Converting lowercase letters to capitals

问题

I am a newbie in Haskell and have some problem of defining a function that would convert all small letters to capital and leave the rest intact.

I tried solving this question in my book so far:

capitalise :: String -> String
capitalise xs = [capitalise2 ch| ch<-xs]

capitalise2 :: Char -> Char
capitalise2 ch 
    | isLower ch    = chr (ord ch - 32)
    | otherwise    = ch

I am getting errors:

p3.hs:6:7: Not in scope: `isLower'
p3.hs:6:23: Not in scope: `chr'
p3.hs:6:28: Not in scope: `ord'

Any help would be much appreciated.

回答1:

First, you need to import Data.Char to use those functions it is complaining about.

Right, you are missing the otherwise case in the new function. Try it with an if .. then .. else construct. Experienced Haskellers do not use that construct very much; I would probably do it with a helper function:

capitalize cs = [ toUpper c | c <- cs ]
    where
    toUpper ...

which is pretty much the same as what you already have, the main difference being the scope of the helper function.

回答2:

Has the book explained the standard library yet?

import Data.Char (toUpper)

capitalise  =  map toUpper

回答3:

You need to make isLower part of the expression, instead of using it as a filter.

[if isLower ch then chr (ord ch - 32) else ch | ch <- xs]

Or, move the helper function inside.

capitalise = map capitalise'
             where capitalise' ch
                      | isLower ch = chr (ord ch - 32)
                      | otherwise  = ch

来源：https://stackoverflow.com/questions/8700564/converting-lowercase-letters-to-capitals

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haskell