Non-exhaustive patterns error with empty list

Question

I am declaring a new type for memory and then use a function to update it. The program compiles and works fine when I add values to the list, but if my list is empty I get the e

Answer 1

This is because your code doesn't cover the empty list case. In particular this: h:t == [] will never evaluate to True. h:t is a pattern which will only match a non-empty list: it binds h to the head of the list and t to the rest of the list.

So your function needs to handle three cases:

update n x [] = (n,x):[]                        -- empty list
update n x (h:t) | n == fst h = (n,x):t         -- key equal to n
                 | otherwise  = h:update n x t  -- key not equal to n