Sohail Aslam Compiler Construction Notes Set:2-1
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An LR(1) item is a pair [X → α•β, a] where X → αβ is a production and a∈T
(terminals) is look-ahead symbol. The model uses a set of LR(1) items to represent each
parser state. The model is called the canonical collection (
CC
) of set of LR(1) items.
Canonical Collection
Each set in
CC
represents a state in the eventual parser DFA. The construction of CC
begins by building a model of parser’s initial state. The initial state consists of the set of
LR(1) items that represent the parser’s initial state, along with any items that must also
hold in the initial state. To simplify the task of building this initial state, the construction
requires that the grammar have a unique goal symbol. The convention is to add a new
start symbol S to grammar and a production
S → E
This leads to the augmented grammar
S → E
E → E + (E) | int
The Closure Procedure
The item [S → •E, $] describes the parser’s initial state. It represents a configuration in
which recognizing S followed by $ would be a valid parse. This item, i.e., [S → •E, $]
becomes the core of the first state in CC, labeled I0. If the grammar has several distinct
productions for the start symbol, each of them generates an item in this initial core of I0.
The procedure closure does this.
closure(s) =
repeat
for each [X → α•Yβ, a] ∈ s
for each production Y → α
for each b ∈ FIRST(βa)
s ← s ∪ [Y → • γ, b]
until s is unchanged
Let’s apply this procedure to the augmented grammar.
The first set is I0 = closure({[S → •E, $] }). Equating the terms in the procedure,
s = {[S → •E, $]}, [X → α •Yβ, a] ⇔ [S → •E, $], X = S, α = ε, Y = E,
β = ε, a = $, Y → γ ⇔E → E + (E) and E → intFIRST(βa) = FIRST($) = $.
