2023/09/28

Pretty print indentation

Yet another small thing good to have. You can now specify base indentation to the pretty printer (ref:pprint). It is applied to the second line and after.

gosh> (pprint (make-list 100 'abc) :indent 20)
(abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc)
#<undef>

To say more precisely, when the pretty printer spills data to another line, it inserts "a newline + whitespace * indent", plus any indent computed by the pretty printer.

The benefit easiest to see is when the pretty printer is used inside format. When a pretty printing triggered by the ~:w directive, it sets the base indentation at the column it starts printing. Hence the entire pretty print is indented to align nicely:

gosh> (format #t "Long list: ~:w\n" (make-list 100 'abc))
Long list: (abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc
            abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc
            abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc
            abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc
            abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc
            abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc
            abc abc)
#<undef>

Since pretty printing is built-in to the core printer (pprint is just a simple interface to use), other output routines such as write can also use base indentation. You can set indent slot of a write-controls.

gosh> (write (make-list 100 'abc) (make-write-controls :pretty #t :indent 20 :width 79))
(abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc abc abc abc abc abc abc abc abc abc abc abc abc
                     abc abc)#<undef>

Tags: 0.9.13, pretty-printing

2023/09/26

Segmented completion

Another little something on REPL. It can now complete symbols like call-with-current-continuation from c-w-c-c. This is an old tradition of Lisp environment.

We added a new module text.segmented-match (draft:text.segmented-match) to support this.

Tags: 0.9.13, REPL

2023/09/26

Hints for unbound variable error

While working on REPL, sometimes you accidentally try to evaluate a variable that isn't visible from your current module. It is a bit annoying if you know the module is loaded, just you forget to use it in the current module.

So we added a little feature. When REPL reports an unbound varriable error, it also lists if there are variables of that name, exported from modules that are loaded into the process but not visible from the evaluating module:

gosh$ (thread-start! (make-thread (^[] (print "Hi"))))
*** UNBOUND-VARIABLE-ERROR: unbound variable: make-thread
    NOTE: `make-thread' is exported from the following module:
     - gauche.threads
Stack Trace:
_______________________________________
  0  (report-error e)
  1  (make-thread (^ () (print "Hi")))
        at "(input string port)":1

It may be nice to show modules that aren't even loaded, too, but that would be too costly so we avoided it. It also doesn't show non-exported variables, which is debatable--sometimes you forgot to export one and that caused this error. Let's use this for a while and see if we need non-exported ones, too.

This is realized in a general mechanism in error reporting. We haven't documented it yet, for we may tweak the interface, but I'll show it to give the general idea.

The error message in REPL, including the stack trace, is produced by report-error (ref:report-error). It prints *** ... line, with the condition class name and error message, then calls a generic function report-additional-condition on the condition. We have a specialized method for <unbound-variable-error> which searches the name in the loaded modules and prints the hint.

If the thrown condition is a compound condition, report-additional-condition is called over each component of the compound condition. This allows custom report for each component. When you load a file that has a statically detectable error, you get the additional information (While compiling ...). It is also realized by the same mechanism. The compiler and the loader adds the location information as a compound condition, and report-error calls report-additional-condition on them, which shows those additional messages.

gosh> ,l ./foo
*** ERROR: wrong number of arguments: cons requires 2, but got 1
    While compiling "./foo.scm" at line 1: (define (bar x) (cons x))
    While loading "./foo.scm" at line 2
Stack Trace:
_______________________________________
  0  (report-error e)
  1  (errorf "wrong number of arguments: ~a requires ~a, but got ~"...
  2  (pass1/expand-inliner program id gval cenv)

Tags: 0.9.13, REPL, report-error

2023/04/13

:immutable slot option

Immutability is far more valued nowadays than when CLOS was designed. Back then, basically the system allows programmers to change almost everything, assuming he knows what he was doing.

However, specifying something immutable is not only to protect users from shooting their own foot; it communicates to the readers. Here the readers can be a programmer who reads and uses the code years later (who can be the same programmer who wrote it bot has forgotten the details), or a program-processing programs such as optimizer to take advantage of immutablity.

CLOS (and Gauche's object system) have enough flexibility to implement immutable slots, but it is somewhat awkward. It's not as simple as having a custom setter that throws an error; for, the slot setter is also called in the instance initializer which runs at the instance construction. You have to distinguish whether the slot setter is invoked during initialization or outside initialization, but such dynamic introspection would be costly.

We came up an alternative mechanism which is effectively realizes immutable slots in practical situations, but does not require to distinguish whether it's in initialization or not.

If a slot has a true value for :immutable slot option, the slot can only be initialized once--that is, the setter sets the value if the slot is previously unbound, but throws an error if not. If you give the slot an initial value, either with :init-keyword or :init-value etc., then that one chance to set the value is used within initializer. Uninitialized immutable slots don't make much sense, so we expect almost always immutable slots are initialized this way.

It is possible that the initializer leaves the slot unbound, and later the user call slot-set! to set it once. It can be viewed as delayed initialization.

(We first named this option init-once, for the slot can be set once, but changed our mind for it could be confusing.)

Tag: ObjectSystem

2022/10/05

Source info propagation with macro expansion

Gauche tracks source code location information and shows it in the stack trace. However, what if the source is generated by macros? In 0.9.12, the macro expander re-attached the original source info to the outermost form of the macro output. However, if a runtime error occurred in constructed code other than the outermost one, stack trace couldn't find the info and had to show "[unknown location]". It was annoying especially when the code was the result of nested macro expansions, that you didn't get a clue about where the error came from.

I was annoyed enough, so from 0.9.13, you can have better stack trace. (Well, if you're familiar with other Scheme that employs syntax-case macro expander, you're already familiar with such a feature. Yes, Gauche finally caught up.)

Let's show it with a somewhat contrived example. The following cxr macro expands to cxxx...xxr according to the given sequence of a or d.

;; (cxr a r obj) == (car obj)
;; (cxr a a r obj) == (caar obj)
;; (cxr a d a r obj) == (cadar obj)
;;etc.
(define-syntax cxr
  (syntax-rules (a d r)
    [(_ r obj) obj]
    [(_ a xs ...) (car (cxr xs ...))]
    [(_ d xs ...) (cdr (cxr xs ...))]
    [(_ . xs) (syntax-error "Malformed cxr:" (cxr . xs))]))

In 0.9.12, if you pass something that causes a runtime error, you get the annoying unknown location:

gosh$ (cxr a a a a r '(1 2 3 4))
*** ERROR: pair required, but got 1
Stack Trace:
_______________________________________
  0  (car (cxr a r '(1 2 3 4)))
        [unknown location]
  1  (eval expr env)
        at "/usr/share/gauche-0.98/0.9.12/lib/gauche/interactive.scm":336

In 0.9.13, you'll get this:

gosh$ (cxr a a a a r '(1 2 3 4))
*** ERROR: pair required, but got 1
Stack Trace:
_______________________________________
  0  (car (cxr a r '(1 2 3 4)))
        at "/home/shiro/src/Gauche/test/macro-source-info.scm":15
        expanded from (cxr a a r '(1 2 3 4))
        at "/home/shiro/src/Gauche/test/macro-source-info.scm":15
        expanded from (cxr a a a r '(1 2 3 4))
        at "/home/shiro/src/Gauche/test/macro-source-info.scm":15
        expanded from (cxr a a a a r '(1 2 3 4))
        at "(standard input)":34
  1  (eval expr env)
        at "/home/shiro/src/Gauche/src/../lib/gauche/interactive.scm":354

This works with ER-macro, too. Suppose we have another macro, c*r, in which you can give a's and d's in a single symbol. That is, (c*r aada obj) is (caadar obj). We also let the code print the given symbol, just for the sake of making things complicated.

;; (c*r aa obj) == print 'aa' and return (caar obj)
;; (c*r addar obj) == print 'addar' and return (caadr obj)
;; etc.
(define-syntax c*r
  (er-macro-transformer
   (^[form rename cmp]
     (match form
       [(_ xs obj)
        (let1 cs (map ($ string->symbol $ string $)
                      (string->list (symbol->string xs)))
          (quasirename rename
            `(begin
               (print ',xs)
               (cxr ,@cs r ,obj))))]))))

Here's 0.9.12:

gosh$ (c*r aad '(1 2 3 4))
aad
*** ERROR: pair required, but got 2
Stack Trace:
_______________________________________
  0  (car (cxr a d r '(1 2 3 4)))
        [unknown location]
  1  (eval expr env)
        at "/usr/share/gauche-0.98/0.9.12/lib/gauche/interactive.scm":336

And HEAD:

gosh$ (c*r aad '(1 2 3 4))
aad
*** ERROR: pair required, but got 2
Stack Trace:
_______________________________________
  0  (car (cxr a d r '(1 2 3 4)))
        at "/home/shiro/src/Gauche/test/macro-source-info.scm":15
        expanded from (cxr a a d r '(1 2 3 4))
        at "/home/shiro/src/Gauche/test/macro-source-info.scm":60
        expanded from (quasirename rename `(begin (print ',xs) (cxr ,@cs r  ...
        at "/home/shiro/src/Gauche/test/macro-source-info.scm":57
  1  (eval expr env)
        at "/home/shiro/src/Gauche/src/../lib/gauche/interactive.scm":354

Now, if you're user of syntax-case or syntax-rules, there's no wonder how it can be done. Macro output is constructed as syntactic objects, which can carry any sideband information. But with ER-macro, you construct the output as a simple S-expression, so it's not obviouhs where those information comes from.

Gauche has an extended pair that can carry extra information other than car and cdr. Those sideband data isn't visible as far as you're treating it as a pair, nor it affects equal?-ity of the pairs. Source code information is stored there by read procedure and its families.

gosh$ (read-from-string "(a b c d)")
(a b c d)
gosh$ (pair-attributes *1)
((source-info "(input string port)" 1))

If you consturct lists with cons or list, those information won't be attached. However, quasirename does the trick. It extracts the original source info from the input, and re-attaches it to the constructed form.

Note that, for a macro expander, we need to consider two kinds of source information: One is of the macro definition, and another is of the macro input. The source info of the macro definition is available through the argument of quasirename. But how can it get the macro input information? The macro input is already deconstructed by the time quasirename is called.

We use another sideband mechanism, procedure tags. Srfi-229 defines a general mechanism to attach an arbitrary Scheme object to a procedure. Gauche has more general mechanism (although not documented yet) that a procedure can have multiple tags, and the macro input is attached to the rename procedure as one of such tags. Then quasirename extracts that information from the rename procedure and applies it to the output.

In the definition of quasirename, the output construction code looks like this:

;; in src/libmacro.scm

   (if-let1 si (pair-attribute-get objs 'source-info #f)
     (let1 orig (assoc-ref ((with-module gauche.internal %procedure-tags-alist) r)
                           'macro-input)
       `(,extended-cons. ,xx ,yys '((source-info ,@si)
                                    ,@(cond-list
                                       [orig `(original . ,orig)]))))
     `(,cons. ,xx ,yys)))))

The pair attribute source-info holds the source info of macro definition, and original holds the macro input form.

The disadvantage of having source info in the sideband data of pairs is, obviously, that you can't attach source info to other objects than pairs. I find it not a big issue in practice, for most expressions that need attention are function calls, macro calls or special forms.

On the other hand, it has an advantage that quoted literal lists can have source code information. It can't be done with syntax objects, for quote strips any syntax wrappings. It is handy when you put a literal nested structure as DSL and let its walker signals an error with the location of the literal structure.

This is a desired feature and I'm happy to have it. However, I'm feeling a bit of ambivalence, too.

The reason I prefer ER-macro to syntax-case is that ER-macro is explicit---input and output are raw S-expression which you can direclty touch and rearrange. With syntax-case, things are wrapped in opaque syntax object, and even though you can unwrap and rewrap the objects, that opaqueness bothers me.

However, with this quasirename modification, I did introduce an implicit operation; even though the output of quasirename can be treated as an ordinary S-expression, it does more to it than just consing.

If I feel comfortable with this, maybe I can also feel comfortable with syntax-case, too. I don't know yet. Let's see.

Tags: Macro,, 0.9.13,, quasirename