Question

I have written this small function to retrieve the tail of a list:

let getTail l = if length l > 0 then tail l else "empty list"

Passing []to getTail returns empty list but passing [1,2,3] gives the following error:

<interactive>:1:14:
No instance for (Num Char)
  arising from the literal `3'
Possible fix: add an instance declaration for (Num Char)
In the expression: 3
In the first argument of `getTail', namely `[1, 2, 3]'
In the expression: getTail [1, 2, 3]

I can’t understand what that error means. What is the problem? Using GHCi 7.0.4

Answer 1

Let’s think about the type of your getTail function. Initially, we can imagine l to be any list. The return type, however, has to be a String because you sometimes return "empty list". Since that’s part of an if statement where you might also return tail l, it means l has to be a String.

So your getTail function has type String -> String.

When you call getTail with [1,2,3], it expects a String, which is just a [Char]. Remember that numeric literals are overloaded: [1,2,3] is equivalent to calling [fromInteger 1, fromInteger 2, fromInteger 3]. So, given this list, Haskell is trying to get a number from a Char. Since Chars are not part of the Num class, this fails, giving you your error.

A good solution would be to return a Maybe type and give Nothing for the error rather than returning "empty list". So rewrite your function as:

let getTail l = if length l > 0 then Just (tail l) else Nothing

Now its type is [a] -> Maybe [a]. Using this will force anybody using your function to check whether getTail succeeded before being able to use the result.

Another option is to return [] in place of “empty list”. In this case, your getTail function will work exactly like drop 1.