Table-driven parsing

1
Table-driven parsing

• Parsing performed by a finite state machine.
• Parsing algorithm is language-independent.
• FSM driven by table (s) generated automatically
  from grammar.
• Language generator
  tables

Input
parser
stack
tables
2
Pushdown Automata

• A context-free grammar can be recognized by a
  finite state machine with a stack a PDA.
• The PDA is defined by set of internal states and
  a transition table.
• The PDA can read the input and read/write on the
  stack.
• The actions of the PDA are determined by its
  current state, the current top of the stack, and
  the current input symbol.
• There are three distinguished states
• start state nothing seen
• accept state sentence complete
• error state current symbol doesnt belong.

3
Top-down parsing

• Parse tree is synthesized from the root (sentence
  symbol).
• Stack contains symbols of rhs of current
  production, and pending non-terminals.
• Automaton is trivial (no need for explicit
  states)
• Transition table indexed by grammar symbol G and
  input symbol a. Entries in table are terminals or
  productions P ABC

4
Top-down parsing

• Actions
• initially, stack contains sentence symbol
• At each step, let S be symbol on top of stack,
  and a be the next token on input.
• if T (S, a) is terminal a, read token, pop symbol
  from stack
• if T (S, a) is production P ABC.,
  remove S from stack, push the symbols A, B, C on
  the stack (A on top).
• If S is the sentence symbol and a is the end of
  file, accept.
• If T (S, a) is undefined, signal error.
• Semantic action when starting a production,
  build tree node for non-terminal, attach to
  parent.

5
Table-driven parsing and recursive descent parsing

• Recursive descent every production is a
  procedure. Call stack holds active procedures
  corresponding to pending non-terminals.
• Stack still needed for context-sensitive legality
  checks, error messages, etc.
• Table-driven parser recursion simulated with
  explicit stack.

6
Building the parse table

• Define two functions on the symbols of the
  grammar FIRST and FOLLOW.
• For a non-terminal N, FIRST (N) is the set of
  terminal symbols that can start any derivation
  from N.
• First (If_Statement) if
• First (Expr) id, (
• FOLLOW (N) is the set of terminals that can
  appear after a string derived from N
• Follow (Expr) , ),

7
Computing FIRST (N)

• If N e First (N) includes e
• if N aABC First (N) includes a
• if N X1X2 First (N) includes First
  (X1)
• if N X1X2 and X1 e,
• First (N) includes
  First (X2)
• Obvious generalization to First (a) where a is
  X1X2...

8
Computing First (N)

• Grammar for expressions, without left-recursion
• E TE T
• E TE e
• T FT F
• T FT e
• F id (E)
• First (F) id, (
• First (T) , e First (T)
  id, (
• First (E) , e First (E)
  id, (

9
Computing Follow (N)

• Follow (N) is computed from productions in which
  N appears on the rhs
• For the sentence symbol S, Follow (S) includes
  
• if A a N b, Follow (N) includes First
  (b)
• because an expansion of N will be followed by an
  expansion from b
• if A a N, Follow (N) includes Follow
  (A)
• because N will be expanded in the context in
  which A is expanded
• if A a N B , B e, Follow (N) includes
  Follow (A)

10
Computing Follow (N)

• E TE T
• E TE e
• T FT F
• T FT e
• F id (E)
• Follow (E) ), Follow (E) ),
• Follow (T) First (E ) Follow (E) , ),
  
• Follow (T) Follow (T) , ),
• Follow (F) First (T) Follow (T) , ,
  ),

11
Building LL (1) parse tables

• Table indexed by non-terminal and token. Table
  entry is a production
• for each production P A a loop
• for each terminal a in First (a) loop
• T (A, a) P
• end loop
• if e in First (a), then
• for each terminal b in Follow (a) loop
  T (A, b) P end loop
• end if
• end loop
• All other entries are errors.
• If two assignments conflict, parse table cannot
  be built.

12
LL (1) grammars

• If table construction is successful, grammar is
  LL (1) left-to right, leftmost derivation with
  one-token lookahead.
• If construction fails, can conceive of LL (2),
  etc.
• Ambiguous grammars are never LL (k)
• If a terminal is in First for two different
  productions of A, the grammar cannot be LL (1).
• Grammars with left-recursion are never LL (k)
• Some useful constructs are not LL (k)

13
Bottom-up parsing

• Synthesize tree from fragments
• Automaton performs two actions
• shift push next symbol on stack
• reduce replace symbols on stack
• Automaton synthesizes (reduces) when end of a
  production is recognized
• States of automaton encode synthesis so far, and
  expectation of pending non-terminals
• Automaton has potentially large set of states
• Technique more general than LL (k)

14
LR (k) parsing

• Left-to-right, rightmost derivation with k-token
  lookahead.
• Most general parsing technique for deterministic
  grammars.
• In general, not practical tables too large
  (106 states for C, Ada).
• Common subsets SLR, LALR (1).

15
The states of the LR(0) automaton

• An item is a point within a production,
  indicating that part of the production has been
  recognized
• A a . B b ,
• seen the expansion of a, expect to see expansion
  of B
• A state is a set of items
• Transition within states are determined by
  terminals and non-terminals
• Parsing tables are built from automaton
• action shift / reduce depending on next symbol
• goto change state depending on synthesized
  non-terminal

16
Building LR (0) states

• If a state includes
• A a . B b
• it also includes every state that is the start of
  B
• B . X Y Z
• Informally if I expect to see B next, I expect
  to see anything that B can start with, and so on
• X . G H I
• States are built by closure from individual items.

17
A grammar of expressions initial state

• E E
• E E T T -- left-recursion ok
  here.
• T T F F
• F id (E)
• S0 E .E, E .E T, E .T,
• F .id, F . ( E )
  ,
• T .T F, T .F

18
Adding states

• If a state has item A a .a b,
• and the next symbol in the input is a, we
  shift a on the stack and enter a state that
  contains item
• A a a.b
• (as well as all other items brought in by
  closure)
• if a state has as item A a. , this
  indicates the end of a production reduce action.
  
• If a state has an item A a .N b, then
  after a reduction that find an N, go to a state
  with A a N. b

19
The LR (0) states for expressions

• S1 E E., E E. T
• S2 E T., T T. F
• S3 T F.
• S4 F (. E), S0 (by closure)
• S5 F id.
• S6 E E . T, T .T F, T .F, F
  .id, F .(E)
• S7 T T . F, F .id, F .(E)
• S8 F (E.), E E. T
• S9 E E T., T T. F
• S10 T T F., S11 F
  (E).

20
Building SLR tables

• An arc between two states labeled with a terminal
  is a shift action.
• An arc between two states labeled with a
  non-terminal is a goto action.
• if a state contains an item A a. , (a
  reduce item)
• the action is to reduce by this production, for
  all terminals in Follow (A).
• If there are shift-reduce conflicts or
  reduce-reduce conflicts, more elaborate
  techniques are needed.

21
LR (k) parsing

• Canonical LR (1) annotate each item with its own
  follow set
• (A -gt a a.b , f )
• f is a subset of the follow set of A, because it
  is derived from a single specific production for
  A
• A state that includes A -gt a a.b is a reduce
  state only if next symbol is in f fewer reduce
  actions, fewer conflicts, technique is more
  powerful than SLR (1)
• Generalization use sequences of k symbols in f
• Disadvantage state explosion impractical in
  general, even for LR (1)

22
LALR (1)

• Compute follow set for a small set of items
• Tables no bigger than SLR (1)
• Same power as LR (1), slightly worse error
  diagnostics
• Incorporated into yacc, bison, etc.