Data.Map.unzip?

[Joachim Breitner]

Hi,


Am Samstag, den 06.12.2014, 13:08 +0100 schrieb Joachim Breitner:
> on a first glance, I agree. But I wouldn’t trust my first glance until I
> investigated the issue.

I investigate the case for lists, which have a specialized unzip:

        unzip    :: [(a,b)] -> ([a],[b])
        {-# INLINE unzip #-}
        unzip    =  foldr (\(a,b) ~(as,bs) -> (a:as,b:bs)) ([],[])
        

I started with defining
> let list1 = map (\c -> (c, Data.Char.ord c)) "hallo"
> let (uz1, uz2) = unzip list1
and then forcing list1 and uz1, but not uz2.

This gave me unzip.pdf, and you can clearly see that uz2 still
references all all of the values, and a whole bunch of thunks and other
annoying stuff.

But: That is not due to unzip, but rather due to the lazy binding in
> let (uz1, uz2) = unzip list1

If I then also force "head uz2", I get unzip_nice.pdf, where indeed uz2,
despite not being forced, does not evaluate anything from uz1 any more.
The same with
> let !(uz1, uz2) = unzip list1

I assume this works due to GHC “Fixing some space leaks with a garbage
collector“.

So my conclusion is: Yes, specialized unzips do behave better behavior,
but exploiting this behavior requires non-trivial knowledge about
Haskell. In particular, the difference between a lazy and a strict tuple
pattern.

(Disclaimer: It is likely that "let (uz1, uz2) = unzip list1" would have
worked in compiled code better than in GHCi due to either strictness
analysis and/or “Fixing some space leaks with a garbage collector“
kicking in.)

Greetings,
Joachim


-- 
Joachim “nomeata” Breitner
  mail at joachim-breitner.de • http://www.joachim-breitner.de/
  Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0xF0FBF51F
  Debian Developer: nomeata at debian.org

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