CODEWITHSUNDEEP
haskellfunctional-programmingtuples
Reed Oei
Reed Oei

Reputation: 1518

How to create a base case for a tuple in Haskell?

I have the following function:

encode_single :: (Eq a) => [a] -> (Int, a)
encode_single (x:xs) = (count xs + 1, x)

However, Haskell complained about needing a base case, but I don't know how to do this because of the generic a type.

Thanks!

Upvotes: 0

Views: 1136

Answers (3)

karakfa
karakfa

Reputation: 67507

if you cannot change the type signature, you can piggyback on head i.e.

encode_single :: [a] -> (Int, a)
encode_single a = (length a, head a)

obviously, empty list input is not handled properly.

Upvotes: 0

J. Abrahamson
J. Abrahamson

Reputation: 74374

Simply enough: at the type you desire you cannot write a total function of that specification. You need to change the type.

You can either add a default a or indicate partiality using Maybe.

encode_single :: a -> [a] -> (Int, a)
encode_single :: [a] -> Maybe (Int, a)

Upvotes: 2

Bakuriu
Bakuriu

Reputation: 101989

First of all, what you received is only a warning, not an error. Haskell does not need the base case of the empty list, it just suggests it.

Partial functions are most often an anti-pattern in functional programming so it just points out something that may be wrong. You can avoid the warning in different ways.

The first one is to make your function safe: if it cannot always return a value it's return type shouldn't be (Int, a) but Maybe (Int, a), so you could do:

encode_single :: (Eq a) => [a] -> Maybe (Int, a)
encode_single [] = Nothing
encode_single (x:xs) = Just (count xs + 1, x)

Otherwise you'd have to return a meaningful value for the empty case (just returning undefined isn't better than not defining that case). It might be appropriate to do somethign like:

encode_single [] = (0, undefined)

However this assumes that any code that uses the result of encode_single will not evaluate the second element of the tuple if the first element is zero (note that if the list isn't empty the first element is always positive, so 0 can be used as a sentinel value).

This may or may not be the case. However one thing is sure: this is not compile-time safe so you may receive some run-time errors when calling such a function.

Upvotes: 5

Related Questions