How to program haskell with ghci?

Question

I've been reading some material and here I have a question: I saw a fragment of code is like this:

>getNthElem 1 xs = head xs
>getNthElem n [] = error "'n' is greater than the length of the list"
>getNthElem n xs
>    | n < 1     = error "'n' is non-positive"
>    | otherwise = getNthElem (n-1) (tail xs)

Should I type all these lines exactly the same into ghci or should I create a .hs file and put them in, then load it in the ghci?

Answer 1

There are 2 ways to do this:

1. Use multiline mode within ghci by setting the flag as:

    Prelude> :set +m
    Prelude> let getNthElem 1 xs = head xs
    Prelude|     getNthElem n [] = error "error1"
    Prelude|     getNthElem n xs
    Prelude|                | n < 1 = error "error2"
    Prelude|                | otherwise = getNthElem (n-1) (tail xs)
    Prelude|
    Prelude>       
   

2. Create a file and load it as a module to access the types and functions defined in it as

   Prelude> :l myModule.hs
   

   And file contents:

   getNthElem :: Int -> [a] -> a
   getNthElem 1 xs = head xs
   getNthElem n [] = error "'n' is greater than the length of the list"
   getNthElem n xs
                  | n < 1     = error "'n' is non-positive"
                  | otherwise = getNthElem (n-1) (tail xs)
   

I would recommend using the second option since it's quite easy to mess up indentation in multiline mode within GHCI. Also, make it a habit of adding type signatures before you start defining the function body.