Slides for our talk at JVMCon. Code for the example is available at: https://github.com/NRBPerdijk/dsl-for-the-dense

1. WRITING A DSL FOR THE DENSE
Nathan Perdijk (@nrbperdijk)
Jan-Hendrik Kuperus (@jhkuperus)

2. Maths / CS / History / Philosophy
DSL for Mortgage Calculations
Authors of ScalaRules
Board Game Geeks
JAN-HENDRIK NATHAN

3. WHAT IS A DSL ? (1/2)
Formal de nition: A domain-speci c language (DSL)
is a computer language specialized to a particular
application domain. (...) the term "domain-speci c
language" has become more popular due to the rise
of domain-speci c modeling. (Wikipedia, consulted
19 January 2018)

4. WHAT IS A DSL ? (2/2)
Our de nition: Providing easy to use and/or
aesthetically pleasing ways to access a (public) API to
improve its domain expressiveness and expand its
potential userbase.

5. PURPOSES OF A DSL
Abstract initialisation logic away
Increase readability of code
Guide unfamiliar users through an API
Increase 'con gurability' of a library

6. DON'T MAKE A DSL ...
Just because you can ...
If API changes frequently

7. WHY USE SCALA?
Optional dots and parenthesis element toString
Implicit conversions 4
In x Type Constructor new (Dollar Per Hour):
Per[Dollar, Hour]
Singleton Instances
Code Completion in IntelliJ
Macros ...
. ()
intToString( )

8. DESIGNING A DSL
Determine your ubiquitous language
Find your expressive freedom
Fit it into a 'rhythm'
Decide what your DSL returns
Lots of trial and error

9. FITTING THE RHYTHM
Basic premise: use odd numbers of words
Hello.there(JVMCon).how(do).you(do)
object Hello { def there(JVMCon.type): HelloThere }
class HelloThere { def how(do.type): HowPart }
class HowPart { def you(do.type): String }
object JVMCon {}
object do {}

10. EXAMPLES SPEAK LOUDER ...
Simple Work ow System
Business Rules determine validity of actions
Multiple ways to achieve success

11. WOLF & SHEEP VEGETABLE LOGISTICS LTD.
Cabbage and Animal Transport across streams
Some issues in Startup Phase
Shipments were eaten
Locally sourced transport (really tiny boat)

15. MODELING THE BUSINESS
Cargo: Wolf, Sheep, Cabbage
Activities: Enter boat, Cross the stream, Exit boat
Outcomes: Cargo eaten, No eating at all

16. TO THE CODE-MOBILE

17. WHAT'S NEXT
Scala Meta offers greater Power
Check out Scala-Rules (2.11 only)
https://github.com/NRBPerdijk/dsl­for­
the­dense

18. SCALA MACROS/META/...
Compile-time methods
AST Transformations
Highly experimental

19. SCALA-RULES
Forward-chaining rule engine
DSL to express data ow
Scala Macros & Meta annotations
(so only works in 2.11)

20. WRITING A DSL FOR THE ...