[Haskell] Haskell [x] and x notation - as-pattern example
Chernin, Nadav
CherninN at corning.com
Wed Apr 3 11:05:49 CEST 2013
Hi,
Write firstLetter all@(x:xs) = "The first letter of " ++ all ++ " is " ++ [x] ++ " otherbit " ++ xs
Nadav
From: haskell-bounces at haskell.org [mailto:haskell-bounces at 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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