Understanding Attribute Grammar: A Comprehensive Introduction to Theoretical Foundations

1. Part 1 : Transformation Technique Lecture 1: Attribute Grammar Dr. Ir. I.S.W.B. Prasetya wishnu@cs.uu.nl A. Azurat S.Kom. ade@cs.uu.nl

2. Plan • Introduction • AG Approach • AG Tool • Examples • Conclusion

3. Transformation transformation result sentence "abba" semantic function parser P parse tree A A P B P B a b b a

4. Example data Form = Form `AND` Form | Form `IMP` Form | Var String | Const Bool Example:(Var "a" `AND` Var "b") `IMP` Var "a"

5. Example simp (Var a) = Var a simp (Const c) = Const c For every semantic function, you have to code the recursion by hand. simp (p `AND` q) = case (simp p, simp q) of ... (Const True , q') -> q' (p' , Const True) -> p' otherwise -> Nothing

6. a computation on tree Composite Semantic Function • In composition: sequencing computations can be subtle • Upgrading a computation: may introduce various intermediate administrative data stucture whose relation can be quite subtle repmin t = replace (minT t) t

7. AG tool AG description sem. function • repmin • simplifier • type checker • compiler • test generator • ... • in the same class as compiler compiler (YACC) compiler generator • example AG tool: LRC, AG Attribute Grammar Approach It is a formalism to abstractly describe semantic functions

8. x inherited attribute And And Var y Attributes countVar (Const _) x = 0 countVar (Var y) x = if x==y then 1 else 0 countVar (And p q) x = countVar p x + countVar q x synthesized attribute

9. Defining countVar in AG DATA Form | Const Bool | Var String | And p,q: Form

10. Defining countVar in AG ATTR Form [ x : String | | result : Int ] SEM Form | Const lhs .result = 0 | Var lhs .result = if @lhs.x == @string then 1 else 0

11. Defining countVar in AG SEM Form ... | And p .x = @lhs.x q .x = @lhs.x lhs .result = @p.result + @q.result

12. Generating the Sem Function • Write AG source in file-name.ag • file-name must begin with CAPITAL letter • c:/app/hugs/runhugs -98 –mdcfs file-name • Will generate file-name.hs • Set the buffered number of lines in your DOS-window to 200 ...

13. Result data Form = And (Form) (Form) | Const (Bool) | Var (String) sem_Form ((And (_p) (_q))) = ... ... sem_Form_Const (_bool) (_lhs_x) = 0 sem_Form (And (Var "x") (Var "y") ) "x" = 1

14. cryptic notation ? currently ... each AG description only generate 1 sem-function per data structure. Hurdles SEM ... | Var lhs .result = if @lhs.x == @string then 1 else 0 ... sem_Form ((And (_p) (_q))) = ... ...

15. substitution list Example: Renaming rename [ x  a, y  b ] x /\ y /\ z = a /\ b /\ z

16. Writing Rename with AG imports { import List } { type SubsList = [ ( String, String ) ] subst :: String -> SubsList -> String subst x s = case find (\(y,z)-> y==x) s of Just (_,z) -> z otherwise -> x } ATTR Form [ subs : SubsList | | result : Form ]

17. can be omitted  copy rule Writing Rename with AG SEM Form | Const lhs .result = Const @bool | Var lhs .result = Var (subst @string @lhs.subs) | And lhs .result = And @p.result @q.result p .subs = @lhs.subs q .subs = @lhs.subs

18. Example: Normalizing Names normalizeNames x /\ c /\ z = a /\ b /\ c

19. PASS 1: compute subs-list first ... [ (x, a), (c, b), (z, c) ] PASS 2: rename Strategy normalizeNames x /\ c /\ z = a /\ b /\ c

20. for generating new (normalized) name Writing Pass 1 with AG { alreadyIn :: String -> SubsList -> Bool x `alreadyIn` s = find (\(y,k)-> y==x) s /= Nothing } ATTR Form [ | cntVar : Int nsubs : SubsList | ]

21. can be omitted  copy rule cntVar, nsubs cntVar, nsubs Const b Pass 1 SEM Form | Const lhs .nsubs = @lhs.nsubs .cntVar = @lhs.cntVar

22. Pass 1 SEM Form ... | Var loc .seen = @string `alreadyIn` @lhs.nsubs .nsubs' = @lhs.nsubs ++ [(@string, [chr(ord 'a' + @lhs.cntVar)])] .cntVar' = @lhs.cntVar + 1 lhs .nsubs = if @seen then @lhs.nsubs else @nsubs' .cntVar = if @seen then @lhs.cntVar else @cntVar'

23. Can be omitted  copy rule ! Pass 1 SEM Form ... | And p .cntVar = @lhs.cntVar .nsubs = @lhs.nsubs q .cntVar = @p.cntVar .nsubs = @p.nsubs lhs .cntVar = @q.cntVar .nsubs = @q.nsubs

24. Pass or Aspect ? • Rather than sequencing passes, we will combine aspects ! • Each SEM computation may consist of several aspects, rather than passes DATA Root | Root f : Form ATTR Root [ | | result : Form ]

25. no imperative sequencing of passes! copy rule! Combining Aspects SEM Root | Root f .cntVar = 0 .nsubs = [ ] .subs = @f.nsubs lhs .result = @f.result

26. Resulting AG Code ... SEM Form | Const lhs.result = Const @bool | Var lhs.result = Var (subst @string @lhs.subs) | And lhs.result = And @p.result @q.result SEM Form | Var loc .seen = @string `alreadyIn` @lhs.nsubs .nsubs' = @lhs.nsubs ++ [(@string, [chr(ord 'a' + @lhs.cntVar)])] .cntVar' = @lhs.cntVar + 1 lhs .nsubs = if @seen then @lhs.nsubs else @nsubs' .cntVar = if @seen then @lhs.cntVar else @cntVar' SEM Root | Root f .cntVar = 0 .nsubs = [] .subs = @f.nsubs

27. Conclusion • Abstract • recursion is generated • copying and chaining information is generated • sequencing passes is automatic • Highly compositional • computation can be split in aspects • Highly extensible • extending a computation is just adding a new aspect