Roelof Wobben
As a exercise I need to rewrite the tail function into a safetail function by using conditions.
So I made this : http://codepad.org/bKcCUdqy
But as you can see there is a error message.
Can anyone explain me how to solve this ?
Haskell is all about types. To understand Haskell, you really must understand how types work. The square brackets are a syntactic construct to easy constructing lists. As David pointed out, the underlying representation is different. If the types don't make sense, the whole program makes no sense. Let's see: [1, [2, 3, 4]] This cannot be right, because lists in Haskell are homogenous, so all elements have the same type, but here you have a list of two elements, one being 1 (a number) and the other being [2, 3, 4] (a list of numbers). The types obviously don't fit, so the program cannot be compiled. You have this: x = 1 y = [2, 3, 4] To put the element x in front of the list of elements y, you need a function of the following type: a -> [a] -> [a] The (:) function has exactly that type, so you can write: (:) x y or to use the usually more convenient infix syntax: x : y This is about list construction. What you need here however is the other direction: destruction. This is done by pattern matching. Again as David pointed out a list is exactly a chain of (:) applications. The list y would have the following real representation: y = 2 : (3 : (4 : [])) Since (:) is right-associative you can leave out the parentheses to have a slightly more readable equation. You can decompose this list by pattern matching, which gives you access to the building blocks of the list: safeTail (x:xs) = xs The (:) function is the "cons" or "prepended to" function. So safeTail applied to x prepended to xs is equal to xs. One more equation and your safeTail function is done. One further note: I have intentionally left out almost all of the type signatures here, but I strongly advise you to write them. In fact write type signatures before writing the actual functions. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/