At the online REPL, you have to write something like:

    let primes = sieve [2..] where sieve (p:xs) = p : sieve [x| x <- xs, x `mod` p /= 0] in primes

Which is rather annoying.

Not only that, but you have to do something like

    let primes = ... in take 5 primes

... if you want the computation to complete without hanging the browser. It would be nice if the REPL supported multi-line expressions.