[HASKELL]Couldn't match expected type `[a0]' with actual type `[a1] -> Bool'

Question

I`m trying to write a function for reading line by line from a file:

readMyFile = do 
          contents <- readFile "input.txt"
          if(null sStringV == True)
                then do
                    let sStringV = lines contents
                    let sString = head sStringV
                    let sStringV = tail sStringV
                    return sString
                else do
                    let sString = head sStringV
                    let sStringV = tail sStringV
                    return sString

and I declared sStringV as null

sStringV    = null

When I compile this code I'm getting the following error.

Couldn't match expected type `[a0]' with actual type `[a1] -> Bool'
In the first argument of `null', namely `sStringV'
In the first argument of `(==)', namely `null sStringV'
In the expression: (null sStringV == True)

I don't understand where my problem is...

Answer 1

null is a function [a] -> Bool and returns whether the input list is empty. Therefore sStringV has type [a] -> Bool .

In the line if (null sStringV == True)

the argument to null should be a list, not the null function itself.

It seems you should change the declaration of sStringV to something like

sStringV :: String
sStringV = ""

However, you should be aware that let sStringV = lines contents does not assign a new value to sStringV - it only declares a new variable sStringV which hides the old definition. You can't modify sStringV from within your readMyFile function.

It looks like you're trying to use Haskell like an imperative language.

Answer 2

null() does not test whether a variable is null. null() tests whether a list is empty. The key word is list , ie you have to call null on a list. So you have two choices:

1) You can call null() on an empty list:

null []  -->True

2) You can call null() on a list that contains something:

null [1, 2, 3]  --> False

Also note that writing:

if(null sStringV == True)

is redundant. null() takes a list as an argument and returns True if the list is empty, and False if the list contains something. Therefore, all you need to write is:

if(null sStringV)
then do ....   --executed if sStringV is an empty list

else do ...    --excuted if sStringV is a list that contains something

Here is an example:

dostuff:: [a] -> IO () 
dostuff alist = if null alist
                then putStrLn "hello"
                else putStrLn "goodbye"

ghci>:l 1.hs
[1 of 1] Compiling Main             ( 1.hs, interpreted )
Ok, modules loaded: Main.
ghci>dostuff []
hello
ghci>dostuff [1, 2, 3]
goodbye
ghci>