parser

In other words, it can create a valid parse tree even upon syntax error.
This means a handwritten parser can detect and handle multiple syntax errors simultaneously.
In generated parsers, this could be achieved to a certain extend with extensive customizations, but is probably not able to fully support resilient parsing.

However, the benefit of abstracting out the reader from the lexer is that, based on the input, we are able to plug-in different readers, to the same lexer.
And the lexer doesn’t have to worry about handling various kinds of inputs.
Can support incremental parsing — Parse only a part of the code, upon an update to the foundation.
However, it doesn’t have the benefits of a handwritten parser have .
An easy task to maintain — Updating the grammar rule and regenerating the parser is all you must do.

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from the input, and remove it from the input.
This means, calling the consume() method multiple times will return a fresh character at each invocation.

• However, due to the complication like these, the parser is also the most complex component to implement in a parser-architecture.
• However, the benefit of abstracting out the reader from the lexer is that, depending on the input, we can plug-in different readers, to exactly the same lexer.
• Often, an abstract syntax tree is confused with a parse/syntax tree.

Leaf-nodes or the terminal-nodes are treated as a special kind of nodes, which is called a ‘token’.
A parser is the initial thing that involves our mind whenever we speak of compiler-development/compiler-construction.
Rightly so, a parser plays an integral role in a compiler architecture and may also be considered because the entry-point to a compiler.
Before we enter the details of how to write a parser, let’s see what

What Is Parsing

The lexer reads characters from the input/character reader and produces tokens.
Usually, the implementation of the lexer is slightly complex compared to the character-reader, but easier than the parser.
Given the point that our objective would be to implement a parser that’s both resilient and provides proper error messages, it is quite an important facet of the parser to properly handle syntax errors.
A syntax error is really a case where an unexpected token is reached, or basically, the next token will not match the defined grammar, during the parsing.

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The parser is responsible for reading the tokens from the lexer and producing the parse-tree.
It gets the next token from the lexer, analyzes it, and compare it against a defined grammar.
Then decides which of the grammar rule should be considered, and continue to parse in line with the grammar.
However, this is not always very straightforward, as sometimes it is not possible to find out which way to take only by considering the next token.
Thus, the parser may need to check a few tokens in to the future to as well, to decide the road or the grammar rule to be considered.

Peek()/peek — Obtain the next character /next k-th character from the input.
This is used to look ahead the characters without consuming/removing them from the input stream.
Calling the peek() method more than once will return exactly the same character.
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It is also possible to embed the input-reader capabilities to the lexer itself.

The Parser

However, because of the complication like these, the parser can be the most complex element of implement in a parser-architecture.
Despite the fact that a parser often refers to as a single component inside a compiler architecture, it consists of several components, including however, not limited by a lexer, a parser, and many other abstractions such as for example input/character reader and an error handler.
The below diagram shows the components and how they’re connected to one another, inside our parser implementation.
As discussed in the previous section, the aim of an error handler is to recover upon a syntax error.