syntax in expression - Haskell

Question

Im new to Haskell!! I wrote this code:

import Data.List
inputIndex :: [String] -> [String] -> Bool
inputIndex listx input = and [x `elem` listx |x <- input]
inputIndex = if inputIndex == true
          then putStrLn ("ok")

It works fine without the if statement but when I put the if statement the following error is shown:

Syntax error in expression (unexpected `}', possibly due to bad layout)

What am I doing wrong here?

Thanks

Answer 1

A couple of things are wrong here:

• You will need an else clause.
• True must be capitalized.
• inputIndex must always take two arguments (right now it does not, in the last case).

I guess you want something like this...

inputIndex :: [String] -> [String] -> IO ()
inputIndex listx input = if inputIndex' listx input
                             then putStrLn ("ok")
                             else putStrLn ("not ok")
  where
    inputIndex' :: [String] -> [String] -> Bool
    inputIndex' listx input = and [x `elem` listx |x <- input]

(Here I defined a new function with a near-identical name, by appending a prime/apostrophe. By defining it in the where clause, it is only visible to the outer inputIndex function. You can call this a helper-function, if you will. I could also have chosen a completely different name, but I'm uncreative.)

You could also condense this to the following (which is also more general):

allPresent :: (Eq t) => [t] -> [t] -> IO ()
allPresent xs ys = putStrLn (if and [y `elem` xs | y <- ys] then "ok" else "not ok")

Answer 2

1. It's "True", not "true".
2. Your second inputIndex implementation is not compatible with the first one. All your pattern cases for a function must have the same signature ([String] -> [String] -> Bool)
3. The error you show here is not generated by this code, because there is no '}' here.
4. putStrLn has signature String -> IO() while your inputIndex looks like it's supposed to be pure - just return the value and print it somewhere else.