Shift
LR Parsers
Implementations
IR Forms
Runtime Environment
and activation records
doesnt matter if prefix is T or NT
Using GNF we can convert
Left recursive grammar to
right recursive
first(a1) ∩ first(a2)
first(a1) ∩ follow(X)
L to R parse
RMD
all
these parsers have the ability
to parse some ambiguous grammar
BUP pushes tokens into stack
And then decides if it can reduce the TOS
string gets accepted when stack has
only Start symbol, and remaining inp symbol is $
Add LR(O) items to S
until there are no more to add.
expected to see something that A reduces to
Of
stored in the stack
Since SLR doesn't reduce blindly,
conflict in LR(0) might not be there
take care of $
while seeing reduced states
in SLR table (& while parsing), reduce only where (next)
inp symb comes in follow (of TOS)
anything w/ a reduced
even .∈
or
indicate a severe internal inconsistency within the compiler itself
Non-Recoverable Impact unlike other stages
connonical
number of shift entries in SLR(1) and LALR(1) is equal
LR(1) item :
An item 𝑋 → 𝛼. 𝛽, a
• parser is looking to reduce to X if next input symbol a
• It has already seen 𝛼 (also 𝛼 is on top of stack)
• Expects to find string derived from 𝛽 in the next input
In an LR(1) machine with a shift/reduce conflict, we should shift if the priority of the rule is
lower than the priority of the incoming symbol, or if the priorities are the same and are both
right associative.
Lookahead means first() of immediate part of variable whose closure has been put
Some grammars can be LR(1) but not LALR
RR conflict in LALR, but none in LR1
📍[[GT]]
AST omitted the syntactic details of parse tree but not semantic.
LR1 grammar exists for all DCFG
Only
(Parent is taking value from child attribute)
&
Both
(child is taking value from Parent/sibling)
Follow
(only if 1st contains ∈)
would cause ∞ loop
ambiguity in decision-making
postpones the decision about which
production to use until enough input
has been seen to make an informed
choice
🌐
https://www.youtube.com/watch?v=0q1BEXFUaBE
simple & Synthesized
Left-to-right
attr. val.
∈ free LL1 is also SLR(1)
∈ is added to First of X
only if all Y's in X -> Y1Y2...Yk have ∈
RDP
general
RDP
Bruteforce
No
because of stack space
Non recursive or table
procedural
E → T | T ∗ E
T → int | int ∗ T | (E)
inherited attributes of Xi
depends on any attributes of
X1, X2,...., X_{i-1}
but ONLY inherited attributes of
parent A
for any sentential form
this is just next input symbol, not lookahead
lookahead comes into play only when the dot is at the end of a production,
signaling a reduce action.
Interactive Graph