How domain specific are Domain Specific Languages?

1. How domain specific are Domain Specific Languages? Greg Michaelson School of Mathematical & Computer Sciences Heriot-Watt University

2. Introduction • tiresome writing lots of small programs for same problem area • end up using same set of: • programming tropes • abstractions • types • data structures • control

3. Introduction • construct an ad hoc command based framework • scripts to invoke and configure individual programs • nice to deploy some consistent notation oriented to problem area • with built in constructs capturing specialised abstractions • domain specific language - DSL

4. Embeded DSL (EDSL) • DSL embeded in extant host language 1. add library to host language • call through API • can use arbitrary host language constructs • not a DSL?

5. Embeded DSL 2. add abstract syntax • interpret abstract syntax trees • call interpreter components from arbitrary host language constructs • shallow embeding

6. Embeded DSL 3. design concrete syntax • add concrete syntax -> AST compiler • call interpreter component with parsed strings • can embed interpreter component calls in arbitrary host language constructs • shallow embeding. 4. extend extant language syntax and semantics • deep embeding

7. Embeded DSL • host language exposed • not Domain Specific? • EDSL inherits semantics of host language • if host language is not Turing complete (TC) then EDSL may be more expressive • can’t be less or differently expressive • if host language is TC then so is EDSL • not Domain Specific?

8. Implemented DSL • expose parser/interpreter only as stand alone language processor • can only use domain specific syntax • have implemented DSL in host language

9. Implemented DSL • tend to want familiar general purpose programming language abstractions as well: • arithmetic & logic • sequence/selection/iteration • sub programs • sequences etc • as DSL grows, tends to become less and less DS...

10. Language • language = syntax + semantics • semantics: program * state -> state • semantics transforms initial state to final state guided by structure of program instance

11. Language and program • program: input * state -> output * state • treat outputs as part of final state • program: input * state -> state • program changes initial state to final state depending on input structure • define input structure with syntax • program is semantics of inputs • does every program define a DSL?

12. What is domain specificity? • Light Bulb Language program -> switch switch -> SWITCH |ε m [SWITCH] ON = OFF m [SWITCH] OFF = ON m [ε] ON = ON m [ε] OFF = OFF

13. What is domain specificity? • Linear Light Bulbs Language program -> row | row program row -> switch | switch row si ϵswitch biϵ {ON,OFF} m’ [s1,...,sN] {b1,...,bN} = {m s1 b1,...,m sNbN}

14. What is domain specificity? • Grid Light Bulbs Language program -> grid grid -> row | row grid riϵrow briϵ b* m’’ [r1,...,rN] {br1,...,brN} = {m’ r1br1,...,m’ rNbrN}

15. What is domain specificity? • is GLBL domain specific...? • light bulbs? • B/W image inversion? • negating boolean values? • not very DS...

16. What is domain specificity? • computer pioneers thought machine code was for configuring hardware • 60’s language designers thought languages were purpose specific: • FORTRAN - sums • COBOL - accounts • ALGOL - algorithms • LISP - symbols • IMP - systems

17. What is domain specificity? • designers of new languages think they’ve enabled something other languages can’t do • new TC language ≡ old TC language + syntax + library • is every language really an embeded DSL with a TC host?

18. DSL is about pragmatics • DSL abstractions & constructs make it easier to express particular things • what may be complex in an arbitrary TC language may become simpler in a DSL • domain may frame choice of DSL abstractions & constructions • abstractions and constructions from one domain may be appropriate for other domains