[Haskell-cafe] Re: bizarre memory usage with data.binary

[Don Stewart]

aeyakovenko:
> Program1:
> 
> module Main where
> 
> import Data.Binary
> import Data.List(foldl')
> 
> 
> main = do
>   let sum' = foldl' (+) 0
>   let list::[Int] = decode $ encode $ ([1..] :: [Int])
>   print $ sum' list
>   print "done"
> 
> vs
> 
> Program2:
> 
> module Main where
> 
> import Data.Binary
> import Data.List(foldl')
> 
> 
> main = do
>   let sum' = foldl' (+) 0
>   let list::[Int] = [1..]
>   print $ sum' list
>   print "done"
> 
> neither program is expected to terminate.  The point of these examples
> is to demonstrate that Data.Binary encode and decode have some strange
> memory allocation patters.

The encode instance for lists is fairly strict:

    instance Binary a => Binary [a] where
        put l  = put (length l) >> mapM_ put l
        get    = do n <- get :: Get Int
                    replicateM n get

This is ok, since typically you aren't serialising infinite structures.

Use a newtype, and a lazier instance, if you need to do this.

-- Don