Converting lowercase letters to capitals

Question

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.

Be aware that this sort of strategy will not work in certain languages, because some characters actually become more/less characters when making uppercase/lowercase. — singpolyma, Sep 26 '12 at 14:46

score 17 · Accepted Answer · answered Jan 02 '12 at 12:12

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.

See also Data.Char.toUpper.

This may also be a good opportunity to break free of list comprehensions and start to play with higher order functions. Try writing this function with map instead of a list comprehension.

score 7 · Answer 2 · edited Feb 12 '15 at 08:28

7

Has the book explained the standard library yet?

import Data.Char (toUpper)

capitalise  =  map toUpper

edited Feb 12 '15 at 08:28

Sridhar Ratnakumar

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answered Jan 02 '12 at 12:11

Fred Foo

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@iPC better `import Data.Char`. If you're using the haskell98 module `Char`, you cannot use any modules from `base` with newer GHCs. – Daniel Fischer Jan 02 '12 at 13:06

score 2 · Answer 3 · answered Jan 02 '12 at 12:11

2

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

answered Jan 02 '12 at 12:11

Cat Plus Plus

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Thanks! So I guess I won't be needing use another function right? or do I? – Amjad Jan 02 '12 at 12:21
5

Using `chr`, `ord`, and `-32` is not the right way of doing this. Use `toUpper`. – augustss Jan 02 '12 at 13:20
3

@CatPlusPlus It gives the right result for ASCII, but it's not very descriptive. You should use the proper tool for the job. – augustss Jan 02 '12 at 15:08

Converting lowercase letters to capitals

3 Answers3

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