On 07 April 2006 22:38, Andy Gill wrote:
On Apr 7, 2006, at 3:59 AM, Rene de Visser wrote:
Hello,
As deepSeq has a non local effect, I think it requires a non-local source transformation to implement it. One option would be for the compiler to create a second deepSeq version of every function definition.
e.g.
If the user defines a function f
f x = g h x
then the compile creates an additional function !!f
!!f x = temp `seq` temp where temp = !!g !!h x
which uses the compiler generated functions !!g and !!h.
It looks like library writers are increasingly doing this manually. Creating a strict and non strict version of a number of the functions provided. This would automate that.
Rene.
It depend on the semantics of deepSeq. If deepSeq just performs seq on all constructors recursively, then that can be implemented as a runtime primitive. If deepSeq is making all embedded partial applications strict, then yes this might be a non-local effect.
What are the semantics of !!(\ x -> ...)?
I am calling for the version of deepSeq/strict that evaluates all thunks, but does not strictify the arguments to partial application, because - I believe this is straightforward to implement
It's not *completely* straightforward to implement, at least in GHC, and at least if you want to implement it in a modular way (i.e. without touching lots of different parts of the system). The obvious way to "add a bit to a closure" is to use the LSB of the info pointer, which currently is always 0. However, that means masking out this bit every time you want to get the info pointer of a closure, which means lots of changes to the runtime. The price seems pretty high. An alternative is to have two info tables for every constructor, one normal one and one "deepSeq'd", and the normal one probably needs to point to the deepSeq'd version. This doesn't require masking out any bits, but it does increase code size (one extra info table + entry code for every constructor, except possibly those that don't contain any pointer fields), and one extra field in a constructor's info table. Plus associated cache pollution. Yet another alternative is to store fully evaluated data in a segregated part of the heap. The garbage collector could do this - indeed we already do something similar, in that data that has no pointer fields is kept separate. Checking the "deepSeq" bit on a closure is then more complicated - but this has the advantage that only the GC and storage manager are affected. None of these solutions is as simple and self-contained as I'd like :-( Cheers, Simon