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

Tyson Whitehead twhitehead at gmail.com
Thu Nov 8 16:04:34 UTC 2018

On Thu, 8 Nov 2018 at 01:09, Tyson Whitehead <twhitehead at gmail.com> wrote:
> Sorry for all the noise.  I believe I finally tracked down the eat all the memory/don't eat all the memory trigger.  It is the view pattern.

There was a request to post both codes as it seems a bit unexpected
that a view pattern would make that difference.

Here they are.  I compiled both with `ghc file.hs` using the standard
GHC 8.4.3 from NixOS 18.09.

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)

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