Map vs Filter over infinite lists?

Question

I'm learning Haskell and i've run into something i can't understand and i can't find an explanation. So, I was testing some functions over infinite lists to see how they worked, and i found a diference between map and filter that I'd like to understand.

Prelude.map definition:

map _ []     = []
map f (x:xs) = f x : map f xs

Prelude.filter definition:

filter _pred []    = []
filter pred (x:xs)
  | pred x         = x : filter pred xs
  | otherwise      = filter pred xs

If I run this:

map (==5) [1..]

The output starts and it never ends, until i stop it. Wich makes sense since the list is infinite.

But now if i run this:

filter (==5) [1..]

I see nothing, not even [5,. Which also makes sense since the list is infinite too, but i want to understand what's the difference beetwen map and filter that makes this. Thank you and sorry for my english!

Edit: I was using tryhaskell.org and that was the problem!

Answer 1

As discovered in the comments, this is due to using Try Haskell. From experimentation, it appears that it will wait for the program to terminate, 3 seconds to elapse, or 1024 characters of output to be produced, before ending and sending you the result. Unfortunately, filter (==5) [1..] will only produce a measly two characters ([5), not meeting the 1024 character limit, and for some reason or another Try Haskell won't send [5 back to you. Running it in a real GHCi should work just fine.

Answer 2

Its related to the buffer settings. When you compile it, the BufferMode of stdin and stdout is by default often set to LineBuffering, printing only on a newline, buffer overflow or manual flush.

GHCi's BufferMode is set to NoBuffering¹, which will print your result immediately.

In case of LineBuffering:

• map (==5) [1..] prints its result, because its large output leads to lots of buffer overflows.
• filter (==5) [1..] never fills the buffer or writes a newline. So its not printed.

¹ if possible. NoBuffering for stdin might not work on Windows cmd, when compiled with GHC.