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Re: Some advice on a "simple" thing...

From: emacstheviking
Subject: Re: Some advice on a "simple" thing...
Date: Wed, 9 Apr 2014 17:42:03 +0100

Wow, I didn't realise how little I understood!

I am *guessing* now that by writing A=B in the list is the same as =(A,B) and so in the code:

has_flag(NameFlags, Value, Name) :-
    member(Name=Mask, NameFlags),
    Value /\ Mask =\= 0.

It could be rewritten as:

has_flag(NameFlags, Value, Name) :-
    member(=(Name, Mask), NameFlags),
    Value /\ Mask =\= 0.

at which point my brain goes, "Aha! It is *just* unification after all and not magic".

Is that a correct intrepretation?  And why '=', is it the case that any functor would have done, for example could I have as easily written this:

has_flag(NameFlags, Value, Name) :-
    member(foo(Name,Mask), NameFlags),
    Value /\ Mask =\= 0.

and this:

has_flag(NameFlags, Value, Name) :-
    member(foo(Name, Mask), NameFlags),
    Value /\ Mask =\= 0.

Did you use '=' because it is builtin?? I am guessing I could use op() to define my own, I will just for the hell of it and learn some more. (I *will* try this but I am in work right now so I can't!) I think tonight I advance my Prolog skills a little more. I will read LPN! on line about functors. II left my Prolog books home this week!

Thanks once again for your time.


On 9 April 2014 17:03, Daniel Diaz <address@hidden> wrote:
Le 09/04/2014 15:07, Sean Charles a écrit :
Some clarificaation…. a while back you helped me write a testing framework…here is the code:
get_all_tests(Tests) :-
setof(Line-Name, get_one_test(Name, Line), Tests).
get_one_test(Name, Line) :-
atom_concat('test_', _, Name),
predicate_property(Name, prolog_line(Line)).
It uses ‘pairs’ as I understand them, using the ‘-‘ to combine the key and value, so in your response, what is ‘=‘ doing in this context in the list of flags????

There is no special meaning for '-' (except keysort/2 which sorts a list of pairs of the form Key-Value according to the Keys). In the above code
I used a functor - but a = would work as well. We only need a structure to group 2 values (the functor does not care in this case).


Just when I thouthgh I was beginning to understand! LMAO

Thanks once again,


On 9 Apr 2014, at 09:18, Daniel Diaz <address@hidden> wrote:

Hello Sean,

you can do this with your own loop or use findall like this:

flag(public, 0x0001).
flag(final, 0x0010).
flag(super, 0x0020).
flag(interface, 0x0200).
flag(abstract, 0x0400).
flag(synthetic, 0x1000).
flag(annotation, 0x2000).
flag(enum, 0x4000).

get_java_flags(Value, Flags) :-
    findall(Name, has_flag(Value, Name), Flags).

has_flag(Value, Name) :-
    flag(Name, Mask),
    Value /\ Mask =\= 0.

then call it with:

| ?- get_java_flags(0x421, Flags).

Flags = [public,super,abstract]

I have detailed a bit to be more comprehensive: the has_flag/2 predicate could be inlined inside the findall surrounding with ().

If you prefer to have the flags as a parameter pass them as a list and use member to handle backtracking (done by get_flags/3 here):

get_java_flags(Value, Flags) :-
    FlagNames = [public = 0x0001,
                final = 0x0010,
                super = 0x0020,
                interface = 0x0200,
                abstract = 0x0400,
                synthetic = 0x1000,
                annotation = 0x2000,
                enum = 0x4000],
    get_flags(FlagNames, Value, Flags).

get_flags(FlagNames, Value, Flags) :-
    findall(Name, has_flag(FlagNames, Value, Name), Flags).

has_flag(NameFlags, Value, Name) :-
    member(Name=Mask, NameFlags),
    Value /\ Mask =\= 0.


Le 09/04/2014 01:02, Sean Charles a écrit :
Hello list,

I have almost completed a predicate called ‘jread’ which parses a Java .class file into a term that will allow me to create a database of all of the methods, fields, superclass and interfaces for that class and ultimately create a database for a complete “android.jar” file of any required API level.

I am, for educational purposes and my own learning, trying to build a system like Hoogle/Hayoo for Android. I do a lot of Android and I wanted to create a sytem I can wire into Emacs/IntelliJ or anything for that matter, a simple HTTP server that can supply a list of methods that have a certain type signature using a si mple query language.  Searching by types it very very useful when using Haskell and I wanted to improve my Prolog so I figured why not do something like that in GNU Prolog? I have already started a simple HTTP library in pure GNU Prolog as well but this comes first now.

Progress so far is good…if I run it like this from a folder containing the unpacked android.jar file...

| ?- jread('javax/net/SocketFactory.class',X).

X = javaclass(super('java/lang/Object'),class('javax/net/SocketFactory'),implements([]),
returns('(Ljava/net/InetAddress;ILjava/net/InetAddress;I)Ljava/net/Socket;'),[attr(18,[0,1,0,19])])])) ? 

The place I am at now is decoding the bit flags for the class (and eventually the methods etc) into a term.

A typical value would be “1057” decimal, 0x421, this contains the flags:

ACC_PUBLIC 0x0001 Declared public; may be accessed from outside its package.
ACC_FINAL 0x0010 Declared final; no subclasses allowed.
ACC_SUPER 0x0020 Treat superclass methods specially when invoked by the invokespecial instruction.
ACC_INTERFACE 0x0200 Is an interface, not a class.
ACC_ABSTRACT 0x0400 Declared abstract; must not be instantiated.
ACC_SYNTHETIC 0x1000 Declared synthetic; not present in the source code.
ACC_ANNOTATION 0x2000 Declared as an annotation type.
ACC_ENUM 0x4000 Declared as an enum type.

So you can see that 0x421 means “ACC_PUBLIC, ACC_SUPER and ACC_ABSTRACT”. I would like to produce a term something like this:


It’s not that I don’t know how to do it or that I can’t do it but I am not sure what is the *most elegant* way to do it in Prolog! Heaven knows I have written bit shifting loops to test for flags in a dozen languages but not in Prolog.

So, there’s the challenge, what is the most elegant way to turn a list of bits into a list of atom terms. Ideally I would make the list of atoms a parameter so that I can reuse it for the other flag based values.

Consider the gauntlet thrown!   In the meantime I am continuing to research the possibilities for myself. I will of course put it all on GitHub along with my Redis client. I may even create a Redis database with the information!


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