Understanding thunks with Fibonacci:

let fib = 0 : 1 : zipWith (+) fib (tail fib)
take 10 fib  -- [0,1,1,2,3,5,8,13,21,34]

What are Thunks?

In Haskell, (4+2,7) is stored as (_,7) where _ is the thunk - a value not yet computed.

let x = (4+2,7)
:print x  -- ((_t1::Integer),7)

snd x  -- 7
:print x  -- ((_t2::Integer),7)  -- still not evaluated!

fst x  -- 6
x  -- (6,7)  -- now it's evaluated

Thunks are only evaluated when needed. Partial evaluation creates more thunks - values are computed lazily on demand.

From haskell-rascal.