2017/05/23

A heads-up for an incompatible fix in util.match

TL;DR: If you match records with inheritance using $ or struct match pattern, you need to change the code for 0.9.6.

We fixed a bug in the positional record matching pattern of match, existed in 0.9.5 and before. The fix actually breaks previously documented behavior, but we believe the previous behavior was incorrect and decided it's better to fix now.

Background

The $, or struct pattern allows you to extract slot values from objects using match (ref:match):

(define-class <point> ()
  ((x :init-keyword :x)
   (y :init-keyword :y)
   (z :init-keyword :z)))

(match (make <point> :x 1 :y 2 :z 3)
  [($ <point> a b c)
   (list a b c)])  => (1 2 3)

However, Gauche's object system isn't designed to access slots with their positions. You use slot names instead. In match, you can use object pattern (or @ in sort) to match with slot values, using slot names.

(match (make <point> :x 1 :y 2 :z 3)
  [(@ <point> (x a) (y b) (z c))
   (list a b c)])  => (1 2 3)

The reason we provided $ was for the compatibility of original Wright's match, which aimed at struct types provided in some Scheme impelementations. We didn't give much thought to it; just made the pattern match with the slot values of the order of class-slots (ref:class-slots). It works just fine with srfi:9 records:

(define-record-type pare (make-pare fst snd) pare?
  (fst get-fst)
  (snd get-snd))

(match (make-pare 1 2)
  [($ pare a b)
   (list a b)])  => (1 2)

Problem

Things got complicated when inheritance enters the picture. How the inherited slots are laid out depends on the implementation of metaclass (ref:compute-slots generic function), and because of multiple inheritance, the slot layout of class S doesn't necessarily a subsequence of the layout of class T that inheriting S. This is highly confusing, and we've always recommended using object match in such a case, in the manual.

However, srfi:99 records only allows single inheritance chain, and the default constructor takes initial value of inherited slot first. So it is a natural call to make positional match in the same way.

(define-record-type S make-S S?
  a b)

(define-record-type (T S) make-T T?   ;; inherit S
  c d)

(make-S 1 2)      ;; Initialize a=1, b=2

(make-T 1 2 3 4)  ;; Initialize a=1, b=2, c=3, d=4

;; Then, ($ T w x y z) should match with w=1, x=2, y=3, z=4.

It hadn't been so. The compute-slots method of <record> placed the direct slots first, followed by the inherited slots. It needs to do so to be consistent with that "fields in derived record types shadow fields of the same name in a parent record type", as defined in srfi-99.

Thus, ($ T w x y z) pattern in the above example matched w=3, x=4, y=1, and z=2. This wasn't inconsistent with the manual, which stated that positional match was done with the order of class-slots. It was an unintended artifact of implementation that was overlooked, unfortunately.

It also had a defect when duplicate slot names existed. When a subclass defines a slot with the same name as inherited slot, the standard compute-slots merges them into one, which is also CLOS's behaviro. However, srfi:99 record types allow subtype to have slots with the same name but as independent slots.

(define-record-type S #t #t
  a)

(define-record-type (T S) #t #t
  a)

(define t (make-T 1 2))

(T-a t)  ;=> 2    ; accesses T's a
(S-a t)  ;=> 1    ; accesses S's a in T

(slot-ref t 'a) ;=> 2  ; named access takes the subtype's slot

The existing implementation of positional matching needed to rely on named slot access, and didn't work on such record types.

Fix

We introduced a generic function to be specialized with metaclass, that handles positional access within match. We keep the underlying mechanism undocumented for now; changing the way of positional matching should be rare and based on well-established customs. The order of record types fits this criteria, and made to work as expected:

(define-record-type S make-S S?
  a b)

(define-record-type (T S) make-T T?   ;; inherit S
  c d)

(match (make-T 1 2 3 4)
  [($ T w x y z) (list w x y z)]) => (1 2 3 4)

Now it also works with record types having duplicate slot names:

(define-record-type S #t #t
  a)

(define-record-type (T S) #t #t
  a)

(match (make-T 1 2)
  [($ T x y) (list x y)])   ;=> (1 2);  was (2 2) before

We hope few have used positional match with inherited records--- the old behavior seems apparently wrong---so we decided to fix this now.

If you happen to have the code that relies on the previous behavior, and need to make it work with both versions, you can switch to use named match (object or @).

Tags: 0.9.6, util.match, gauche.record

2017/04/25

Method call optimization - skipping sort-applicable-methods

It's not that we don't sort methods. It's just that if we have only one applicable method, we don't need to call sort-applicable-methods at all. Obvious, right?

I thought it didn't matter, for that part is written in C (we call Scm_SortMethods) and it just returns without sorting at all when we have only one method. But lo and behold, it is quite effective. Here's the average of several runs, calling ref on a sparse vector 10M times.

Baseline:   3.065 real, 3.863 cpu
Optimized:  2.431 real, 2.793 cpu

Scm_SortMethods uses shell sort on C array to avoid allocation, and we have a setup overhead (convert list to array). That was a waste when we have only one method. And in many cases, we do.

Not bad for just a few lines of change.

https://github.com/shirok/Gauche/commit/556fee985246eed0dd0189b3763d662bf0b0075b

We could also optimize for the cases we have just two or three applicable methods, in which case we can use hard wired comparison instead of using general sorting. It depends on how often we get such cases; some benchmark is required.

Tags: 0.9.6, sort-applicable-methods

2017/04/22

Method call optimization - avoiding next-method

Gauche's object system is a direct descendant of stklos, which is based on TinyCLOS. Each method takes an implicit parameter, next-method, which is bound to a next-method object and can be used to invoke the less specific method in a method chain. It is similar to the super method in class-oriented object systems. However, the next-method object must remember the actual arguments used in the method's invocation. That is, when it is called without arguments, it should pass the original arguments to the next method:

gosh> (define-method foo ((a <number>)) `((number ,a)))
#<generic foo (2)>
gosh> (define-method foo ((a <real>)) (cons `(real ,a) (next-method)))
#<generic foo (2)>
gosh> (foo 3)
((real 3) (number 3))

Here, (next-method) in (foo <real>) passes 3 to the next method, (foo <number>). Note that the next-method object is a first-class object, so for example, it can be saved to a global variable:

gosh> (define nm #f)
nm
gosh> (define-method foo ((a <real>)) (set! nm next-method) nm)
#<generic foo (2)>
gosh> (foo 3)
#<next-method (#<method (foo <number>)>)0 (3)>
gosh> nm
#<next-method (#<method (foo <number>)>)0 (3)>

And invoked later:

gosh> (nm)
((number 3))

This means that a next-method object must be allocated for every invocation of a method. In many cases, though, next-method isn't used at all in the method body, so it's a waste. Can we do something?

An easy path would be to let programmers indicate whether the method want to use next-method or not, probably using a CL-style method qualifier. But it's kind of silly---whether the method uses next-method or not is already shown in the code; why the programmer need to bother to say it again?

Fortunately, during compilation, we track how many times each local variable is referenced. So at some point in the compiler passes, we know whether next-method is used in the method body or not. We can extract that information and save in the method object, and when the method is invoked we check it and avoid next-method creation if possible.

The define-method form is expanded as follows:

(define-method name ((arg spec) ...) <method-body>)

  |
  v

(add-method! name
             (make <method>
               ...
               :specializers (spec ...)
               :body (lambda (next-method arg ...)
                        <method-body>)))

The body of the method is just a plain lambda form; it has nothing special about being a method body. So the compiler can't treat method body differently.

Instead, we introduced a general mechanism to save a list of unused argument in each closure; it is a meta-information and saved along source-code information.

The <method> initializer looks that information to find out whether next-method argument is used. If not, it sets method-leaf flag of the method object.

The method invocation code looks at the flag and omits creation of next-method object.

Let's see how effective this optimization is. This is the benchmark script.

(use gauche.time)

;; method without next-method
(define-method foo (x) #t)

;; method with next-method (for comparison)
(define-method bar (x) (when x (next-method)))

($ time-these/report 10000000
   `((foo . ,(cut foo #f))
     (bar . ,(cut bar #f))))

On 0.9.5:

Benchmark: ran foo, bar, each for 10000000 times.
  foo: 2.021 real, 3.550 cpu (3.470 user + 0.080 sys)@2816901.41/s n=10000000
  bar: 2.099 real, 3.680 cpu (3.570 user + 0.110 sys)@2717391.30/s n=10000000

On HEAD:

Benchmark: ran foo, bar, each for 10000000 times.
  foo: 1.313 real, 1.740 cpu (1.730 user + 0.010 sys)@5747126.44/s n=10000000
  bar: 2.051 real, 3.670 cpu (3.570 user + 0.100 sys)@2724795.64/s n=10000000

In wall-clock time, it's about 35% speedup (actually, averaging multiple runs show it's more like 40%). It's also interesting that CPU time is halved---since GC runs in parallel, it indicates this modification generates a lot less garbage, hence less GC time.

Note: Since 0.9.6's built-in methods will be pre-compiled by the 0.9.5 compiler, you won't see the effect of this optimization on built-in methods in the 0.9.6 release. You'll need to rebuild the source with 0.9.6.

There are still lots of room of optimization in our method dispatch mechanism, and we'll explore it more in the comping releases.

Tags: 0.9.6, next-method

2017/04/05

Another little improvement of online doc

Here's another little feature to be in 0.9.6. You can search info entries using regexp.

gosh> ,i #/^symbol-/
symbol->string           Symbols:62
symbol-append            Symbols:96
symbol-hash              Hashing:154
symbol-interned?         Symbols:54
symbol-sans-prefix       Symbols:87

Since gauche.interactive.info module builds the table from entry names to locations in info documents, it's just easy to pick entry names that matches the given regexp.

This raises an interesting question: We already have apropos which can search symbols. What's the difference?

On systems such as CL or Clojure, where docstring is tied to symbols, it's reasonable to let apropos for searching, and documentation/doc for retrieving the actual document.

In Gauche, we chose not to adopt docstring convention; instead, we provide a way to lookup separately provided document. This allows you to browse the document of symbols that are not loaded into the current repl, which is handy, since often you need to read doc before finding which module to import.

We can consider apropos more as an introspection tool into the current running process. With that view, there could be some options to enhance apropos---e.g. showing visibility of each binding from the current module, and if it's visible, why (e.g. this is visible because the current module imports this module which inherits this module, etc.)

Tags: Documentation, repl

2017/04/02

Little improvement of online doc

As of 0.9.5, online document (info command on REPL) only shows the specified entry, instead of the entire info page that contains the entry.

gosh> ,info make-queue
 -- Function: make-queue
     Creates and returns an empty simple queue.

It is easier to read than the entire page, but has one drawback---from the entry alone, it is not clear which module I should import. If you're reading it in info, it's easy to look up which section you're in, and the section shows the module name. The online doc is out of such context.

I've been mulling about it and finally decided to go for a kind of brute-force solution; add the module name to every entry. In HEAD (which is to be 0.9.6), it will show as follows:

gosh> ,info make-queue
 -- Function: make-queue
     {data.queue} Creates and returns an empty simple queue.

It may be a bit annoying when you're reading it in info document, for the module name is repeated in every entry. But it is more frustrating that necessary info isn't shown.

Tags: Documentation, repl