As we study the table-construction algorithm, we will see how those problems arise, how to cure them, and how to understand the kinds of diagnostic information that parser generators produce. In practice, the parser generator identifies the productions that introduce ambiguity or that need more than a one word lookahead to distinguish a shift action from a reduce action. With an parser generator, the compiler writer's role is to define the grammar and to ensure that the grammar has the property. The next section presents one algorithm that can be used to construct parse tables. Programs that automate this construction are widely available. However, the table-construction algorithm requires scrupulous bookkeeping it is a prime example of the kind of task that should be automated and relegated to a computer. The compiler writer can build and tables by hand. While the number of such sequences is huge, the grammar itself constrains the order in which reductions can occur. The tables encode all of the legal reduction sequences that can arise in a reverse rightmost derivation for the given grammar. The key to lr parsing lies in the construction of the and tables. Cooper, Linda Torczon, in Engineering a Compiler (Third Edition), 2023 Using LR Parsers Thus, the additional cost is directly proportional to the number of terminal symbols in the program.

This adds an extra reduction for every terminal symbol. For example, to perform an action whenever the parser shifts the terminal symbol Bit, a compiler writer can add a productionĪnd replace every occurrence of Bit with ShiftedBit. To force actions on shifts, a compiler writer can either move them into the scanner or add a production to hold the action. When the first part reduces, the parser invokes the action.

A higher-level production that sequences the first part, then the second part, is added. To reduce in the middle of a production, they can break the production into two pieces around the point where the action should execute. To accomplish this, compiler writers can transform the grammar so that it performs a reduction at each point where an action is needed. Compiler writers might also need to perform an action in the middle of a production or on a shift action.

Writing a compiler