How can I use map with a string got from getLine?[Haskell] - haskell

Question

I want to read a String and toUpper all the characters.
import Data.Char
main = do
a <- getLine
b <- getLine
map toUpper a
if (a == b)
then print 0
else if (a < b)
then print (-1)
else print 1
Then I got this
Couldn't match expected type `IO a0' with actual type `[b0]'
In the return type of a call of `map'
In a stmt of a 'do' expression: map toUpper a
In the expression:
do { a <- getLine;
b <- getLine;
map toUpper a;
if (a == b) then
print 0
else
if (a < b) then print (- 1) else print 1 }
Hou can I use map with a String got from getLine?
Or there is another way to read a String and toUpper all the characters ?

Answer 1

You are not assigning the "result" of your map call to anything at all. This is causing the type error you are getting, which is telling you that you are trying to return a string (the result of the map call), when it really needs to be some IO type.
A direct fix would look something like this:
import Data.Char
main = do
a <- getLine
b <- getLine
let c = map toUpper a
if (c == b)
then print 0
else if (c < b)
then print (-1)
else print 1
If you use fmap you can toUpper all the chars and get the line of input at the same time (preventing the need for a c).
import Data.Char
main = do
a <- fmap (map toUpper) getLine
b <- getLine
if a == b
then print 0
else if a < b
then print (-1)
else print 1

Answer 2

Others have corrected your program in the minimal way, but I want to point out a C-ism that Haskell has improved:
if (a == b)
then print 0
else if (a < b)
then print (-1)
else print 1
Has it ever bothered you that numbers were appropriated for recording how a thing compared? It's certainly bothered me. Fortunately, defining new data types in Haskell is so cheap that we do it all the time. In the standard library, there's a type defined as follows:
data Ordering = LT | EQ | GT
And there's a standard function
compare :: Ord a => a -> a -> Ordering
So why not use this beautiful Haskell artifact?
main = do
a <- getLine
b <- getLine
print (compare (map toUpper a) b)

Answer 3

in Haskell it's a good practice to separate non-monadic code from monadic one.
A very minimal improvement is to move print outwards:
print $ if (a == c)
then 0
else if (a < b)
then (-1)
else 1
As for an idiomatic solution, think of separating all non-monadic code in a separate function (both comparison and uppercasing).
Also, if you see elsif in your code, think about guards:
c_compare a c
| a == c = 0
| a < c = -1
| otherwise = 1
A case implementation is also possible:
c_compare a c = case (compare a c) of
LT -> -1
EQ -> 0
GT -> 1

Answer 4

Remember, everything in Haskell is immutable, so calling map toUpper a doesn't actually modify a. If you'd like to save that result, you'll have to bind it to a variable in a let clause. So you might want to change your code to something like this:
import Data.Char
main = do
a <- getLine
b <- getLine
let c = map toUpper a
if (a == c)
then print 0
else if (a < b)
then print (-1)
else print 1