CODEWITHSUNDEEP
haskellprintingcompiled
Toby Storli
Toby Storli

Reputation: 5

Haskell print a return value (Yet Another Haskell Tutorial, askForWords)

So I'm trying to print out a line of text to the terminal window using a returned value in a compiled code. The program looks like this:

module Main
    where

import System.IO

main = do
  askForWords

askForWords = do
  putStrLn "Pleas enter a word:"
  word <- getLine
  if word == ""
    then return []
    else do
      rest <- askForWords
      return (word ++ " " ++ rest)

When I run it in the GHCi it works fine

*Main> main 
Pleas enter a word:
Hello
Pleas enter a word:
World
Pleas enter a word:

"Hello World "
*Main>

When I try to run the Unix executable file the program don't print the last string

% /Users/tobyone/Workspace/Haskell/yaht/Yaht ; exit;
Pleas enter a word:
Hello
Pleas enter a word:
World
Pleas enter a word:


[Process completed]

I have tried printing out askForWords in main with putStrLn but get the error

<interactive>:2:10: error:
    Couldn't match type ‘IO [Char]’ with ‘[Char]’
    ...
    ...

Upvotes: 0

Views: 232

Answers (1)

chepner
chepner

Reputation: 532238

You aren't printing the output, you are only returning it. Your executable effectively ignores the return value of the main, which conventionally has type IO () to emphasize this.

module Main
    where

import System.IO

main = do
  result <- askForWords
  putStrLn result

askForWords = do
  putStrLn "Pleas enter a word:"
  word <- getLine
  if word == ""
    then return []
    else do
      rest <- askForWords
      return (word ++ " " ++ rest)

In GHCi (like REPLs in most languages), the value of an expression is printed to the terminal.

main could also be defined more simply as

main = askForWords >>= putStrLn

Upvotes: 5

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