[Haskell-cafe] Why doesn't this consume all the computer's memory?

[Tom Ellis]

I must admit I'm stumped!  I don't see any significant difference between
those two programs.

On Thu, Nov 08, 2018 at 11:04:34AM -0500, Tyson Whitehead wrote:
> Constant memory code (RES 6MB):
> 
>    {-# LANGUAGE ViewPatterns #-}
> 
>    module Main (main) where
> 
>    import Data.Either
> 
>    (length -> lx,length -> ly) = partitionEithers (repeat $ Left ())
> 
>    main = do
>        print lx
>        print ly
> 
> Unbounded memory:
> 
>    module Main (main) where
> 
>    import Data.Either
> 
>    (xs, ys) = partitionEithers (repeat $ Left ())
> 
>    main = do
>        print $ length xs
>        print $ length ys
> 
> Cheers!  -Tyson
> 
> PS:  The constant-memory view-pattern version seems to compile down to
> 
>    lxly = case partitionEithers (repeat $ Left ()) of
>      (xs,ys) -> (length xs,length ys)
> 
>    main = do
>        print (case lxly of (lx,_) -> lx)
>        print (case lxly of (_,ly) -> ly)
> 
> while the unbounded-memory non-view-pattern one compiles down to
> 
>    xsys = partitionEithers (repeat $ Left ())
>    xs = case xsys of (xs,_) -> xs
>    ys = case xsys of (_,ys) -> ys
> 
>    main = do
>        print (length xs)
>        print (length ys)