Add expression evaluator example for EBNF

extra/peg/expr/authors.txt

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+Chris Double

extra/peg/expr/expr.factor

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+! Copyright (C) 2008 Chris Double.
+! See http://factorcode.org/license.txt for BSD license.
+USING: kernel arrays strings math.parser sequences
+peg peg.ebnf peg.parsers memoize math ;
+IN: peg.expr
+
+: operator-fold ( lhs seq -- value )
+ #! Perform a fold of a lhs, followed by a sequence of pairs being
+ #! { operator rhs } in to a tree structure of the correct precedence.
+ swap [ first2 swap call ] reduce ;
+
+<EBNF
+
+times    = ("*") [[ drop [ * ] ]]
+divide   = ("/") [[ drop [ / ] ]]
+add      = ("+") [[ drop [ + ] ]]
+subtract = ("-") [[ drop [ - ] ]]
+
+digit    = "0" | "1" | "2" | "3" | "4" |
+           "5" | "6" | "7" | "8" | "9" 
+number   = ((digit)+) [[ concat string>number ]]
+
+value    = number | ("(" expr ")") [[ second ]] 
+product = (value ((times | divide) value)*) [[ first2 operator-fold ]]
+sum = (product ((add | subtract) product)*) [[ first2 operator-fold ]]
+expr = sum
+EBNF>
+
+: eval-expr ( string -- number )
+  expr parse parse-result-ast ;

extra/peg/expr/summary.txt

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+Simple expression evaluator using EBNF

extra/peg/expr/tags.txt

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+parsing