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C H A P T E R 16
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Lexing and Parsing
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In this chapter, you take a closer look at lexing and parsing, topics introduced briefly in s 9 and 11. In particular, the chapter introduces the lexer and parser generators, fslex and fsyacc, that come with the F# distribution. A typical scenario when these techniques and tools can come in handy is the following: You want to read user-readable input that has a well-defined syntax. You have a type (often an abstract syntax tree [AST] type) to represent this input.
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The typical task is to parse the user input into your internal representation by breaking down the input string into a sequence of tokens (a process called lexical analysis) and then constructing an instance of your internal representation based on a grammar (via syntactic analysis). Lexing and parsing don t have to be separated, and there are often convenient .NET methods for extracting information from text in particular formats, as shown in this chapter. Nevertheless, it s often best to treat the two processes separately. The goal of this chapter is to provide the background you need to use the built-in lexing and parsing facilities of .NET and F# effectively and to understand the options for lexing and parsing other input formats: For lexing, we cover simple line-based techniques to crack data formats using the .NET libraries directly. You then learn how to use fslex to break text into simple tokens, strings with escape characters, and nested comments, and you see tablebased token generation and stateful lexing (passing state as a parameter as opposed to using mutable state). For parsing, we cover the core parsing topics of languages, grammar formalisms, and various parser types. We explain the typical problems for recursive-descent parsing and introduce fsyacc by giving a parser for Kitty, a small BASIC-like language. This highlights how to parse lists of symbols, how to assign precedence andassociativity to your tokens and production rules, and how you can recover from parsing errors. You also learn about conflicts that can arise in the fsyacc specifications and how to resolve them.
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Finally, the chapter covers combinator-based techniques, which are particularly useful for writing parsers for binary formats without relying on fslex and fsyacc.
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CHAPTER 16 LEXING AND PARSING
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Lexical analysis is concerned with identifying those tokens that make up a given input. A token is a piece of the input text that constitutes a word from the lexer s perspective. This can be a number, an identifier, a special word, or any sequence of characters deemed to make a unit. During syntax analysis, you check whether the input (a series of tokens) is structured according to a set of grammar rules that make up your language. For instance, the F# construct let a = b*2 in .. is syntactically correct, but it s a semantically valid expression only if the variable b is bound in the preceding scope. The notion of scope and binding depends on the semantics of your language, and these are the topics of interest for semantic analysis. In a typical compiler, for instance, source programs go through the following stages: Lexing Parsing Semantic analysis Optimization(s)/Transformations Code generation s 9 and 11 cover examples of semantic analysis and optimization/transformation.
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A common, simple case of parsing and lexing occurs when you re working with an existing line-based text-file format. In this case, parsing is often as easy as splitting each line of input at a particular separator character and trimming whitespace off the resulting partial strings: > let line = "Smith, John, 20 January 1986, Software Developer";; val line : string = "Smith, John, 20 January 1986, Software Developer" > line.Split [| ',' |];; val it : string [] = [|"Smith"; " John"; " 20 January 1986"; " Software Developer"|] > line.Split [| ',' |] |> Array.map (fun s -> s.Trim());; val it : string [] = [|"Smith"; "John"; "20 January 1986"; "Software Developer"|] You can then process each column in the data format: let splitLine (line: string) = line.Split [| ',' |] |> Array.map (fun s -> s.Trim()) let parseEmployee (line: string) = match splitLine line with | [| last; first; startDate; title |] -> last, first, System.DateTime.Parse(startDate), title | _ -> failwithf "invalid employee format: '%s'" line The type of this function is as follows: val parseEmployee : string -> string * string * System.DateTime * string
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