Haskell ReplicateM IO

Question

I'm trying to create a function which allows the user to input a list of strings. The function takes the length and allows the user to input length-1 more lines. Then each line is checked to ensure it is the same length as the original line. However, I'm having a few problems and that I can't find a solution to.

The problems are that I can input more than count-1 lines and the length isn't being calculated as I expected.. for example if I input ["12","13"] and then ["121","13"] the error is given, although they are the same length!

read :: IO [Line]
read = do
  line <- getLine
  let count = length line
  lines <- replicateM (count-1) $ do
    line <- getLine
    if length line /= count
    then fail "too long or too short"
    else return line
  return $ line : lines

Line is of type String.

readLn gives a parse error.

Your code works fine for me (except indentation issue with if-then-else block). — Matvey Aksenov, Mar 12 '12 at 11:24
As clarification, is the first line of input meant to be a number that tells you how many other lines there are? Or is that number actually based on the length (number of characters) of the first line as you have written here? — huon, Mar 12 '12 at 11:25
The strings `["12","13"]` and `["121","13"]` don't have the same length; the latter has one more character than the first. Do you want to read strings, or lists of strings? — danr, Mar 12 '12 at 11:29
The number is based on the length of the first line, although looking at the comments I think an issue could be with my use of length. Is it based on characters? In prelude length ["12","13"] and ["121","13"] both return 2. — gdrules, Mar 12 '12 at 11:32
As Daniel Wagner reports in his answer, from `getLine` you get a `String`, so length is the number of characters in the string. To try this in ghci, try `length (show ["12","13"])`. — danr, Mar 12 '12 at 13:26

score 5 · Accepted Answer · answered Mar 12 '12 at 11:35

It sounds to me like you're getting confused about the difference between getting a line as a String and reading/parsing a line of input as a custom type. You are using getLine, which always returns exactly the String that the user types. Compare:

Prelude> fmap length getLine
["12","13"]
11
Prelude> length "[\"12\",\"13\"]" -- explanation of the 11
11
Prelude> fmap length (readLn :: IO [String])
["12","13"]
2
Prelude> length ["12", "13"] -- explanation of the 2
2

As demonstrated here, you probably want to use readLn, which first gets a line of input and then parses it with read.

-- defined in the Prelude
readLn = do
    s <- getLine
    return (read s)

If I modify your code to include the imports and definitions below:

import Control.Monad
type Line = [String]

...and to call readLn instead of getLine, then I can type the literal lines ["12","13"] and ["121","13"] without errors.

Thanks for the explanation, I see now - although my type Line = String and I can't change it to [String] as it is used in various other functions I've defined. So when using readLn in this instance a parse error is returned — gdrules, Mar 12 '12 at 11:45
@gdrules: so change your other functions. Or add some glue code. — rampion, Mar 12 '12 at 13:27

Haskell ReplicateM IO

1 Answers1