RE: [Caml-list] Why List.map does not be implemented tail-recursively?

["Jon Harrop" <jon@ffconsultancy.com>]

What happens when the first allocation in the creation of a cyclic immutable structure incurs a minor collection?

let rec xs = 0::ys and ys = 1::xs

-----Original Message-----
From: stedolan@stedolan.net [mailto:stedolan@stedolan.net] On Behalf Of Stephen Dolan
Sent: 02 October 2014 11:10
To: Gabriel Scherer
Cc: Pierre Chambart; Goswin von Brederlow; caml users
Subject: Re: [Caml-list] Why List.map does not be implemented tail-recursively?

On Mon, Sep 29, 2014 at 10:00 PM, Gabriel Scherer <gabriel.scherer@gmail.com> wrote:
>> Please, don't do that hack ! The compiler assumes immutable data are 
>> not mutated and optimise knowing that.
>
> My understanding was that it is unsafe to Obj.magic an immutable data 
> structure into a mutable one (one mental model is that immutable data  
> might be allocated in read-only memory, although that's not what the 
> current implementation does), but that on the contrary it is safe to 
> Obj.magic a mutable data-structure into an immutable one. The 
> Obj.magic-using code for List.map, implemented in Extlib and inherited 
> by Batteries, is careful to use an unsafe cast in exactly the second 
> situation. This is a feature that other languages (eg. Mezzo) safely provide.

This trick is unsound in our implementation of multicore OCaml: the system assumes that immutable objects only point to older objects, and thus that a shared immutable object cannot point to a private immutable object. This can be hacked around (we have a similar hack for recursive immutable objects), but the point is that such tricks are fragile and depend on very implementation-specific behaviour.

Even in single-threaded OCaml, this trick is broken by quite reasonable compiler optimisations (although not ones that OCaml does today). For instance, if ocamlopt were to perform alias analysis and optimise based on the results, the first thing the alias analyser would do is conclude that references to immutable and mutable fields do not alias. This would allow it to swap the order of references to the immutable and mutable fields, and after inlining would mean that the cons cell is read before it is initialised.

Stephen

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