70 lines
2.0 KiB
Factor
70 lines
2.0 KiB
Factor
USING: accessors kernel locals math math.parser peg.ebnf ;
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IN: rosetta-code.arithmetic-evaluation
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! http://rosettacode.org/wiki/Arithmetic_evaluation
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! Create a program which parses and evaluates arithmetic
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! expressions.
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! Requirements
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! * An abstract-syntax tree (AST) for the expression must be
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! created from parsing the input.
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! * The AST must be used in evaluation, also, so the input may not
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! be directly evaluated (e.g. by calling eval or a similar
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! language feature.)
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! * The expression will be a string or list of symbols like
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! "(1+3)*7".
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! * The four symbols + - * / must be supported as binary operators
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! with conventional precedence rules.
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! * Precedence-control parentheses must also be supported.
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! Note
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! For those who don't remember, mathematical precedence is as
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! follows:
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! * Parentheses
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! * Multiplication/Division (left to right)
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! * Addition/Subtraction (left to right)
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TUPLE: operator left right ;
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TUPLE: add < operator ; C: <add> add
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TUPLE: sub < operator ; C: <sub> sub
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TUPLE: mul < operator ; C: <mul> mul
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TUPLE: div < operator ; C: <div> div
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EBNF: expr-ast
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spaces = [\n\t ]*
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digit = [0-9]
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number = (digit)+ => [[ string>number ]]
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value = spaces number:n => [[ n ]]
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| spaces "(" exp:e spaces ")" => [[ e ]]
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fac = fac:a spaces "*" value:b => [[ a b <mul> ]]
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| fac:a spaces "/" value:b => [[ a b <div> ]]
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| value
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exp = exp:a spaces "+" fac:b => [[ a b <add> ]]
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| exp:a spaces "-" fac:b => [[ a b <sub> ]]
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| fac
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main = exp:e spaces !(.) => [[ e ]]
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;EBNF
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GENERIC: eval-ast ( ast -- result )
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M: number eval-ast ;
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: recursive-eval ( ast -- left-result right-result )
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[ left>> eval-ast ] [ right>> eval-ast ] bi ;
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M: add eval-ast recursive-eval + ;
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M: sub eval-ast recursive-eval - ;
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M: mul eval-ast recursive-eval * ;
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M: div eval-ast recursive-eval / ;
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: evaluate ( string -- result )
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expr-ast eval-ast ;
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