add simple Peggy expression parser
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{-# Language TemplateHaskell, QuasiQuotes, FlexibleContexts #-}
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import Text.Peggy
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import AST
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[peggy|
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top :: AST = expr !.
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expr :: AST
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= expr "+" fact { Add $1 $2 }
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/ expr "-" fact { Sub $1 $2 }
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/ fact
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fact :: AST
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= fact "*" term { Mul $1 $2 }
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/ fact "/" term { Div $1 $2 }
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/ term
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term :: AST
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= "(" expr ")"
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/ number
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number ::: AST
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= [1-9] [0-9]* { IntConst $ read ($1 : $2) }
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|]
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main :: IO ()
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main = print . parseString top "<stdin>" =<< getContents
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