# Two small questions from the section "Making a safe RPN calculator" in LYAH Classic List Threaded 2 messages Reply | Threaded
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## Two small questions from the section "Making a safe RPN calculator" in LYAH

 Dear List, Question 1: In the section "Making a safe RPN calculator" of LYAH (Chapter 14 -- http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions) there is an expression: read numberString:xs, take from the last line of the function      foldingFunction :: [Double] -> String -> [Double]      foldingFunction (x:y:ys) "*" = (x * y):ys      foldingFunction (x:y:ys) "+" = (x + y):ys      foldingFunction (x:y:ys) "-" = (y - x):ys      foldingFunction xs numberString = read numberString:xs My first question is why is the read function called before the cons operator? Question 2: The same section of the book also introduces the reads function which is used to implement the readMaybe function and a refactored foldingFunction      readMaybe :: (Read a) => String -> Maybe a      readMaybe st = case reads st of [(x,"")] -> Just x                                      _ -> Nothing      foldingFunction :: [Double] -> String -> Maybe [Double]      foldingFunction (x:y:ys) "*" = return ((x * y):ys)      foldingFunction (x:y:ys) "+" = return ((x + y):ys)      foldingFunction (x:y:ys) "-" = return ((y - x):ys)      foldingFunction xs numberString = liftM (:xs) (readMaybe numberString) I'd like to know why the foldingFunction returns Nothing in the following example: ghci> foldingFunction [] "1 wawawawa" Nothing Considering that reads "1 wawawawa" does not return Nothing, as follows ghci> reads "1 wawawawa" :: [(Double,String)] [(1.0," wawawawa")] Regards, - Olumide _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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## Re: Two small questions from the section "Making a safe RPN calculator" in LYAH

 1)  Not entirely sure what you mean.  The expression is parsed as (read numberString) : xs, which means that the result number is prepended to the front of the list.2) Because [(x,"")] does not match [(1.0," wawawawa")].  An empty string does not match a string with characters in it.On Wed, Jul 11, 2018 at 1:24 PM, Olumide wrote:Dear List, Question 1: In the section "Making a safe RPN calculator" of LYAH (Chapter 14 -- http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions) there is an expression: read numberString:xs, take from the last line of the function     foldingFunction :: [Double] -> String -> [Double]     foldingFunction (x:y:ys) "*" = (x * y):ys     foldingFunction (x:y:ys) "+" = (x + y):ys     foldingFunction (x:y:ys) "-" = (y - x):ys     foldingFunction xs numberString = read numberString:xs My first question is why is the read function called before the cons operator? Question 2: The same section of the book also introduces the reads function which is used to implement the readMaybe function and a refactored foldingFunction     readMaybe :: (Read a) => String -> Maybe a     readMaybe st = case reads st of [(x,"")] -> Just x                                     _ -> Nothing     foldingFunction :: [Double] -> String -> Maybe [Double]     foldingFunction (x:y:ys) "*" = return ((x * y):ys)     foldingFunction (x:y:ys) "+" = return ((x + y):ys)     foldingFunction (x:y:ys) "-" = return ((y - x):ys)     foldingFunction xs numberString = liftM (:xs) (readMaybe numberString) I'd like to know why the foldingFunction returns Nothing in the following example: ghci> foldingFunction [] "1 wawawawa" Nothing Considering that reads "1 wawawawa" does not return Nothing, as follows ghci> reads "1 wawawawa" :: [(Double,String)] [(1.0," wawawawa")] Regards, - Olumide _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners