3 ms·

> Composing datalog natively I'm naive. What does this mean and why is it important?

by all2 4y ago

pharmakom 4y ago
Just look at any template heavy c++ library.
- Blackthorn 4y ago
  That really doesn't answer the question. (I'm not sure what it really means either, it's a good question.)
deleted 4y ago
[deleted]
jorkadeen 4y ago
Datalog programs are values. You can store them in local variables, pass them as arguments, and return them. If you have two Datalog values you can combine them into one (effectively it's just the union of them). This allows you to write small "Datalog fragments" and stitch them together into a larger program. The type system ensures that this stays meaningful. (I am one of the authors of Flix.)
machiaweliczny 4y ago
In short you work with tree (AST) and can compose instead of stiching strings (builder pattern) Good thing of having common AST in language is libs interoperability. It’s like normal lanugages had SQL support built-in (except you can’t do that easily as SQL is order dependent)
jitl 4y ago
Datalog is a logic programming language for relational data. It is much easier to express recursive relationships in Datalog compared to SQL. The most obvious application of Datalog is for working with graphs. The starter example I've usually seen is "graph reachability", which something like "given a list of edges between nodes, tell me if I can reach node X from node Y". // Given an edge x -> y, there is a path x -> y. path(x, y) :- edge(x, y). // Given an edge x -> z and a path z -> y, there is a path x -> y. path(x, y) :- edge(x, y: z), path(x: z, y). Here is the equivalent recursive SQL (SQLite syntax): WITH RECURSIVE path(x, y) AS ( SELECT edge.x AS x, edge.y AS y FROM edge UNION SELECT edge.x AS x, path.y AS y FROM edge, path WHERE edge.y = path.x ) SELECT DISTINCT * FROM path If you want to play with Datalog, https://percival.ink https://percival.ink is an online notebook with a very gentle introduction. I also published a version of percival.ink that converts simple Datalog queries to recursive SQLite queries here: https://percival.jake.tl https://percival.jake.tl (the example above is from percival.jake.tl). As jorkadeen said in their answer (I added emphasis to the last sentence): > Datalog programs are values. You can store them in local variables, pass them as arguments, and return them. If you have two Datalog values you can combine them into one (effectively it's just the union of them). This allows you to write small "Datalog fragments" and stitch them together into a larger program. The type system ensures that this stays meaningful. Why this is important: From my perspective is that a very large percent of industrial code interacts with data (in databases) by building query strings. In most languages, we throw away all the nice features of the language when it comes to data access. To regain some level of composability and safety, developers pour years into libraries like [Arel in Ruby](https://www.rubydoc.info/gems/arel https://www.rubydoc.info/gems/arel) or [Calcite in Java](https://calcite.apache.org/docs/algebra.html https://calcite.apache.org/docs/algebra.html). The most notable example of language native database access is [Language Integrated Query (LINQ)](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/ https://docs.microsoft.com/en-us/dotnet/csharp/programming-g...) in C#. But all of these tools are hampered by the constraints of SQL. SQL is very good for flat, normalized data with one or two joins. Once you begin working with trees or graphs, tools based around SQL become cumbersome. Using datalog instead of SQL means you have a logical langauge that is inherently more accepting of composition in a theoretical way. But, most systems that speak datalog today are extremely specialized. [Souffle](https://souffle-lang.github.io/simple https://souffle-lang.github.io/simple) and [differential-datalog](https://github.com/vmware/differential-datalog https://github.com/vmware/differential-datalog) are useful ahead-of-time compilers for datalog, but you have to design your whole program around using them. The datalog variant [inside Datomic](https://docs.datomic.com/on-prem/query/query.html https://docs.datomic.com/on-prem/query/query.html) is dynamic, but is only usable for accessing data in a data store. Good luck using it for analysis of data in memory. Including datalog into the language as first-class values mean that you can access the power of datalog and actually compose it in practice. [Note: I've never used Flix – just a datalog fan.]
- newbye3 4y ago
  in prolog the definition of path is something like: path(x,y) := edge(x,y). path(x,z) := edge(x,y),path(y,z). So to me it seems a little strange the definition of path in datalog from a prolog user point of view.
- newbye3 4y ago
  At first sight, in the examples: inc1(7) pure, inc2(8) impure, what's the meaning of inc2? (some kind of increment?) Then in the definion of map there is an argument function f, and in the definition there is a new function g that comes from nowhere, so for me at first sight the examples are not insightful.

> Composing datalog natively I'm naive. What does this mean and why is it important?

by all2 4y ago

> Composing datalog natively I'm naive. What does this mean and why is it important?

pharmakom 4y ago

Just look at any template heavy c++ library.

Blackthorn 4y ago
That really doesn't answer the question. (I'm not sure what it really means either, it's a good question.)

deleted 4y ago

[deleted]

jorkadeen 4y ago

Datalog programs are values. You can store them in local variables, pass them as arguments, and return them. If you have two Datalog values you can combine them into one (effectively it's just the union of them). This allows you to write small "Datalog fragments" and stitch them together into a larger program. The type system ensures that this stays meaningful. (I am one of the authors of Flix.)

machiaweliczny 4y ago

In short you work with tree (AST) and can compose instead of stiching strings (builder pattern) Good thing of having common AST in language is libs interoperability. It’s like normal lanugages had SQL support built-in (except you can’t do that easily as SQL is order dependent)

jitl 4y ago

Datalog is a logic programming language for relational data. It is much easier to express recursive relationships in Datalog compared to SQL. The most obvious application of Datalog is for working with graphs. The starter example I've usually seen is "graph reachability", which something like "given a list of edges between nodes, tell me if I can reach node X from node Y". // Given an edge x -> y, there is a path x -> y. path(x, y) :- edge(x, y). // Given an edge x -> z and a path z -> y, there is a path x -> y. path(x, y) :- edge(x, y: z), path(x: z, y). Here is the equivalent recursive SQL (SQLite syntax): WITH RECURSIVE path(x, y) AS ( SELECT edge.x AS x, edge.y AS y FROM edge UNION SELECT edge.x AS x, path.y AS y FROM edge, path WHERE edge.y = path.x ) SELECT DISTINCT * FROM path If you want to play with Datalog, https://percival.ink https://percival.ink is an online notebook with a very gentle introduction. I also published a version of percival.ink that converts simple Datalog queries to recursive SQLite queries here: https://percival.jake.tl https://percival.jake.tl (the example above is from percival.jake.tl). As jorkadeen said in their answer (I added emphasis to the last sentence): > Datalog programs are values. You can store them in local variables, pass them as arguments, and return them. If you have two Datalog values you can combine them into one (effectively it's just the union of them). This allows you to write small "Datalog fragments" and stitch them together into a larger program. The type system ensures that this stays meaningful. Why this is important: From my perspective is that a very large percent of industrial code interacts with data (in databases) by building query strings. In most languages, we throw away all the nice features of the language when it comes to data access. To regain some level of composability and safety, developers pour years into libraries like [Arel in Ruby](https://www.rubydoc.info/gems/arel https://www.rubydoc.info/gems/arel) or [Calcite in Java](https://calcite.apache.org/docs/algebra.html https://calcite.apache.org/docs/algebra.html). The most notable example of language native database access is [Language Integrated Query (LINQ)](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/ https://docs.microsoft.com/en-us/dotnet/csharp/programming-g...) in C#. But all of these tools are hampered by the constraints of SQL. SQL is very good for flat, normalized data with one or two joins. Once you begin working with trees or graphs, tools based around SQL become cumbersome. Using datalog instead of SQL means you have a logical langauge that is inherently more accepting of composition in a theoretical way. But, most systems that speak datalog today are extremely specialized. [Souffle](https://souffle-lang.github.io/simple https://souffle-lang.github.io/simple) and [differential-datalog](https://github.com/vmware/differential-datalog https://github.com/vmware/differential-datalog) are useful ahead-of-time compilers for datalog, but you have to design your whole program around using them. The datalog variant [inside Datomic](https://docs.datomic.com/on-prem/query/query.html https://docs.datomic.com/on-prem/query/query.html) is dynamic, but is only usable for accessing data in a data store. Good luck using it for analysis of data in memory. Including datalog into the language as first-class values mean that you can access the power of datalog and actually compose it in practice. [Note: I've never used Flix – just a datalog fan.]

newbye3 4y ago
in prolog the definition of path is something like: path(x,y) := edge(x,y). path(x,z) := edge(x,y),path(y,z). So to me it seems a little strange the definition of path in datalog from a prolog user point of view.
newbye3 4y ago
At first sight, in the examples: inc1(7) pure, inc2(8) impure, what's the meaning of inc2? (some kind of increment?) Then in the definion of map there is an argument function f, and in the definition there is a new function g that comes from nowhere, so for me at first sight the examples are not insightful.