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wisent: Example

 
 2.2 Example
 ===========
 
 Here is an example to parse simple infix arithmetic expressions.  See
 (bison)Infix Calc, in the Bison manual for details.
 
      '(
        ;; Terminals
        (NUM)
 
        ;; Terminal associativity & precedence
        ((nonassoc ?=)
         (left ?- ?+)
         (left ?* ?/)
         (left NEG)
         (right ?^))
 
        ;; Rules
        (input
         ((line))
         ((input line)
          (format "%s %s" $1 $2))
         )
 
        (line
         ((?;)
          (progn ";"))
         ((exp ?;)
          (format "%s;" $1))
         ((error ?;)
          (progn "Error;")))
         )
 
        (exp
         ((NUM)
          (string-to-number $1))
         ((exp ?= exp)
          (= $1 $3))
         ((exp ?+ exp)
          (+ $1 $3))
         ((exp ?- exp)
          (- $1 $3))
         ((exp ?* exp)
          (* $1 $3))
         ((exp ?/ exp)
          (/ $1 $3))
         ((?- exp) [NEG]
          (- $2))
         ((exp ?^ exp)
          (expt $1 $3))
         ((?\( exp ?\))
          (progn $2))
         )
        )
 
    In the bison-like “WY” format (Wisent Semantic) the grammar
 looks like this:
 
      %token <number> NUM
 
      %nonassoc '=' ;; comparison
      %left '-' '+'
      %left '*' '/'
      %left NEG     ;; negation--unary minus
      %right '^'    ;; exponentiation
 
      %%
 
      input:
          line
        | input line
          (format "%s %s" $1 $2)
        ;
 
      line:
          ';'
          {";"}
        | exp ';'
          (format "%s;" $1)
        | error ';'
          {"Error;"}
        ;
 
      exp:
          NUM
          (string-to-number $1)
        | exp '=' exp
          (= $1 $3)
        | exp '+' exp
          (+ $1 $3)
        | exp '-' exp
          (- $1 $3)
        | exp '*' exp
          (* $1 $3)
        | exp '/' exp
          (/ $1 $3)
        | '-' exp %prec NEG
          (- $2)
        | exp '^' exp
          (expt $1 $3)
        | '(' exp ')'
          {$2}
        ;
 
      %%
 

Info Catalog wisent: Grammar format wisent: Wisent Grammar wisent: Compiling a grammar