datamatrix.net c# example I Note The regular expressions used in the lexer are not the same as the ones used with the .NET BCL in Font

Encode DataMatrix in Font I Note The regular expressions used in the lexer are not the same as the ones used with the .NET BCL

I Note The regular expressions used in the lexer are not the same as the ones used with the .NET BCL
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Regex class. You can find more details about the syntax at http://strangelights.com/FSharp/ Foundations/default.aspx/FSharpFoundations.FSLexRegEx.
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Although the actions can be any valid F# expressions, it s normal to return the token declarations that you will make in the fsyacc file. See the next section, Generating a Parser: Fsyacc, for more information about this. If you want to use the lexer on its own, you will place whatever logic you want to happen here, such as writing the token to the console or storing the token found in a list. The following example shows a file that is capable of tokenizing your little language. You usually do one of two things in the actions. If you re interested in the match, you return a token that has been defined in the parser file. These are the identifiers in block capitals like RPAREN or MULTI. If you re not interested, you call token with the special lexbuf value function to start the parsing again. The lexbuf value is automatically placed in your parser definition and represents the text stream being processed. It is of type Microsoft.FSharp.Tools.FsLex.LexBuffer. Also notice how, in places where you re actually interested in the value found rather than just the fact a value was found, you use a function lexeme from the module Microsoft.FSharp.Compatibility.OCaml.Lexing to get the string representing the match from the lexbuf. Table 11-1 summarizes other useful functions from this module. { open System open Pars open Lexing } let digit = ['0'-'9'] let whitespace = [' ' '\t' ] let newline = ('\n' | '\r' '\n') rule token = parse | whitespace | newline | "(" | ")"
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{ { { {
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token lexbuf } token lexbuf } LPAREN } RPAREN }
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CHAPTER 11 I LANGUAGE-ORIENTED PROGRAMMING
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"*" { MULTI } "/" { DIV } "+" { PLUS } "-" { MINUS } '['[^']']+']' { ID(lexeme lexbuf) } ['-'] digit+('.'digit+) (['e''E']digit+) { FLOAT (Double.Parse(lexeme lexbuf)) } | eof { EOF }
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Table 11-1. Useful Functions from the Lexing Module
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Function
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lexeme lexeme_bytes lexeme_char lexeme_end from_text_reader from_string
Description
Gets the current lexeme from the lexbuf as a string Gets the current lexeme from the lexbuf as a byte array Takes an integer and returns the character at this index from the current lexeme in the lexbuf as a char from the lexbuf as a string Gets the absolute position of the lexeme, which is useful when trying to provide error information to the user Creates a lexbuf from a text reader, which is useful when using the lexer from F# code Creates a lexbuf from a string, which is useful when using the lexer from F# code
A lexer can contain several rules; any further rules are separated from each other by the keyword and, then a name for the rule, then an equals sign, and then the keyword parse. After this come the definitions of the regular expressions that make up this rule. This is often useful if you want to implement comments in your language. Comments often produce false positives in lexers, since they can contain any text. To deal with this, when a start-comment token is detected, it is customary to switch to another rule that looks only for an end-comment token and ignores all other input. The following example shows a simple parser that either finds strings that are like F# identifiers or discards C#-style multiline comments. Notice how when you find the start of a comment, you call the comment function to hop into the comment rule, and to find the end of it, you return unit to hop out of it. { open Lexing } rule token = parse | "/*" { comment lexbuf; token lexbuf } | ['_''a'-'z''A'-'Z']['_''a'-'z''A'-'Z''0'-'9']* { lexeme lexbuf } and comment = parse | "*/" | eof { () } | _ { comment lexbuf }
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