Just use GHCi's :print command =). Prelude> let xs = [1..42] :: [Int]; x = 17; ys = [ y | y <- xs, y >= x ] Prelude> :print xs xs = (_t1::[Int]) Prelude> :print ys ys = (_t2::[Int]) Initially nothing is evaluated (as expected). Let's try forcing ys to WHNF: Prelude> ys `seq` () -- forcing to WHNF () Prelude> :print xs xs = 1 : 2 : 3 : 4 : 5 : 6 : 7 : 8 : 9 : 10 : 11 : 12 : 13 : 14 : 15 : 16 : 17 : (_t3::[Int]) Prelude> :print ys ys = 17 : (_t4::[Int]) So xs had to be forced until the first element of ys appeared (since WHNF decides between [] and _:_). We didn't explicitly evaluate ys's first element but as it needed to be evaluated before it already appears evaluated now. Same thing if you continue forcing ys: Prelude> tail ys `seq` () -- forcing tail to WHNF () Prelude> :print xs xs = 1 : 2 : 3 : 4 : 5 : 6 : 7 : 8 : 9 : 10 : 11 : 12 : 13 : 14 : 15 : 16 : 17 : 18 : (_t5::[Int]) Prelude> :print ys ys = 17 : 18 : (_t6::[Int]) Cheers, -- Felipe.