Re: [Haskell-cafe] How to print a string (lazily)

On Jan 3, 2006, at 6:30 PM, Sebastian Sylvan wrote:

On 1/3/06, Daniel Carrera wrote:

...

Neil Mitchell wrote:

...

All Haskell functions are lazy, hence there is no need to "write a lazy version" of your print_list function. I think the function you probably want is:

putStr (unlines xs)

Hhmm... that does work, and I'm a bit surprised that it does. I guess I'm still stuck in the eager computation mindset. I would expect putStr to have to wait for the (unlines xs) to be finished before doing any printing, but it doesn't.

Some day I'll get the hang of this lazy evaluation thing. :)

It does, in a sense, but since unlines is lazy (just like *everything else* in Haskell) it won't actually *do* anything until putStr demands an element from the result.

... and, significantly, putStr will demand those elements one at a time; each element of the list is, in turn, evaluated lazily, producing (a) a next element and (b) a thunk representing the rest of the list. In your example 'join' function, join [] = "" join (x:xs) = (show x) ++ "\n" ++ join xs the caller will demand the first element of the list, which will force the evaluation of (show x) but it will not immediately force the evaluation of (join xs). That part stays as it is, living life as a thunk, until the caller has demanded all the preceding elements and still wants more. hth, Ezra