All Haskell compilers use pointers internally. The idea is that because Haskell is referentially transparent and side effect free, you can overwrite a function application with its result. For example,
let x = [1..1000] in foo (A x) (B x)
Will internally have "x" pointing to the function application [1..1000], when this function application is evaluated it will overwrite the memory cell "x" points to with the result. So eventually it will look like this:
+---+---+ +---+---+ | A | x | | B | x | +---+---+ +---+---+ | | +---+-------+ | V +---+---+------+ | : | 1 | tail |---> The list 2..1000 +---+---+------+
Where each block is a memory cell and each arrow is a pointer.
What does it mean to have a letter ("A", "x", or a number or "tail", etc) inside a box?
A book that describes this a lot better is:
Simon Peyton-Jones. The implementation of functional programming languages. Prentice-Hall, 1987
Are they always implemented that way these days? -- J