Hi, Write firstLetter all@(x:xs) = "The first letter of " ++ all ++ " is " ++ [x] ++ " otherbit " ++ xs Nadav
From: haskell-boun...@haskell.org [mailto:haskell-boun...@haskell.org] On Behalf Of Angus Comber Sent: Wednesday, April 03, 2013 12:01 PM To: Haskell Mailing List Subject: [Haskell] Haskell [x] and x notation - as-pattern example I am reading Learn you a Haskell for great good and on page 40 - as-patterns. I have changed the example slightly to be: firstLetter :: String -> String firstLetter "" = "Empty string, oops" firstLetter all@(x:xs) = "The first letter of " ++ all ++ " is " ++ [x] ++ " otherbit " ++ xs Then can use like this: *Main> firstLetter "Qwerty" "The first letter of Qwerty is Q otherbit werty" But I was confused about the difference between [x] and x and why I have to use [x] in the above example. For example if I change to firstLetter :: String -> String firstLetter "" = "Empty string, oops" firstLetter all@(x:xs) = "The first letter of " ++ all ++ " is " ++ x ++ " otherbit " ++ xs I get error: Couldn't match expected type `[Char]' with actual type `Char' In the first argument of `(++)', namely `x' In the second argument of `(++)', namely `x ++ " otherbit " ++ xs' In the second argument of `(++)', namely `" is " ++ x ++ " otherbit " ++ xs' I can use xs to print "werty" but have to use [x] to print "Q". Why is that? What does [x] mean? In the (x:xs) : just delimits each element. so x is the first element. Why can I not print by using x? Also xs is of what type? list of values? So does this mean x is an element and xs must be of type list? Confused...
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