CODEWITHSUNDEEP
listhaskell
user2415706
user2415706

Reputation: 982

Why does adding the error case break my implementation of last?

This code works perfectly fine:

last' :: [a] -> a
last' (x:[]) = x
last' (x:xs) = last xs

But if I try to add:

last' [] = error "Empty list"

Anywhere. I get this error: 

"Couldn't match type 'a' with [Char] -> a0' 
'a' is a rigid type variable bound by the type signature for last' :: [a] -> a
In the expression: error 
in any equation for last': last [] = error "Empty list"

Why is this? My implementations of head, tail and init did not scream when I put in the case for the empy list.

I'm an idiot. I had typos. Thanks!

Upvotes: 0

Views: 66

Answers (1)

Sibi
Sibi

Reputation: 48766

Adding error is not breaking your code. This should be the correct implementation:

last' :: [a] -> a
last' (x:[]) = x
last' (x:xs) = last' xs  -- xs not x
last' [] = error "Empty list"

Upvotes: 2

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