datamatrix net example type stmt = | Assign | While | Seq | IfThen | IfThenElse | Print in VB.NET

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type stmt = | Assign | While | Seq | IfThen | IfThenElse | Print
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type prog = Prog of stmt list
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CHAPTER 16 LEXING AND PARSING
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The Lexer for Kitty
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Listing 16-6 shows a lexer for the language in the file KittyLexer.fsl. It s similar to lexers developed earlier in this chapter. The one exception is that you use a keyword table. Matching against lexemes to identify tokens is a sensible solution only if there are relatively few cases. Tokenizing a large set of keywords and operators using explicit rules can lead to large lexers. This situation is often handled using tables that contain the possible lexeme matches and the tokens to be returned. Listing 16-6 uses simple dictionaries (maps). Listing 16-6. KittyLexer.fsl: Lexer for Kitty { module KittyLexer open System open KittyParser open Microsoft.FSharp.Text.Lexing let lexeme = LexBuffer<_>.LexemeString let ids = [ ("while", ("begin", ("end", ("do", ("if", ("then", ("else", ("print", WHILE); BEGIN); END); DO); IF); THEN); ELSE); PRINT);]
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let idsMap = Map.ofList ids let ident lexbuf tokenText = if Map.containsKey tokenText idsMap then Map.find tokenText idsMap else ID tokenText } let let let let let let num alpha ident integer whitespace newline = = = = = = ['0'-'9']+ ['a'-'z' 'A'-'Z'] alpha+ (alpha | ['_' '$'])* '-' num ' ' | '\t' '\n' | '\r' '\n' token lexbuf } lexbuf.EndPos <- lexbuf.EndPos.NextLine; token lexbuf } LPAREN } RPAREN } PLUS } MINUS } TIMES } SEMI } ASSIGN } ident lexbuf (lexeme lexbuf) } INT (Int32.Parse(lexeme lexbuf)) } EOF }
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rule token = parse | whitespace { | newline { | "(" { | ")" { | "+" { | "-" { | "*" { | ";" { | ":=" { | ident { | integer { | eof {
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CHAPTER 16 LEXING AND PARSING
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Note that at compilation time, the lexer depends on the parser defined later in Listing 16-7. This is because the lexer must return the type of tokens required by the parser. You can generate the lexer by calling fslex: fslex KittyLexer.fsl --unicode This produces KittyLexer.fs, which contains the implementation of the lexer.
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The Parser for Kitty
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Listing 16-7 shows the parser specification for the Kitty language in the file KittyParser.fsy. Listing 16-7. KittyParser.fsy: Parser for Kitty %{ open Ast %} // The start token becomes a parser function in the compiled code. %start start // These are the terminal tokens of the grammar along with the types of // the data carried by each token: %token <string> ID %token <int> INT %token PLUS MINUS TIMES LPAREN RPAREN IF THEN ELSE %token WHILE DO BEGIN END PRINT SEMI ASSIGN EOF // Associativity and Precedences - Lowest precedence comes first %left PLUS MINUS %left TIMES // This is the type of the data produced by a successful reduction // of the 'start' symbol: %type <prog> start %% start: Prog Prog: StmtList Expr: | | | | | ID INT Expr PLUS Expr Expr MINUS Expr Expr TIMES Expr LPAREN Expr RPAREN { $1 } { Prog (List.rev $1) } { { { { { { Val $1 } Int $1 } Plus ($1, $3) } Minus ($1, $3) } Times ($1, $3) } $2 } { Assign ($1, $3) } { While ($2, $4) }
Stmt: ID ASSIGN Expr | WHILE Expr DO Stmt
CHAPTER 16 LEXING AND PARSING
| | | |
BEGIN StmtList END { IF Expr THEN Stmt { IF Expr THEN Stmt ELSE Stmt { PRINT Expr {
Seq (List.rev $2) } IfThen ($2, $4) } IfThenElse ($2, $4, $6) } Print $2 }
StmtList: | Stmt { [$1] } | StmtList SEMI Stmt { $3 :: $1
You can generate the parser by calling fsyacc: fsyacc KittyParser.fsy --module KittyParser This produces KittyParser.fs, which contains the implementations of the parser, along with a signature file KittyParser.fsi. The generated parser has one entry point for each %start symbol (here there is only one). The type of this entry point is as follows: val start : (LexBuffer<'a> -> token) -> LexBuffer<'a> -> Ast.prog You see how to use this function a little later in this chapter. In a roundabout way, the type says, If you give me a lexing function that generates tokens and give me a LexBuffer to supply to that lexing function, then I ll generate you an Ast.prog. The generic type variable indicates that the parser is independent of the characters manipulated by the LexBuffer. We now cover in more detail the different aspects of fsyacc parsing illustrated by this example.
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