Wow, Alex nice investigation!
Thanks for looking into that.
On Wed, Oct 30, 2013 at 7:46 PM, Alex Vondrak <ajvond...@gmail.com> wrote:
> Let me preface by saying you generally want to avoid `execute` and
> `call`, except when implementing combinators
> (http://docs.factorcode.org/content/article-combinators.html). As
> you've already seen, they might have some finicky behavior. It's not
> quite the same level of antipattern as "eval is evil", but it's always
> felt similar to me. Perhaps the following is a bit better:
>
> ```
> USING: arrays combinators formatting fry kernel sequences
> unicode.normalize ;
>
> : normalizations ( str -- norms )
> {
> [ nfc ]
> [ nfd ]
> [ nfkc ]
> [ nfkd ]
> } cleave 4array
> [ [ "U+%04X" sprintf ] { } map-as ] map ;
> ```
>
> That said...
>
> The reason this issue was tripped was because of `nfc`---and probably
> the others, too, but `nfc` is the first one done in the `map`.
>
> `nfc` is defined like this:
>
> ```
> USING: unicode.normalize.private ;
> IN: unicode.normalize
> : nfc ( string -- nfc ) [ (nfd) combine ] with-string ;
> ```
>
> The `with-string` combinator that it uses is defined like this:
>
> ```
> USING: accessors kernel ;
> IN: unicode.normalize.private
> : with-string ( str quot -- str )
> over aux>> [ call ] [ drop ] if ; inline
> ```
>
> Because the above word is inlined, `nfc` is "really" behaving like this:
>
> ```
> : nfc ( string -- nfc )
> [ (nfd) combine ] over aux>> [ call ] [ drop ] if ;
> ```
>
> Thus, if the input string doesn't have an `aux` slot set, we don't do
> any computation because the `[ (nfd) combine ]` quotation just gets
> `drop`ped. It's as though we defined it as just a no-op, like this:
>
> ```
> : nfc ( string -- nfc ) ;
> ```
>
> So, just take a look:
>
> ```
> IN: scratchpad "Factor" aux>> .
> f
> IN: scratchpad "\xF1" aux>> .
> B{ 0 0 }
> ```
>
> Because of the above, the call to `"Factor" nfc` becomes a no-op, and
> it *looks* to the Factor VM like `nfc` had the stack-effect `( -- )`.
> So, the `execute( -- )` doesn't raise an error in this example.
>
> To get really technical, I think the reason the VM thinks the
> stack-effect looks OK is because of how this primitive is implemented:
> https://github.com/slavapestov/factor/blob/master/vm/contexts.cpp#L274
> Basically, the datastack before & after the call to `nfc` is exactly
> the same, so it thinks the stack effect was `( -- )`. But with
> "\xF1", this isn't the case, and the error is thus caught.
>
> Hope that helps,
> --Alex Vondrak
>
> On Wed, Oct 30, 2013 at 3:24 PM, OwnWaterloo <ownwater...@gmail.com>
> wrote:
> > I'm new to Factor and don't know how to minimize the code while get
> > the similar behaviour. Sorry for that.
> > But thanks to the expressiveness and good libraries of Factor the code
> > won't be too long, I hope...
> >
> > Firstly, the primary goal is to normalize some unicode string.
> > Formating the result will make it more clear. And I also want to learn
> > the "Fried quotations".
> > So I use the three vocabularies:
> >
> > IN: scratchpad USING: unicode.normalize formatting fry ;
> > Loading resource:basis/formatting/formatting.factor
> > Loading resource:basis/formatting/formatting-docs.factor
> >
> > Then the implement it with correct stack effect declaration in
> "execute(" :
> >
> > IN: scratchpad : normalizations ( str -- norm )
> > '[ _ swap execute( x -- x ) ] ! correct stack effect declaration
> > { nfc nfd nfkc nfkd } swap map ! normalize str with four methods
> > [ >array [ "U+%04X" sprintf ] map ] map ; ! make it more clear
> >
> > It gives me the expected results except the last two(but it maybe not
> > important to this question):
> >
> > IN: scratchpad "Factor" normalizations .
> > {
> > { "U+0046" "U+0061" "U+0063" "U+0074" "U+006F" "U+0072" }
> > { "U+0046" "U+0061" "U+0063" "U+0074" "U+006F" "U+0072" }
> > { "U+0046" "U+0061" "U+0063" "U+0074" "U+006F" "U+0072" }
> > { "U+0046" "U+0061" "U+0063" "U+0074" "U+006F" "U+0072" }
> > }
> > IN: scratchpad "\xF1" normalizations .
> > {
> > { "U+00F1" }
> > { "U+006E" "U+0303" }
> > { "U+00F1" }
> > { "U+006E" "U+0303" }
> > }
> > IN: scratchpad "\x6E\u000303" normalizations .
> > {
> > { "U+00F1" }
> > { "U+006E" "U+0303" }
> > { "U+00F1" }
> > { "U+006E" "U+0303" }
> > }
> > IN: scratchpad "\u01D160" normalizations .
> > {
> > { "U+1D158" "U+1D165" "U+1D16E" }
> > { "U+1D158" "U+1D165" "U+1D16E" }
> > { "U+1D158" "U+1D165" "U+1D16E" }
> > { "U+1D158" "U+1D165" "U+1D16E" }
> > }
> > IN: scratchpad "\u01D158\u01D165\u01D16E" normalizations .
> > {
> > { "U+1D158" "U+1D165" "U+1D16E" }
> > { "U+1D158" "U+1D165" "U+1D16E" }
> > { "U+1D158" "U+1D165" "U+1D16E" }
> > { "U+1D158" "U+1D165" "U+1D16E" }
> > }
> >
> > But with the wrong stack effect declaration(actually I use the wrong
> > declaration first and the correct one latter during the study):
> >
> > IN: scratchpad : normalizations ( str -- norm )
> > '[ _ swap execute( -- ) ] ! wrong stack effect declaration
> > { nfc nfd nfkc nfkd } swap map
> > [ >array [ "U+%04X" sprintf ] map ] map ;
> >
> > The first case will pass:
> >
> > IN: scratchpad "Factor" normalizations .
> > {
> > { "U+0046" "U+0061" "U+0063" "U+0074" "U+006F" "U+0072" }
> > { "U+0046" "U+0061" "U+0063" "U+0074" "U+006F" "U+0072" }
> > { "U+0046" "U+0061" "U+0063" "U+0074" "U+006F" "U+0072" }
> > { "U+0046" "U+0061" "U+0063" "U+0074" "U+006F" "U+0072" }
> > }
> >
> > And the rest will get an error:
> >
> > IN: scratchpad "\xF1" normalizations .
> > Quotation's stack effect does not match call site
> > quot [ \ nfc execute ]
> > call-site ( -- )
> >
> > Type :help for debugging help.
> >
> > The same thing will happen in "\x6E\u000303" "\u01D160"
> > "\u01D158\u01D165\u01D16E".
> >
> > I promise that I will not depend on this behaviour and will use the
> > correct stack effect declaration in real code.
> > I'm just curious that why "execute(" will check it in some cases but
> > won't in others?
> >
> > Thanks!
> >
> >
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paper to learn more about secure code signing practices that can help keep
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