I really do think that if the engineering challenges can be overcome, this would be a very useful representation to have on hand. There are many situations where the only way to get a sufficiently light memory representation is to use hand coded hacks that try to implement similar schemes using the Obj module. It wound be far better to have this as a first class part of the language. On Jan 24, 2014 5:16 AM, "Alain Frisch" wrote: > Revised description: there is no need to keep the tag on B or C values > when applying the A constructor, and one can skip the 0 integer as the > second field when applying the B/C constructor. > > B (x, y) ----> b0 = 1:(id_t,x, y) > A (B (x, y)) ----> b1 = 0:(id_t, b0) > > C ----> c0 = 2:(id_t) > A C ----> c1 = 0:(id_t, c0) > > > This simplifies the criterion for checking if a value of type t has the > B/C constructor (tag = 1 or 2) or the A constructor (tag = 0, and the > argument is the second field of the block if the first is id_t, and the > value itself otherwise). > > -- Alain > > > On 01/24/2014 11:06 AM, Alain Frisch wrote: > >> On 01/17/2014 10:10 AM, Gabriel Scherer wrote: >> >>> There have been recurrent discussions of optimizing `'a option` to >>> avoid allocation in some cases, which is interesting when it is used >>> as a default value for example. (The nice recent blog post by Thomas >>> Leonard also seems to assume that `'a option` is somehow optimized.) >>> >>> My strictly personal opinion is that I doubt this would be a good >>> idea, because I expect a fair share of the programming practice that >>> currently use ('a option) to move to something like (('a, >>> error-description) either) later in their lifetime, and I wouldn't >>> want people to avoid to do that for performance concerns. >>> Historically, we've rather come to see special-case representation >>> optimizations (eg. array of floats) as a mistake -- but on the other >>> hand there is not much downside to record of floats. >>> >> >> It could be argued the role of option types is important enough to >> justify a special treatment for them. But maybe one could think (just >> for the fun of it) about a more general optimized representation for sum >> types where one constructor should behave (mostly) as the identity at >> runtime. >> >> To take an example, consider a type: >> >> type ('a, 'b) t = >> | A of 'a >> | B of 'b * 'b >> | C >> >> with some marker to tell the compiler to optimize the representation of A. >> >> If one wants the constructor A to be the identity at runtime (in most >> cases), we still need to distinguish C from A C, A (A C), A (A (A C)), >> etc, and B (x, y) from A (B (x, y)), A (A (B (x, y))), etc. Here is >> one possible implementation: let's allocate a fresh value to represent >> the identity of the t type: >> >> id_t = 0:(0) >> >> that is, a block of size 1, tag 0, with a single 0 field (equivalent to: >> id_t = ref ()). (This value would be generated by the compiler and >> passed along in modules which re-export the type t.) >> >> The value (B (x, y)) would be represented as a block b0 = 1:(id_t, 0, x, >> y) (block with tag 1 and 4 fields). Applying the A constructor to such >> a block b0 would return a new block b1 = 1:(id_t, b0). Applying again >> the A constructor to b1 would return b2 = 1:(id_t, b1). >> >> Similarly, the value C would be represented as a block c0 = 2:(id_t, 0). >> Applying A to such a value would return a block c1 = 1:(id_t, c0), and >> then c2 = 1:(id_t, c1). >> >> So, in general, applying the A constructor to a value x requires to >> check if its argument is a block whose first field is equal to id_t, and >> in this case, it returns a new block with the same tag and the two >> fields id_t and x. In other cases, the constructors simply returns its >> argument. >> >> With this representation, it is not difficult to deconstruct the three >> constructors. For a value of type t: >> >> - If the value is a block whose first field is equal to id_t and its >> second field is 0, then the value comes from the B or C constructor >> (according to the block tag) and the arguments can be found in the block. >> >> - If the value is a block whose first first is equal to id_t and its >> second field is not 0, then the value comes from the A constructor, and >> the argument is the second field of the block. >> >> - Otherwise, the value comes from the A constructor and its argument >> is represented by the same value. >> >> >> There is one correctness problem with this representation, though: >> applying the A constructor to a float value cannot be the identity, >> because of the specific representation for float arrays (which is >> triggered by checking if the value is a float block). This means we >> must also have a special representation for A x, A (A x), etc, where x >> is a float. The scheme above extends naturally to support this >> representation: a0 = 0:(id_t, 0, x), a1 = 0:(id_t, a0), etc. >> >> >> Another drawback is related to the use of the id_t block, which does not >> work well with the generic marshaling, and requires extra plumbing to >> make this value available where the type t can be constructed or >> deconstructed. It's possible to do better for a type with a "global >> name". >> >> >> In case of a constant constructor such as C, one can of course >> pre-allocate the block c0 = 2:(id_t, 0). To avoid passing an extra >> value around, one could store it within id_t itself (id_t = 0:(c0) >> instead of id_t = 0:(0)). >> >> Another optimization is to avoid the allocation when applying the A >> constructor several times to the same B or C value. This can be done by >> memoization. One can add an extra field to all the blocks described >> above, initialized to 0, and updated to point to the "next" application >> of A when requested. >> >> So, we would have: >> >> c0 = 2:(id_t, 0, 0) >> >> When applying A to it, one create c1 >> >> c1 = 2:(id_t, c0, 0) >> >> and update the last field of c0 to be c1: >> >> c0 = 2:(id_t, 0, c1) >> >> If one needs to apply A again to c0, one can reuse the existing value. >> The same applies to non-constant constructors as well. >> >> >> >> -- Alain >> >> > > -- > Caml-list mailing list. Subscription management and archives: > https://sympa.inria.fr/sympa/arc/caml-list > Beginner's list: http://groups.yahoo.com/group/ocaml_beginners > Bug reports: http://caml.inria.fr/bin/caml-bugs >