CMSC 330: Organization of Programming Languages. Context Free Grammars


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1 CMSC 330: Organization of Programming Languages Context Free Grammars 1
2 Architecture of Compilers, Interpreters Source Analyzer Optimizer Code Generator Abstract Syntax Tree Front End Back End Compiler / Interpreter 2
3 Front End Scanner and Parser Front End Token Source Scanner Parser Stream AST Scanner / lexer converts program source into tokens (keywords, variable names, operators, numbers, etc.) using regular expressions Parser converts tokens into an AST (abstract syntax tree) using context free grammars 4
4 ContextFree Grammar (CFG) A way of describing sets of strings (= languages) The notation L(G) denotes the language of strings defined by grammar G Example grammar G is S 0S 1S e which says that string s L(G) iff s = e, or s L(G) such that s = 0s, or s = 1s Grammar is same as regular expression (0 1)* Generates / accepts the same set of strings 5
5 CFGs Are Expressive CFGs subsume REs, DFAs, NFAs There is a CFG that generates any regular language But: REs are often better notation for those languages And CFGs can define languages regexps cannot S ( S ) e // represents balanced pairs of ( ) s As a result, CFGs often used as the basis of parsers for programming languages 6
6 Parsing with CFGs CFGs formally define languages, but they do not define an algorithm for accepting strings Several styles of algorithm; each works only for less expressive forms of CFG LL(k) parsing We will discuss this next lecture LR(k) parsing LALR(k) parsing SLR(k) parsing Tools exist for building parsers from grammars JavaCC, Yacc, etc. 7
7 Formal Definition: ContextFree Grammar A CFG G is a 4tuple (Σ, N, P, S) Σ alphabet (finite set of symbols, or terminals) Ø Often written in lowercase N a finite, nonempty set of nonterminal symbols Ø Often written in UPPERCASE Ø It must be that N Σ = P a set of productions of the form N (Σ N)* Ø Informally: the nonterminal can be replaced by the string of zero or more terminals / nonterminals to the right of the Ø Can think of productions as rewriting rules (more later) S N the start symbol 8
8 Notational Shortcuts S abc // S is start symbol A aa b // A b // A e A production is of the form lefthand side (LHS) right hand side (RHS) If not specified Assume LHS of first production is the start symbol Productions with the same LHS Are usually combined with If a production has an empty RHS It means the RHS is ε 9
9 BackusNaur Form Contextfree grammar production rules are also called BackusNaur Form or BNF Designed by John Backus and Peter Naur Ø Chair and Secretary of the Algol committee in the early 1960s. Used this notation to describe Algol in 1962 A production A B c D is written in BNF as <A> ::= <B> c <D> Nonterminals written with angle brackets and uses ::= instead of Often see hybrids that use ::= instead of but drop the angle brackets on nonterminals 10
10 Generating Strings We can think of a grammar as generating strings by rewriting Example grammar G S 0S 1S e Generate string 011 from G as follows: S 0S // using S 0S 01S 011S 011 // using S 1S // using S 1S // using S e 11
11 Accepting Strings (Informally) Checking if s L(G) is called acceptance Algorithm: Find a rewriting starting from G s start symbol that yields s A rewriting is some sequence of productions (rewrites) applied starting at the start symbol Ø 011 L(G) according to the previous rewriting Terminology Such a sequence of rewrites is a derivation or parse Discovering the derivation is called parsing 12
12 Derivations Notation + indicates a derivation of one step indicates a derivation of one or more steps * indicates a derivation of zero or more steps Example S 0S 1S e For the string 010 S 0S 01S 010S 010 S *
13 Language Generated by Grammar L(G) the language defined by G is L(G) = { s Σ* S + s } S is the start symbol of the grammar Σ is the alphabet for that grammar In other words All strings over Σ that can be derived from the start symbol via one or more productions 14
14 Quiz #1 Consider the grammar S as T T bt U U cu ε Which of the following strings is generated by this grammar? A. ccc B. aba C. bab D. ca 15
15 Quiz #1 Consider the grammar S as T T bt U U cu ε Which of the following strings is generated by this grammar? A. ccc B. aba C. bab D. ca 16
16 Quiz #2 Consider the grammar S as T T bt U U cu ε Which of the following is a derivation of the string bbc? A. S T U bu bbu bbcu bbc B. S bt bbt bbu bbcu bbc C. S T bt bbt bbu bbcu bbc D. S T bt btbt bbt bbcu bbc 17
17 Quiz #2 Consider the grammar S as T T bt U U cu ε Which of the following is a derivation of the string bbc? A. S T U bu bbu bbcu bbc B. S bt bbt bbu bbcu bbc C. S T bt bbt bbu bbcu bbc D. S T bt btbt bbt bbcu bbc 18
18 Quiz #3 Consider the grammar S as T T bt U U cu ε Which of the following regular expressions accepts the same language as this grammar? A. (a b c)* B. abc* C. a*b*c* D. (a ab abc)* 19
19 Quiz #3 Consider the grammar S as T T bt U U cu ε Which of the following regular expressions accepts the same language as this grammar? A. (a b c)* B. abc* C. a*b*c* D. (a ab abc)* 20
20 Designing Grammars 1. Use recursive productions to generate an arbitrary number of symbols A xa ε A ya y // Zero or more x s // One or more y s 2. Use separate nonterminals to generate disjoint parts of a language, and then combine in a production a*b* S AB A aa ε B bb ε // a s followed by b s // Zero or more a s // Zero or more b s 23
21 Designing Grammars 3. To generate languages with matching, balanced, or related numbers of symbols, write productions which generate strings from the middle {a n b n n 0} // N a s followed by N b s S asb ε Example derivation: S asb aasbb aabb {a n b 2n n 0} // N a s followed by 2N b s S asbb ε Example derivation: S asbb aasbbbb aabbbb 24
22 Designing Grammars 4. For a language that is the union of other languages, use separate nonterminals for each part of the union and then combine { a n (b m c m ) m > n 0} Can be rewritten as { a n b m m > n 0} { a n c m m > n 0} S T V T atb U U Ub b V avc W W Wc c 25
23 Practice Try to make a grammar which accepts 0* 1* 0 n 1 n where n 0 S A B A 0A ε B 1B ε S 0S1 ε Give some example strings from this language S 0 1S Ø 0, 10, 110, 1110, 11110, What language is it, as a regexp? Ø 1*0 26
24 Quiz #4 Which of the following grammars describes the same language as 0 n 1 m where m n? A. S 0S1 ε B. S 0S1 S1 ε C. S 0S1 0S ε D. S SS 0 1 ε 27
25 Quiz #4 Which of the following grammars describes the same language as 0 n 1 m where m n? A. S 0S1 ε B. S 0S1 S1 ε C. S 0S1 0S ε D. S SS 0 1 ε 28
26 CFGs for Language Syntax When discussing operational semantics, we used BNFstyle grammars to define ASTs e ::= x n e + e let x = e in e This grammar defined an AST for expressions synonymous with an OCaml datatype We can also use this grammar to define a language parser However, while it is fine for defining ASTs, this grammar, if used directly for parsing, is ambiguous 29
27 Arithmetic Expressions E a b c E+E EE E*E (E) An expression E is either a letter a, b, or c Or an E followed by + followed by an E etc This describes (or generates) a set of strings {a, b, c, a+b, a+a, a*c, a(b*a), c*(b + a), } Example strings not in the language d, c(a), a+, b**c, etc. 30
28 Parse Trees Parse tree shows how a string is produced by a grammar Root node is the start symbol Every internal node is a nonterminal Children of an internal node Ø Are symbols on RHS of production applied to nonterminal Every leaf node is a terminal or ε Reading the leaves left to right Shows the string corresponding to the tree 32
29 Parse Tree Example S S as T T bt U U cu ε S 33
30 Parse Tree Example S as S as T T bt U U cu ε a S S 34
31 Parse Tree Example S as at S as T T bt U U cu ε a S S T 35
32 Parse Tree Example S as at au S as T T bt U U cu ε a S S T U 36
33 Parse Tree Example S as at au acu S as T T bt U U cu ε a S S T U c U 37
34 Parse Tree Example S as at au acu ac S as T T bt U U cu ε a S S T U c U ε 38
35 Parse Trees for Expressions A parse tree shows the structure of an expression as it corresponds to a grammar E a b c d E+E EE E*E (E) a a*c c*(b+d) 39
36 Abstract Syntax Trees A parse tree and an AST are not the same thing The latter is a data structure produced by parsing a*c c*(b+d) Parse trees * a c ASTs Mult(Var( a ),Var( c )) * c + b d Mult(Var( c ),Plus(Var( b ),Var( d ))) 40
37 Practice E a b c d E+E EE E*E (E) Make a parse tree for a*b a+(bc) d*(d+b)a (a+b)*(cd) a+(bc)*d 41
38 Leftmost and Rightmost Derivation Leftmost derivation Leftmost nonterminal is replaced in each step Rightmost derivation Rightmost nonterminal is replaced in each step Example Grammar Ø S AB, A a, B b Leftmost derivation for ab Ø S AB ab ab Rightmost derivation for ab Ø S AB Ab ab 42
39 Parse Tree For Derivations Parse tree may be same for both leftmost & rightmost derivations Example Grammar: S a SbS Leftmost Derivation S SbS abs aba String: aba Rightmost Derivation S SbS Sba aba Parse trees don t show order productions are applied Every parse tree has a unique leftmost and a unique rightmost derivation 43
40 Parse Tree For Derivations (cont.) Not every string has a unique parse tree Example Grammar: S a SbS String: ababa Leftmost Derivation S SbS abs absbs ababs ababa Another Leftmost Derivation S SbS SbSbS absbs ababs ababa 44
41 Ambiguity A grammar is ambiguous if a string may have multiple leftmost derivations Equivalent to multiple parse trees Can be hard to determine 1. S as T T bt U U cu ε 2. S T T T Tx Tx x x 3. S SS () (S) No Yes? 45
42 Ambiguity (cont.) Example Grammar: S SS () (S) String: ()()() 2 distinct (leftmost) derivations (and parse trees) Ø S Þ SS Þ SSS Þ()SS Þ()()S Þ()()() Ø S Þ SS Þ ()S Þ()SS Þ()()S Þ()()() 46
43 CFGs for Programming Languages Recall that our goal is to describe programming languages with CFGs We had the following example which describes limited arithmetic expressions E a b c E+E EE E*E (E) What s wrong with using this grammar? It s ambiguous! 47
44 Example: abc E EE ae aee abe abc E EE EEE aee abe abc Corresponds to a(bc) Corresponds to (ab)c 48
45 Another Example: IfThenElse Aka the dangling else problem <stmt> <assignment> <ifstmt>... <ifstmt> if (<expr>) <stmt> if (<expr>) <stmt> else <stmt> (Note < > s are used to denote nonterminals) Consider the following program fragment if (x > y) if (x < z) a = 1; else a = 2; (Note: Ignore newlines) 50
46 Two Parse Trees if (x > y) if (x < z) a = 1; else a = 2; 51
47 Quiz #5 Which of the following grammars is ambiguous? A. S 0SS1 0S1 ε B. S A1S1A ε A 0 C. S (S, S, S) 1 D. None of the above. 52
48 Quiz #5 Which of the following grammars is ambiguous? A. S 0SS1 0S1 ε B. S A1S1A ε A 0 C. S (S, S, S) 1 D. None of the above. 53
49 Dealing With Ambiguous Grammars Ambiguity is bad Syntax is correct But semantics differ depending on choice Ø Different associativity Ø Different precedence Ø Different control flow Two approaches Rewrite grammar (ab)c vs. a(bc) (ab)*c vs. a(b*c) if (if else) vs. if (if) else Ø Grammars are not unique can have multiple grammars for the same language. But result in different parses. Use special parsing rules Ø Depending on parsing tool 54
50 Fixing the Expression Grammar Require right operand to not be bare expression E E+T ET E*T T T a b c (E) Corresponds to left associativity Now only one parse tree for abc Find derivation 56
51 What if we want Right Associativity? Leftrecursive productions Used for leftassociative operators Example E E+T ET E*T T T a b c (E) Rightrecursive productions Used for rightassociative operators Example E T+E TE T*E T T a b c (E) 57
52 Parse Tree Shape The kind of recursion determines the shape of the parse tree left recursion right recursion 58
53 A Different Problem How about the string a+b*c? E E+T ET E*T T T a b c (E) Doesn t have correct precedence for * When a nonterminal has productions for several operators, they effectively have the same precedence Solution Introduce new nonterminals 59
54 Final Expression Grammar E E+T ET T T T*P P P a b c (E) lowest precedence operators higher precedence highest precedence (parentheses) Controlling precedence of operators Introduce new nonterminals Precedence increases closer to operands Controlling associativity of operators Introduce new nonterminals Assign associativity based on production form Ø E E+T (left associative) vs. E T+E (right associative) Ø But parsing method might limit form of rules 60
55 Conclusion Context Free Grammars (CFGs) can describe programming language syntax They are a kind of formal language that is more powerful than regular expressions CFGs can also be used as the basis for programming language parsers (details later) But the grammar should not be ambiguous Ø May need to change more natural grammar to make it so Parsing often aims to produce abstract syntax trees Ø Data structure that records the key elements of program 61
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