Is everything in Haskell stored in thunks, even simple values?

Question

What do the thunks for the following value/expression/function look like in the Haskell heap?

val = 5                -- is `val` a pointer to a box containing 5?         


        

Answer 1

Short answer: Yes.

Long answer:

val = 5

This has to be stored in a thunk, because imagine if we wrote this anywhere in our code (like, in a library we imported or something):

val = undefined

If this has to be evaluated when our program starts, it would crash, right? If we actually use that value for something, that would be what we want, but if we don't use it, it shouldn't be able to influence our program so catastrophically.

For your second example, let me change it a little:

div x y = x / y

This value has to be stored in a thunk as well, because imagine some code like this:

average list =
  if null list
     then 0
     else div (sum list) (length list)

If div was strict here, it would be evaluated even when the list is null (aka. empty), meaning that writing the function like this wouldn't work, because it wouldn't have a chance to return 0 when given the empty list, even though this is what we would want in this case.

Your final example is just a variation of example 1, and it has to be lazy for the same reasons.

All this being said, it is possible to force the compiler to make some values strict, but that goes beyond the scope of this question.