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# arithmetic parser
require "rsec"
include Rsec::Helpers
def arithmetic
calculate = proc do |(p, *ps)|
ps.each_slice(2).inject(p) do |left, (op, right)|
left.send op, right
end
end
num = prim(:double).fail 'number'
paren = '('.r >> lazy{expr} << ')'
factor = num | paren
term = factor.join(one_of_('*/%').fail 'operator').map &calculate
expr = term.join(one_of_('+-').fail 'operator').map &calculate
expr.eof
end
if __FILE__ == $PROGRAM_NAME
print '1+ 2*4 = '
p arithmetic.parse! '1+ 2*4' #=> 9
print '1+ 2*/4 = '
p arithmetic.parse! '1+ 2*/4' #=> syntax error
end
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