Föreläsning 2: Syntax och grammatiker

Kursen Kompilatorer och interpretatorer | Föreläsningar: 1 2 3 4 5 6 7 8 9 10 11 12


Det här är ungefär vad jag tänker säga på föreläsningen. Använd det för förberedelser, repetition och ledning. Det är inte en definition av kursinnehållet, och det ersätter inte kursboken.

Idag: Mer om syntaktisk analys, "parsning" (engelska: syntactical analysis, parsing)

2.1 Overview

Vi kommer att använda C på labbarna, och därför använder vi C-exemplen från den gamla kursboken ASU-86, i stället för Java-exemplen från den nya kursboken ALSU-07.

A compiler that translates infix to postfix:

Tree Infix notation Postfix notation
An abstract syntax tree for the expression 2 + 3 2 + 3 2 3 +
An abstract syntax tree for the expression 2 + 3 * 4 2 + 3 * 4 2 3 4 * +
An abstract syntax tree for the expression 2 * 3 + 4 2 * 3 + 4 2 3 * 4 +
An abstract syntax tree for the expression 2 * (3 + 4) 2 * (3 + 4) 2 3 4 + *

Source and target as text.

Postfix: Stack machine. Easy to write an interpreter.

The "2.5" program from ASU-86: simple grammar (Sw: "grammatik") (only + and -), simple parser, very simple scanner (one character = one token).
The "2.9" program from ASU-86: more advanced grammar (identifiers, *, /, mod, div), therefore a more complex parser, a "real" scanner.

2.2 Syntax definition

Example: the if statement in C. An instance:

if (a == b)
  printf("Same!\n");
else
  printf("Not same!\n");

This, as you know, is the syntax for the if statement:

if ( some expression ) some statement else some other statement

A rule that could be part of a context-free grammar (Sw: kontextfri grammatik) for C:

statement -> if ( expression ) statement else statement
statement -> if ( expression ) statement
statement -> { statement-list } (forgot what?)
...

"Context-free": a production "X -> ..." can always be used to replace X with "...", no matter what the rest of the program (that is, the context, Sw: kontext, omgivning) looks like.

  1. A set of terminals (Sw: terminaler) = terminal symbols = tokens
  2. A set of non-terminals (Sw: icke-terminaler) = non-terminal symbols (compound grammatical constructs)
  3. A set of productions (Sw: produktioner) = rules: non-terminal -> tokens/non-terminals. A production is for the non-terminal to the left.
  4. What is the start symbol (Sw: startsymbolen)
Other concepts:

Example 2.1 (p. 43)

Ensiffriga tal, plus, minus:
7+3, 7+3-4+6, 3 (but not 17, -3 or 2*2)

digit -> 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
list -> digit
list -> list + digit
list -> list - digit

or

digit -> 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
list -> digit | list + digit | list - digit

Try 9-5+2.
9 -> digit -> list.
5 -> digit.
9-5 -> list - digit -> list
2 -> digit.
9-5+2 -> list + digit -> list

ALSU-87 fig 2.5 = ASU-86 fig 2.2, the parse tree (= concrete syntax tree) and the syntax tree (= abstract syntax tree):

Parse tree for 9-5+2

Why list + digit etc? Asymmetrical and ugly? Why not just list + list, like this:

digit -> 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
string -> digit | string + string | string - string

ALSU-07 fig 2.6 = ASU-86 fig 2.3 (slide!):

Two parse trees for 9-5+2

Operator associativity (Sw: Operatorassociativitet)

ALSU-07 fig 2.7 = ASU-86 fig 2.4:

Parse trees for left- and right-associative operators

Use a grammar like above, that is,
list -> list + digit
for left-associative (Sw: vänsterassociativa) operators. Use a grammar like
list -> digit + list
for right-associative (Sw: högerassociativa) operators, for example:

right -> letter = right
letter -> a | b | c | ... | z

Operator precedence (Sw: Operatorarprioritet, operatorarprecedens)

9 + 5 * 2 = 9 + (5 * 2), not (9 + 5) * 2.
"*" has higher precedence than "+".

Express this in the grammar:

factor -> digit | ( expr )
term -> term * factor | term / factor | factor
expr -> expr + term | expr - term | term

2.3 Syntax-directed translation

Not just the parse tree for a certain construct, but also keep track of attributes of each subtree. An attribute can be any data that we want to keep track of in the tree, such as the data type of an expression, or even generated code.

Two different types:

Syntax-directed definitions

Syntax-directed definition = syntax-styrd definition

Grammar + what to do for each production

Syntax-directed definition = context-free grammar, plus a semantic rule (Sw: semantisk regel) for each production, that specifies how to calculate values of attributes. Example:

Production Semantic rule
term -> 0 term.postfixcode -> "0"
term -> 1 term.postfixcode -> "1"
term -> 2 term.postfixcode -> "2"
... ...
expr -> expr1 + term1 expr.postfixcode -> expr1.postfixcode + term1.postfixcode + " +"
... ...

ALSU-07 fig 2.9 = ASU-86 fig 2.6, an annotated (= with attribute values) parse tree:

Attribute values at nodes in a parse tree

A syntax-directed definition says nothing about how the parser should build the parse tree! Just the grammar, and what to do when the tree is finished (or at least, when we have found which production to use).

Den syntax-styrda definitionen säger inte heller något om i vilken ordning de semantisk reglerna ska "köras", förutom förstås att de värden som används i reglerna måste finnas tillgängliga. (Så man måste till exempel räkna ut expr1.postfixcode och term1.postfixcode innan man slår ihop dem till expr.postfixcode.)

(Syntax-directed) translations schemes

Syntax-directed translation scheme = syntax-styrt översättningsschema

Grammar + what to do for each production embedded in the grammar: in the right-hand side of productions

Syntax-directed definition = context-free grammar, plus semantic actions (Sw: semantiska aktioner, semantiska åtgärder) for each production, that specifies what to do. Example:

expr -> expr1 + term1 { print("+"); }

Generates postfix!

Or, with the action somewhere in the middle:

rest -> + term1 { print("+"); } rest1

The semantic actions are put in the parse tree, just like the "real" parts.
ALSU-07 fig 2.13 = ASU-86 fig 2.12:

An extra leaf is constructed for a semantic action

ALSU-07 fig 2.14 = ASU-86 fig 2.14:

Actions translating 9-5+2 into 95-2+

As with a syntax-directed definition, a syntax-directed scheme says nothing about how the parser should build the parse tree! Just the grammar, and what to do when the tree is finished.

But we don't actually have to build the tree. Just perform the semantic actions as the tree is (not) built!

Till skillnad från med en syntax-styrd definition (som var en tabell med regler) så måste de semantiska aktionerna i ett syntax-styrt översättningsschema utföras i rätt ordning. De är programsnuttar (t ex print-satser och variabeltilldelningar), och det blir förstås fel om de görs i fel ordning!

2.4 Parsing

Se nästa föreläsning.

Kursen Kompilatorer och interpretatorer | Föreläsningar: 1 2 3 4 5 6 7 8 9 10 11 12


Thomas Padron-McCarthy (Thomas.Padron-McCarthy@tech.oru.se) 7 september 2007