currently, I manually match this configuration in the python code, but I want to know if it is feasible to do it directly within gremlin.
-
So let's define a sample graph for my use-case:
We have three types of vertices:
Def (Definition), contains a String property called "signature", which is actually the signature of the template defined by this node.
Impl (Implementation), containing two properties which are pathes to the original source and pre-compiled files.
Var (variable), containing a String property which is the signature of the variable.
Then, a few kind of edges:
Def -> impl_by -> Impl (multiple implementations can exist for a definition, does not contain any property)
Impl -> select_by -> Var (Implementations may be selected through a constraint over a configuration variable's value, each edge of this type contains actually three properties: type, value, and constraint - a comparison operator -)
The selected_by edge (or relationship, following bulflow's vocabulary) describes a selection constraint, and thus has the following properties:
val (value associated to the variable for the origin implementation)
op (comparison operator telling which kind of comparison to make for the constraint to be valid or not)
This translates as a graph such as (I'll omit the types from the selected_by edges in this graph):
-- select_by { value="John", op="="} ---------
| \
(1)--Impl--- select_by { value=12, op=">"} ------ \
| \ \
| \ |- Var("name")
| |- select_by { value="Peter", op="="} -----------/
Def (2)--Impl-- \/
| |- select_by { value=15, op="<"} ---- /\
| \ / \
| |-/----|--- Var("ver")
(3)--Impl--- select_by { value="Kat", op="!="} ------/ /
| /
|--- select_by { value=9, op=">"} ---------/
What I want to do is to select one (or more) Impl depending on their relationship with the Vars. Let's say I have a configuration as follows:
Config 1:
variable="name", value="Peter"
variable="ver", value=16
This would select Impl(3) Since Peter != Kat AND 16 > 9, but not Impl(1) since Peter != John nor Impl(2) since 16 !< 15.
I was blocked on multiple levels, so I was starting to wonder if this was even feasible:
I could not find how to give such arguments (the configuration) to a gremlin script
I could not find how to select the Impl based on conditions over the outgoing edges.
I hope this wasn't too confusing.
Cheers, and thanks !
EDIT:
I managed to make part of my request work, by using repeatedly backtracking and filters. The request (X being the starting vertex, VALUE the value I want to match, and NAME the name of the variable to be matched) looks like this:
Basis of the request:
g.v(X).out('impl').as('implem')
Repeat this part for each couple VALUE/NAME:
.out('select_by').filter{it.value=='VALUE'}
.inV('select_by').filter{it.name=='NAME'}
.back('implem')
The only thing currently missing is that I do not know how to use the select_by edge's 'op' property to determine how to build the filter to use. For instance, thre are cases where I want to match exactly the configuration (and thus, as in this request, I ignore the 'op' property), but there are cases where I want to take the 'op' property into account, and use the associated comparator in the filters.
Is there any way to do that ? (Or should I post another question?)
Related
I have a function that uses diesel to get an object from a DB based off the given ID:
fn get_db_site(pool: web::Data<Pool>, site_id: u32) -> Result<Site, diesel::result::Error> {
let conn = pool.get().unwrap();
dsl::site.find(site_id).get_result::<Site>(&conn)
}
This function is going to be exactly the same for every table I want to run it on so I'm hoping to put it in it's own utils file so I don't have to type the same thing in every time. The only problem is in order to call that find I need to do
crate::schema::site::dsl::site.find and I'm not sure how I can make that call generic to take any type. I know there are type arguments but I don't think that would work here
I normally advise against making diesel things more generic as this leads to really complex trait bounds quite fast. You normally never want to do this in application code. (It's a different thing for libraries that need to be generic). I normally compare the situation with plain SQL. For example if someone complains that users::table.find(pk) feels like duplication, ask yourself the following question: Would you feel that SELECT … FROM users is duplicated in the corresponding query SELECT … FROM users WHERE id = $. (The diesel dsl statement is basically the same).
So to answer your actual question, the generic functions needs to look something like this:
(Not sure if I got all bounds right without testing)
fn get_db_thing<T, U, PK>(pool: web::Data<Pool>, primary_key: PK) -> Result<U, diesel::result::Error>
where T: Table + HasTable<Table = T>,
T: FindDsl<PK>,
U: Queryable<SqlTypeOf<Find<T, PK>>, Pg>
{
let conn = pool.get().unwrap();
T::table().find(primary_key).get_result::<U>(&conn)
}
As you can see the list of trait bounds is already much longer than just having the load inline in the corresponding functions. In addition all the details added while constructing the query would now be required as generic function argument. At least the type for T cannot be inferred by the compiler, so from a code size point of view this solution is not "simpler" than just not making it generic.
Is this possible to get the name of an object as a variable? I’m trying to make a database where each object represents each person. I’ve got objects with [name/1, surname/1], but when I ask e.g.
X::name(john).
it gives me an error. Ofc there is no problem to get the atom by using this method:
object_id::name(X).
The ::/2 message sending control construct indeed requires a bound first argument at call time. But you can enumerate existing objects using the current_object/1 built-in predicate:
| ?- current_object(Person), Person::name(john).
...
However, this solution may also result in errors as we will be enumerating all objects by backtracking and not all of them will understand a name/1 message. A better solution is thus to only enumerate objects that understand the name/1 message. Assuming that all objects representing a person implement (directly or through inheritance) a person_protocol, we can use the conforms_to_protocol/2 built-in predicate:
| ?- conforms_to_protocol(Person, person_protocol),
Person::name(john).
...
See https://logtalk.org/manuals/refman/predicates/conforms_to_protocol_2_3.html for details.
I am struggling trying to traverse an MDLAsset instance created by loading an SCNScene file (.scn).
I want to identify and extract the MDLMeshs as well as camera(s) and lights. I see no direct way to do that.
For example I see this instance method on MDLAsset:
func childObjects(of objectClass: Swift.AnyClass) -> [MDLObject]
Is this what I use?
I have carefully labeled things in the SceneKit modeler. Can I not refer to those which would be ideal. Surely, there is a dictionary of ids/labels that I can get access to. What am I missing here?
UPDATE 0
I had to resort to pouring over the scene graph in the Xcode debugger due to the complete lack of Apple documentation. Sigh ...
A few things. I see the MDLMesh and MDLSubmesh that is what I am after. What is the traversal approach to get it? Similarly for lights, and camera.
I also need to know the layout of the vertex descriptors so I can sync with my shaders. Can I force a specifc vertex layout on the parsed SCNScene?
MDLObject has a name (because of its conformance to the MDLNamed protocol), and also a path, which is the slash-separated concatenation of the names of its ancestors, but unfortunately, these don't contain the names of their SceneKit counterparts.
If you know you need to iterate through the entire hierarchy of an asset, you may be better off explicitly recursing through it yourself (by first iterating over the top-level objects of the asset, then recursively enumerating their children), since using childObjects(of:) repeatedly will wind up internally iterating over the entire hierarchy to collect all the objects of the specified type.
Beware that even though MDLAsset and MDLObjectContainerComponent conform to NSFastEnumeration, enumerating over them in Swift can be a little painful, and you might want to manually extend them to conform to Sequence to make your work a little easier.
To get all cameras,
[asset childObjectsOfClass:[MDLCamera class]]
Similarly, to get all MDLObjects,
[asset childObjectsOfClass:[MDLObjects class]]
Etc.
MDLSubmeshes aren't MDLObjects, so you traverse those on the MDLMesh.
There presently isn't a way to impose a vertex descriptor on MDL objects created from SCN objects, but that would be useful.
One thing you can do is to impose a new vertex descriptor on an existing MDL object by setting a mesh's vertexDescriptor property. See the MDLMesh.h header for some discussion.
I typically code most of my C projects in Vim. I am comfortable with navigation, search and replace, and indexing via Ctags/Cscope.
One feature I would like to have, if possible, is a keymapping that will show the datatype for a variable under the cursor on screen.
For example, if my cursor is on a variable, "test123" (ie: int test123 = 0) is there a way to have the type (int) and some other details about the variable shown in another tab within Vim?
Also, is there something similar that would do the same for a struct variable, and show a list of all its members in the descriptive tab as well as the type (ie: struct)?
I've also noticed that sometimes while coding, I have a tab titled "[Scratch][Preview]" at the top of Vim that appears to fulfill this requirement, but I have no idea what triggers it (searches and Ctag searches don't seem to trigger it). It looks like so:
name: myStruct::instanceOfStrct| 2 cmd: /^ int instanceOfStrct;$/
.. (up a dir) | 3 kind: m
</code/test/test.c | 4 struct: myStruct
|+config/ | 5 access: public
|+lib/ | 6 filename: /code/test/test.c
I think this is something that already exists in Vim to some extent, but I have not idea how to work with it.
Thank you.
I do not know of any plugin that does what you want, however it should be quite possible using libclang. There is a fork of clang_complete that adds 'go to definition' functionality which is close to what you want. However development on that plugin seems to have stagnated.
Also the scratch buffer appears when doing autocompletion to give more information about a specific completion. It can be enabled and disabled using the completeopt setting.
So, I know I can do something like this:
sitelist: &sites
- www.foo.com
- www.bar.com
anotherlist: *sites
And have sitelist and anotherlist both contain www.foo.com and www.bar.com. However, what I really want is for anotherlist to also contain www.baz.com, without having to repeat www.foo.com and www.baz.com.
Doing this gives me a syntax error in the YAML parser:
sitelist: &sites
- www.foo.com
- www.bar.com
anotherlist: *sites
- www.baz.com
Just using anchors and aliases it doesn't seem possible to do what I want without adding another level of substructure, such as:
sitelist: &sites
- www.foo.com
- www.bar.com
anotherlist:
- *sites
- www.baz.com
Which means the consumer of this YAML file has to be aware of it.
Is there a pure YAML way of doing something like this? Or will I have to use some post-YAML processing, such as implementing variable substitution or auto-lifting of certain kinds of substructure? I'm already doing that kind of post-processing to handle a couple of other use-cases, so I'm not totally averse to it. But my YAML files are going to be written by humans, not machine generated, so I would like to minimise the number of rules that need to be memorised by my users on top of standard YAML syntax.
I'd also like to be able to do the analogous thing with maps:
namedsites: &sites
Foo: www.foo.com
Bar: www.bar.com
moresites: *sites
Baz: www.baz.com
I've had a search through the YAML spec, and couldn't find anything, so I suspect the answer is just "no you can't do this". But if anyone has any ideas that would be great.
EDIT: Since there have been no answers, I'm presuming that no one has spotted anything I haven't in the YAML spec and that this can't be done at the YAML layer. So I'm opening up the question to idea for post-processing the YAML to help with this, in case anyone finds this question in future.
The merge key type is probably what you want. It uses a special << mapping key to indicate merges, allowing an alias to a mapping (or a sequence of such aliases) to be used as an initializer to merge into a single mapping. Additionally, you can still explicitly override values, or add more that weren't present in the merge list.
It's important to note that it works with mappings, not sequences as your first example. This makes sense when you think about it, and your example looks like it probably doesn't need to be sequential anyway. Simply changing your sequence values to mapping keys should do the trick, as in the following (untested) example:
sitelist: &sites
? www.foo.com # "www.foo.com" is the key, the value is null
? www.bar.com
anotherlist:
<< : *sites # merge *sites into this mapping
? www.baz.com # add extra stuff
Some things to notice. Firstly, since << is a key, it can only be specified once per node. Secondly, when using a sequence as the value, the order is significant. This doesn't matter in the example here, since there aren't associated values, but it's worth being aware.
As the previous answers have pointed out, there is no built-in support for extending lists in YAML. I am offering yet another way to implement it yourself. Consider this:
defaults: &defaults
sites:
- www.foo.com
- www.bar.com
setup1:
<<: *defaults
sites+:
- www.baz.com
This will be processed into:
defaults:
sites:
- www.foo.com
- www.bar.com
setup1:
sites:
- www.foo.com
- www.bar.com
- www.baz.com
The idea is to merge the contents of a key ending with a '+' to the corresponding key without a '+'. I implemented this in Python and published here.
(Answering my own question in case the solution I'm using is useful for anyone who searches for this in future)
With no pure-YAML way to do this, I'm going to implement this as a "syntax transformation" sitting between the YAML parser and the code that actually uses the configuration file. So my core application doesn't have to worry at all about any human-friendly redundancy-avoidance measures, and can just act directly on the resulting structures.
The structure I'm going to use looks like this:
foo:
MERGE:
- - a
- b
- c
- - 1
- 2
- 3
Which would be transformed to the equivalent of:
foo:
- a
- b
- c
- 1
- 2
- 3
Or, with maps:
foo:
MERGE:
- fork: a
spoon: b
knife: c
- cup: 1
mug: 2
glass: 3
Would be transformed to:
foo:
fork: a
spoon: b
knife: c
cup: 1
mug: 2
glass: 3
More formally, after calling the YAML parser to get native objects from a config file, but before passing the objects to the rest of the application, my application will walk the object graph looking for mappings containing the single key MERGE. The value associated with MERGE must be either a list of lists, or a list of maps; any other substructure is an error.
In the list-of-lists case, the entire map containing MERGE will be replaced by the child lists concatenated together in the order they appeared.
In the list-of-maps case, the entire map containing MERGE will be replaced by a single map containing all of the key/value pairs in the child maps. Where there is overlap in the keys, the value from the child map occurring last in the MERGE list will be used.
The examples given above are not that useful, since you could have just written the structure you wanted directly. It's more likely to appear as:
foo:
MERGE:
- *salt
- *pepper
Allowing you to create a list or map containing everything in nodes salt and pepper being used elsewhere.
(I keep giving that foo: outer map to show that MERGE must be the only key in its mapping, which means that MERGE cannot appear as a top-level name unless there are no other top level names)
To clarify something from the two answers here, this is not supported directly in YAML for lists (but it is supported for dictionaries, see kittemon's answer).
To piggyback off of Kittemon's answer, note that you can create mappings with null values using the alternative syntax
foo:
<< : myanchor
bar:
baz:
instead of the suggested syntax
foo:
<< : myanchor
? bar
? baz
Like Kittemon's suggestion, this will allow you to use references to anchors within the mapping and avoid the sequence issue. I found myself needing to do this after discovering that the Symfony Yaml component v2.4.4 doesn't recorgnize the ? bar syntax.