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 <50295@web.de> 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

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