[Haskell-cafe] New to Haskell - List Comprehension Question

[minh thu]

2009/10/7 Steven1990 <stevenyoung1990 at msn.com>:
>
> Hi, I'm currently learning Haskell, and I've been trying to work out a
> function for the following problem for a couple of days now.
>
> I want to use a list comprehension method to change the first letter of a
> string to upper case, and the rest of the string to lower case.
>
> Eg: "heLLo" -> "Hello"
>
> As I'm trying to learn this, I would appreciate hints rather than the
> explicit solution if possible? I'm sure I'm close to a solution, I must be
> missing something though. Driving me crazy!
>
> My attempts are something similar to this:
>
> upperCase :: String -> String
> upperCase xs = [toUpper(x):toLower(xs) | x <- xs]
>
> I think 'toLower' expects a single character rather than the list which is
> one place I'm going wrong?

Hi,

try to work little things by little things:

$ ghci
Prelude> let f xs = [x:xs | x <- xs]
Prelude> f "hello"
["hhello","ehello","lhello","lhello","ohello"]
Prelude>

Prelude> :m + Data.Char
Prelude Data.Char> :t toLower
toLower :: Char -> Char
Prelude Data.Char> :t toUpper
toUpper :: Char -> Char
Prelude Data.Char>

So xs is the whole list (the "hello" part of each element in the
resilt of f "hello") and x gets the value of each character. toLower
and toUpper have the same type; indeed toLower expects a single
character while you feed it a list.

The part on the left of the pipe is "executed" for each x drawn from
the xs list. This means that if you want to make something specific to
the first element of xs, you have to provide more information: the x
alone is not enough to know it is the first one or not.

The easiest way to do that is with pattern matching on the upperCase argument:
upperCase (x:xs) = ...

(Don't forget for the "other" case, the empty list:
upperCase [] = ...)

Cheers,
Thu