PS Denotational Semantics 8 Introduction to Denotational Semantics

PS — Denotational Semantics 8. Introduction to Denotational Semantics © O. Nierstrasz

PS — Denotational Semantics Roadmap Overview: > Syntax and Semantics > Semantics of Expressions > Semantics of Assignment > Other Issues © O. Nierstrasz 2

PS — Denotational Semantics References > > D. A. Schmidt, Denotational Semantics, Wm. C. Brown Publ. , 1986 D. Watt, Programming Language Concepts and Paradigms, Prentice Hall, 1990 © O. Nierstrasz 3

PS — Denotational Semantics Roadmap Overview: > Syntax and Semantics > Semantics of Expressions > Semantics of Assignment > Other Issues © O. Nierstrasz 4

PS — Denotational Semantics Defining Programming Languages There are three main characteristics of programming languages: 1. Syntax: What is the appearance and structure of its programs? 2. Semantics: What is the meaning of programs? The static semantics tells us which (syntactically valid) programs are semantically valid (i. e. , which are type correct) and the dynamic semantics tells us how to interpret the meaning of valid programs. 3. Pragmatics: What is the usability of the language? How easy is it to implement? What kinds of applications does it suit? © O. Nierstrasz 5

PS — Denotational Semantics Uses of Semantic Specifications Semantic specifications are useful for language designers to communicate with implementors as well as with programmers. A precise standard for a computer implementation: How should the language be implemented on different machines? User documentation: What is the meaning of a program, given a particular combination of language features? A tool for design and analysis: How can the language definition be tuned so that it can be implemented efficiently? Input to a compiler generator: How can a reference implementation be obtained from the specification? © O. Nierstrasz 6

PS — Denotational Semantics Methods for Specifying Semantics Operational Semantics: > [[ program ]] = abstract machine program > can be simple to implement > hard to reason about Axiomatic Semantics: > [[ program ]] = set of properties > good for proving theorems about programs > somewhat distant from implementation Denotational Semantics: > [[ program ]] = mathematical denotation > (typically, a function) > facilitates reasoning > not always easy to find suitable semantic domains Structured Operational Semantics: > [[ program ]] = transition system > (defined using inference rules) > good for concurrency and nondeterminism > hard to reason about equivalence © O. Nierstrasz 7

PS — Denotational Semantics Roadmap Overview: > Syntax and Semantics > Semantics of Expressions > Semantics of Assignment > Other Issues © O. Nierstrasz 8

PS — Denotational Semantics Concrete and Abstract Syntax How to parse “ 4 * 2 + 1”? Abstract Syntax is compact but ambiguous: Expr Op : : = Num | Expr Op Expr : : = + | - | * | / Concrete Syntax is unambiguous but verbose: Expr Term Factor Low. Op High. Op : : = Expr Low. Op Term | Term : : = Term High. Op Factor | Factor : : = Num | ( Expr ) : : = + | : : = * | / Concrete syntax is needed for parsing; abstract syntax suffices for semantic specifications. © O. Nierstrasz 9

PS — Denotational Semantics A Calculator Language Abstract Syntax: Prog Stmt | Expr | | | : : = 'ON' Stmt : : = Expr 'TOTAL' Stmt Expr 'TOTAL' 'OFF' : : = Expr 1 '+' Expr 2 Expr 1 '*' Expr 2 'IF' Expr 1 ', ' Expr 2 ', ' Expr 3 'LASTANSWER' '(' Expr ')' Num The program “ON 4 * ( 3 + 2 ) TOTAL OFF” should print out 20 and stop. © O. Nierstrasz 10

PS — Denotational Semantics Calculator Semantics We need three semantic functions: one for programs, one for statements (expression sequences) and one for expressions. The meaning of a program is the list of integers printed: Programs: P : Program Int* P [[ ON S ]] = S [[ S ]] (0) A statement may use and update LASTANSWER: Statements: S : : Expr. Sequence Int* S [[ E TOTAL S ]] (n) = let n' = E [[ E ]] (n) in cons(n', S [[ S ]] (n')) S [[ E TOTAL OFF ]] (n) = [ E [[ E ]] (n) ] © O. Nierstrasz 11

PS — Denotational Semantics Calculator Semantics. . . Expressions: E : Expression Int E [[ E 1 + E 2 ]] (n) = E [[ E 1 ]] (n) + E [[ E 2 ]] (n) E [[ E 1 * E 2 ]] (n) = E [[ E 1 ]] (n) E [[ E 2 ]] (n) E [[ IF E 1 , E 2 , E 3 ]] (n) = if E [[ E 1 ]] (n) = 0 then E [[ E 2 ]] (n) else E [[ E 3 ]] (n) E [[ LASTANSWER ]] (n) = n E [[ ( E ) ]] (n) = E [[ E ]] (n) E [[ N ]] (n) = N © O. Nierstrasz 12

PS — Denotational Semantics Semantic Domains In order to define semantic mappings of programs and their features to their mathematical denotations, the semantic domains must be precisely defined: data Bool = True | False (&&), (||) : : Bool -> Bool False && x = False True && x = x False || x =x True || x = True not : : Bool -> Bool not True = False not False = True © O. Nierstrasz 13

PS — Denotational Semantics Data Structures for Abstract Syntax We can represent programs in our calculator language as syntax trees: data Program = On Expr. Sequence data Expr. Sequence = Total Expression Expr. Sequence | Total. Off Expression data Expression = Plus Expression | Times Expression | If Expression | Last. Answer | Braced Expression | N Int © O. Nierstrasz 14

PS — Denotational Semantics Representing Syntax The test program “ON 4 * ( 3 + 2 ) TOTAL OFF” can be parsed as: 4 Stmt Prog 3 * () ON + TOTAL OFF 2 And represented as: test = On (Total. Off (Times (Braced (N 4) (Plus (N 3) (N 2))))) © O. Nierstrasz 15

PS — Denotational Semantics Implementing the Calculator We can implement our denotational semantics directly in a functional language like Haskell: pp : : Program -> [Int] pp (On s) = ss s 0 ss : : Expr. Sequence -> Int -> [Int] ss (Total e s) n = let n' = (ee e n) in n' : (ss s n') ss (Total. Off e) n = (ee e n) : [ ] ee : : Expression -> Int ee (Plus e 1 e 2) n = (ee e 1 n) + (ee e 2 n) ee (Times e 1 e 2) n = (ee e 1 n) * (ee e 2 n) ee (If e 1 e 2 e 3) n | (ee e 1 n) == 0 = (ee e 2 n) | otherwise = (ee e 3 n) ee (Last. Answer) n =n ee (Braced e) n = (ee e n) ee (N num) n = num © O. Nierstrasz 16

PS — Denotational Semantics Roadmap Overview: > Syntax and Semantics > Semantics of Expressions > Semantics of Assignment > Other Issues © O. Nierstrasz 17

PS — Denotational Semantics A Language with Assignment Prog Cmd | | Exp : : = | | Bool | : : = Cmd '. ' : : = Cmd 1 '; ' Cmd 2 'if' Bool 'then' Cmd 1 'else' Cmd 2 Id ': =' Exp 1 '+' Exp 2 Id Num : : = Exp 1 '=' Exp 2 'not' Bool Example: z : = 1 ; if a = 0 then z : = 3 else z : = z + a. Input number initializes a; output is final value of z. © O. Nierstrasz 18

PS — Denotational Semantics Representing abstract syntax trees Data Structures: data Program data Command = data Expression = data Boolean. Expr = type Identifier © O. Nierstrasz = Dot Command = CSeq Command | Assign Identifier Expression | If Boolean. Expr Command Plus Expression | Id Identifier | Num Int Equal Expression | Not Boolean. Expr Char 19

PS — Denotational Semantics An abstract syntax tree Example: z : = 1 ; if a = 0 then z : = 3 else z : = z + a. Is represented as: Dot (CSeq (Assign 'z' (Num 1)) (If (Equal (Id 'a') (Num 0)) (Assign 'z' (Num 3)) (Assign 'z' (Plus (Id 'z') (Id 'a'))) ) ) © O. Nierstrasz 20

PS — Denotational Semantics Modelling Environments A store is a mapping from identifiers to values: type Store = Identifier -> Int newstore : : Store newstore id = 0 update : : Identifier -> Int -> Store update id val store = store' where store' id' | id' == id = val | otherwise = store id' © O. Nierstrasz 21

PS — Denotational Semantics Functional updates Example: env 1 = update 'a' 1 (update 'b' 2 (newstore)) env 2 = update 'b' 3 env 1 ‘b’ 2 env 2 ‘b’ 3 env 2 ‘z’ 0 © O. Nierstrasz 22

PS — Denotational Semantics of assignments pp : : Program -> Int pp (Dot c) n = (cc c (update 'a' n newstore)) ‘z’ cc : : Command -> Store cc (CSeq c 1 c 2) s = cc c 2 (cc c 1 s) cc (Assign id e) s = update id (ee e s) s cc (If b c 1 c 2) s = ifelse (bb b s) (cc c 1 s) (cc c 2 s) ee : : Expression -> Store -> Int ee (Plus e 1 e 2) s = (ee e 2 s) + (ee e 1 s) ee (Id id) s = s id ee (Num n) s =n bb : : Boolean. Expr -> Store -> Bool bb (Equal e 1 e 2) s = (ee e 1 s) == (ee e 2 s) bb (Not b) s = not (bb b s) ifelse : : Bool -> a ifelse True x y ifelse False x y © O. Nierstrasz =x =y 23

PS — Denotational Semantics Running the interpreter src 1 = "z : = 1 ; if a = 0 then z : = 3 else z : = z + a. " ast 1 = Dot (CSeq (Assign 'z' (Num 1)) (If (Equal (Id 'a') (Num 0)) (Assign 'z' (Num 3)) (Assign 'z' (Plus (Id 'z') (Id 'a'))))) pp ast 1 10 11 © O. Nierstrasz 24

PS — Denotational Semantics Roadmap Overview: > Syntax and Semantics > Semantics of Expressions > Semantics of Assignment > Other Issues © O. Nierstrasz 25

PS — Denotational Semantics Practical Issues Modelling: > Errors and non-termination: — need a special “error” value in semantic domains > Branching: — semantic domains in which “continuations” model “the rest of the program” make it easy to transfer control > Interactive input > Dynamic typing >. . . © O. Nierstrasz 26

PS — Denotational Semantics Theoretical Issues What are the denotations of lambda abstractions? > need Scott’s theory of semantic domains What are the semantics of recursive functions? > need least fixed point theory How to model concurrency and non-determinism? > abandon standard semantic domains > use “interleaving semantics” > “true concurrency” requires other models. . . © O. Nierstrasz 27

PS — Denotational Semantics What you should know! What is the difference between syntax and semantics? What is the difference between abstract and concrete syntax? What is a semantic domain? How can you specify semantics as mappings from syntax to behaviour? How can assignments and updates be modelled with (pure) functions? © O. Nierstrasz 28

PS — Denotational Semantics Can you answer these questions? Why are semantic functions typically higher-order? Does the calculator semantics specify strict or lazy evaluation? Does the implementation of the calculator semantics use strict or lazy evaluation? Why do commands and expressions have different semantic domains? © O. Nierstrasz 29

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