[Haskell-cafe] more sharing in generated code

Peter Divianszky divipp at gmail.com
Sat Nov 3 13:39:07 CET 2012

Hi Dennis,

 > I think you mean this:
>    r { x = f (x r) }

Yes, I made a typo.

Thanks for advising ghc-vis.

> In GHC: In your example a new record is built, but all its entries (x in
> this case) are shared.

The problem is, that in case of nested records, if an inner record is 
updated, the whole path to that record is copied.

Of course copying just the path to the inner data is a lot better than 
copying the whole tree of data, but my proposal is about to optimize it 
further: don't copy of the path if the updated value is identical to the 
original one. The Agda compiler may benefit by this optimization for 

In fact, the last version of the proposal[^1] will copy the path to keep 
laziness properties, but it does it in such a way that the GC can 
collect the copied path instead of trying to collect the old path (which 
will if it is shared).


Does this make sense to you?


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