Help implementing List.head : how to throw an exception

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#1

Hi,

I am learning Haskell and I come from an imperative programming language background.

I am trying to re implement some functions in the Data.List ghc library like head :

head :: [a] -> a

But when ran with an empty list it should throw an exception :

Prelude> head []
*** Exception: Prelude.head: empty list

This is fairly simple but I have this :

head :: [a] -> a
head [] = ??
head (x:xs) = x

How can I implement something similar in Haskell? I tried to look at the source code but it uses rather complicated structures and use a raise# function that I can’t find in the documentation.

Thanks! :wink:

#2

Hello Joseph,
you definitely can. The simplest way is to use error:

> :t error
error :: String -> a

and if you want to get more fancy, take a look at throw and friends in in Control.Exception.

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#3

Thanks for the answer, this is what I wanted!

#4

I have another question, if I want to print the result of the head function like this :

module List where

head :: [a] -> a
head [] = error "List is empty"
head (x:xs) = x

module Main where

import List

main :: IO ()
main = print (List.head [])

It gives me this :

Main.hs:7:8: error:
    • Ambiguous type variable ‘a0’ arising from a use of ‘print’
      prevents the constraint ‘(Show a0)’ from being solved.

How can I solve that?

By the way the error function takes a String parameter and return a value of type a but what is this value, it means it can be any type?

#5

The ambiguous type variable error is due to the fact that you are applying head to an emty list. This could be an empty list of any type. [] :: [Int] is an empty list of Int while []::[Char] is an empty list of Chars.

For the second question, error produces an exception and “returns” bottom, which is a special value that inhabits every type. I would suggest to read the first few paragraphs of this article.

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