Flix – Safe, reliable, concise, and functional-first programming language

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.)

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 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 (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) or [Calcite in Java](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-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) and [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) 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.]

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)

Just look at any template heavy c++ library.

As someone who loves Haskell a lot and was looking for a functional language on the JVM, the language looks absolutely awesome. If the richer effect system gets implemented this has a shot to becoming a staple of mine! I'm reading the documentation right now, and I had a few questions:

- is there any plans to add restricted forms of instance overlapping, in the spirit of purescript's instance chains (or Haskell's overlappable/overlapping)?

- I couldn't find resources on error handling, does flix use some sort of Either/Result type? and if so, how ergonomic is it? I'm assuming Flix doesn't have special syntax like Scala/Haskell for monadic for(/do) comprehensions, so I fear that Either based error handling would quickly become gnarly with just monadic operators. Is there any example of Result-dense code out there to check out? I couldn't find it in the example (admittedly, I have checked quite thoroughly yet)

- I read somewhere on the docs (I think on the design flaws blog?) that Flix at one point had UFCS, is that still supported? I can't seem to make it work on the playground

- Is there anywhere I can read up on the way instance resolution works? More specifically, I'm interested in whether Flix takes only instance heads or it also takes the constraints into account when choosing an instance (if it does take constraints into account, that makes overlapping instances a lot less useful in a good way, I think?)

In any case, great work on the language! I will for sure try this out the first time I get the chance, it looks great and has basically 99% of the features that I would ever want in a programming language. Keep up the good work!

The color of your function can be "effect polymorphic". In other words, e.g. List.map works with both pure and impure functions. You don't have to write two versions of List.map.

No, effect systems aren't a different function type, in fact you could mitigate the whole function color mess in $lang with an effect system.

I get that I just don’t understand why they chose to make them all so different.