A framework to create embedded Domain-Specific Languages in Scala
Dsl.scala is a framework to create embedded Domain-Specific Languages in Scala. It can be considered as an alternative syntax to for comprehension, Scala Async and Scala Continuations. It unifies monads, generators, asynchronous functions, coroutines and continuations to a single universal syntax, and can be easily integrate to Scalaz, Cats, Scala Collections, Scala Futures, Akka HTTP, Java NIO, or your custom domains.
A DSL author is able to create language keywords by implementing the Dsl trait, which contains only one abstract method to be implemented. No knowledge about Scala compiler or AST macros is required.
DSLs written in Dsl.scala are collaborative with others DSLs and Scala control flows. A DSL user can create functions that contains interleaved DSLs implemented by different vendors, along with ordinary Scala control flows.
We also provide some built-in keywords, including:
Await
keyword for creating memoized asynchronous values as Scala Futures, similar to the await
/ async
keywords in C#, Python and JavaScript.Shift
keyword for creating asynchronous tasks as delimited continuations, similar to the shift
operator in Scala Continuations.AsynchronousIo.Connect
, AsynchronousIo.Accept
, AsynchronousIo.Read
and AsynchronousIo.Write
keywords for performing I/O on an asynchronous channel.Yield
keyword for generating lazy streams, similar to yield
in C#, Python and JavaScript.Fork
keyword for duplicating current context, similar to the fork
system call in POSIX.Return
keyword for early returning, similar to the native return
keyword in Scala.Using
keyword to automatically close resources when exiting a scope, similar to the native using
keyword in C#.Monadic
keyword for creating Scalaz or Cats monadic control flow, similar to the !-notation in Idris.NullSafe
keyword for the null safe operator, similar to the ?
operator in Kotlin and Groovy.NoneSafe
keyword for the None
safe operator, similar to the Maybe
monad in Haskell.All the above keywords can be used together with each others. For example you can perform list comprehension to manipulate native resources in an asynchronous task by using Each
, Using
and Shift
together.
Suppose you want to create a random number generator. The generated numbers should be stored in a lazily evaluated infinite stream, which can be built with the help of our built-in domain-specific keyword Yield
.
So, you need to add the library that contains the implementation of the keyword Yield
:
// Add the "keywords-yield" library in your build.sbt, to use the `Yield` keyword
libraryDependencies += "com.thoughtworks.dsl" %% "keywords-yield" % "latest.release"
// Add other "keywords-xxx" libraries in your build.sbt, to use other keywords
// libraryDependencies += "com.thoughtworks.dsl" %% "keywords-xxx" % "latest.release"
The random number generator can be implemented as a recursive function that produces the next random number in each iteration.
import com.thoughtworks.dsl.keywords.Yield
// Must not annotated with @tailrec
def xorshiftRandomGenerator(seed: Int): LazyList[Int] = reset {
val tmp1 = seed ^ (seed << 13)
val tmp2 = tmp1 ^ (tmp1 >>> 17)
val tmp3 = tmp2 ^ (tmp2 << 5)
!Yield(tmp3)
xorshiftRandomGenerator(tmp3)
}
Note that a keyword is a plain case class. You need a !
prefix to the keyword to activate the DSL.
It's done. We can test it in ScalaTest:
val myGenerator = xorshiftRandomGenerator(seed = 123)
myGenerator(0) should be(31682556)
myGenerator(1) should be(-276305998)
myGenerator(2) should be(2101636938)
The call to xorshiftRandomGenerator
does not throw a StackOverflowError
because the execution of xorshiftRandomGenerator
will be paused at the keyword Yield
, and it will be resumed when the caller is looking for the next number.
Each
keywords to iterate through configuations and keys, as an alternative syntax of for
comprehensions.!
-notation for creating Cats monadic expressions.(Feel free to add your project here)
BangNotation
compiler plugin is inspired by Idris' !-notation.