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It's programs all the way down

This is the first of a series of articles on “Monadic EDSLs in Scala.”

Embedded domain specific languages (EDSLs) are a powerful tool for abstracting complexities such as effects and business logic from our programs. Instead of mixing ad-hoc error handling, database access, and web calls through our code, we isolate each domain into a little language. These little languages can then be used to write “mini-programs” describing, for example, how to create a web page for a user.

Our program then becomes a composition of mini-programs, and running our program becomes interpreting these mini-programs into actions. This is analogous to running an interpreter, itself a program, which turns code into actions.

The following illustrates what an EDSL might look like in Scala.

// An embedded program for fetching data for a user
def process(id: UserId): Program[Page] = for {
  bio  <- getBio(id)
  feed <- getFeed(id)
  page <- createPage(bio, feed)
} yield page

def interpretProgram[A](page: Program[A]): IO[A] = page.interpret {
  case GetBio(id)            => ...
  case GetFeed(id)           => ...
  case CreatePage(bio, feed) => ...
}

Here process defines a program in our embedded language. No action has actually been performed yet, that happens when it gets interpreted by interpretProgram and run at runtime.

In many situations a program in one EDSL is translated into another EDSL, much like a compiler (again another program).

// Translate each term of the program into a database call
def compile[A](program: Program[A]): Database[A] = program.interpret {
  case GetBio(id)            => ...
  case GetFeed(id)           => ...
  case CreatePage(bio, feed) => ...
}

def interpretDatabase(db: Database[A]): IO[A] = db.interpret { ... }

Sometimes you can even optimize programs in an EDSL, much like an optimizing compiler. In the above example, interpretDatabase could deduplicate identical requests and batch requests to the same table.

In this series of articles we will explore a couple approaches to embedding such DSLs in Scala. These techniques will be evaluated against the following axes:

In the next post we’ll take a look at the first of these approaches.

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