CODEWITHSUNDEEP
haskell
w13rfed
w13rfed

Reputation: 375

Haskell Printing Strings

How do you print Strings in Haskell with newline characters?

printString :: String -> String -> String -> String
printString s1 s2 s3 = (s1++"\n"++s2++"\n"++s3)

When using the function it prints the entire line including the newline characters as well

Upvotes: 1

Views: 10995

Answers (3)

MathematicalOrchid
MathematicalOrchid

Reputation: 62808

Your printString function does not print a string, it simply returns a string. Because you're running this in GHCi, it gets printed out. But it's GHCi that's printing it; your function itself prints nothing.

If you were to compile some code that calls printString, you would discover that nothing gets printed.

By default, GHCi prints stuff as expressions. If you want to write the string to the console unaltered, you need to use putStrLn, as the other answers suggest. Compare:

Prelude> print "1\n2"
"1\n2"

Prelude> putStrLn "1\n2"
1
2

Basically when you write an expression in GHCi, it automatically calls print on the result if possible. Because if you're executing something, you probably want to see what the result was. Compiled code doesn't do this; it only prints what you explicitly tell it to.

Upvotes: 0

leftaroundabout
leftaroundabout

Reputation: 120711

As already commented, your code has nothing to do with printing. (It shouldn't have print in it's name either!) Printing is a side-effectful action (it makes something appear observably on your screen!) and hence can't be done without binding in the IO type.

As for the actual task of generating a single three-line string, that is perfectly fulfilled by your suggested solution. "This\nis\ntest" is a three-line string, as witnessed by

Prelude> lines $ printString "This" "is" "test"
["This","is","test"]

The reason why GHCi doesn't actually output three separate lines when you just write

Prelude> "This\nis\ntest"
"This\nis\ntest"

is that the print function that's used for this purpose always guarantees a format that's safe to use again in Haskell code, hence it puts strings in quotes and escapes all tricky characters including newlines.

If you simply want to dump a string to the terminal as-is, use putStrLn instead of print.

Prelude> putStrLn $ printString "This" "is" "test"
This
is
test

Upvotes: 5

jamshidh
jamshidh

Reputation: 12060

In ghci, you can just type

putStrLn "line1\nline2"

If you want to write a program to do this, you need to make sure that putStrLn runs in the IO monad, for instance, by putting it in main

main = do
  <do stuff>
  putStrLn "line1\nline2"
  <do other stuff>

Upvotes: 1

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