Clojure's compilation strategy isn't a matter of single-pass vs multi-pass,
but of what constitutes a compilation unit.
In Clojure, a compilation unit is a top-level form. Within a form, you can
have cyclic references:
(letfn [(f [x] (if (> x 0) (g (- x 2)) :f))
(g [x] (if (> x 0) (f (- x 1)) :g))]
(f 9))
I believe Clojure's compiler is technically a two-pass compiler.
So your question isn't so much about single-pass vs. multi-pass compilers,
but a question about how large a compilation unit should be. Should a
compilation unit be a top-level form, or an entire file, or an entire
project?
Currently Clojure considers a top-level form to be a compilation unit. A
file has no inherent semantic meaning to the compiler; it's just a way of
storing a sequence of top-level forms. Changing this isn't a case of giving
Clojure a two-pass compiler (and it has one already anyway), but
significantly changing how Clojure thinks about compilation.
As for why it chooses this compilation strategy to begin with, I believe
it's because it makes dealing with REPLs and macros somewhat more
straightforward.
Regarding your specific example, you can get around it just by using the
constructor functions:
(defprotocol XP (make-Y [this]))
(defprotocol YP (make-X [this]))
(declare ->Y)
(defrecord X [x] XP (make-Y [this] (->Y x)))
(defrecord Y [y] YP (make-X [this] (->X y)))
- James
On 11 July 2015 at 20:31, Mars0i <[email protected]> wrote:
> My understanding is that all of the drawbacks mentioned below are a
> consequence of Clojure using a single-pass compiler. Feel free to correct
> my misunderstandings, which no doubt exist.
>
>
> 0. Provisos: I don't want Clojure to include every feature that anyone
> wants. I don't even want it to include every feature that I want. The
> feature that I most want is what Clojure provides, which is a small set of
> elegant, flexible functions that allow one to do a lot with a little bit of
> code, without the language being cluttered up by a confusing set of
> non-orthogonal, confusingly named features. I'm immensely grateful to Rich
> and others who've worked on the design and implementation of Clojure. The
> follow is not an attack or a criticism, but a statement of reasons for what
> seems like a small change in a language that I love using.
>
>
> 1. The world contains cyclic dependencies. Not all relationships are
> hierarchical.
>
> 2. Fortunately, Clojure allows cyclic dependencies. They're completely
> unproblematic in many cases due to dynamic typing. Clojure only forbids
> cyclic dependencies when the dependency *explicitly* involves something
> that can be interpreted as a Java class, as defined, for example, by
> defrecord. This seems like an arbitrary distinction.
>
> 3. Allowing cyclic dependencies to work in some cases but not in others is
> confusing, and can lead to puzzling behavior:
>
> - If I add type hints, what was not a forbidden cyclic dependendency can
> suddenly become one and fail to compile.
>
> - If I add forbidden cyclic dependency to one source file, and that source
> file depends on a file that's already been compiled, the first,
> newly-modified source file may compile and the program may run without
> trouble--until I delete the second class file or or try to compile on a
> different machine on which some of the source files have not been compiled
> yet. (So I can give my coworker code that worked on my machine but breaks
> on hers, even though we have the same source code and use the same
> libraries. I may also have made other code depend on the recently-added
> functions that embody the cyclic dependency; when I realize what's gone
> wrong, I'll have to go back and fix all of that code that depends on the
> offending functions in order to fix the problem.)
>
> 4. One shouldn't shouldn't have to organize code into source files so that
> conceptually very distinct things are required to be in the same file, or
> at least in the same namespace, in order to avoid forbidden cyclic
> dependencies. One shouldn't have to create huge source files
> unnecessarily. And whether code has to be organized like that shouldn't
> depend on whether type hints are added: One shouldn't have to turn
> well-organized code into poorly organized code just for the sake of
> optimization. (That should be the compiler's job.)
>
> 5. Some cyclic dependencies won't compile even if all of the code is in
> one file. If I have functions that refer to each other, I can add a
> declare statement, but there's no way to do the same thing for types. The
> following won't compile no matter how I rearrange the definitions:
>
> (ns Foo)
> (defprotocol XP (make-Y [this]))
> (defprotocol YP (make-X [this]))
> (defrecord X [x] XP (make-Y [this] (Y. x)))
> (defrecord Y [y] YP (make-X [this] (X. y)))
>
> A related point is that it may be necessary to rearrange code in a file in
> awkward, unintuitive ways in order to get it to compile. This compounds
> the problem of putting a lot of code in one file.
>
> 6. Changing to two-pass compilation wouldn't add any functions or
> datatypes to the language, so there's no sense in which it clutters up the
> language. And it's hard to believe that a two-pass compiler would be very
> slow compared to the existing compiler, although I don't know much about
> compilers. From the programmer's point of view, changing to a single-pass
> compiler is a trivial change, and breaks no code.
>
> These points concern ways in which coding in Clojure becomes inconvenient,
> awkward, or bug-prone. In another language, I might not mind as much. I
> might just think that programming requires care and skill, and I would take
> pride in my ability to work around such problems, and in the specialized
> knowledge I've gained that allows me to do so. With Clojure, I expect
> more, I guess.
>
> Honestly, I don't expect the single-pass compilation to go away, but I
> don't understand why it persists. The preceding points should be pretty
> obvious to Rich and other members of the core Clojure team. I know that
> some of them have been made before, but I haven't seen all of them made.
>
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