Re: no class-of ?

[Jamison Hope <jrh at theptrgroup dot com>]

OK, so if I understand correctly, I can't use a macro for this, but what
I can do instead is write a function that always gets inlined.  The
function itself doesn't really have to do anything, because the real
work will be done in the validate-apply handler that rewrites the
procedure application to a new Expression.  And there, I can decide what
kind of Expression to construct based upon the Type of the argument
expression.

I think that should work for what I'm trying to accomplish, thanks!

-J

On Sep 22, 2017, at 3:21 PM, Per Bothner <per@bothner.com> wrote:
> 
> On 09/22/2017 11:28 AM, Jamison Hope wrote:
>> On a somewhat related note, is there a way to define a macro that can
>> determine the type of an expression at macro expansion time, without
>> actually evaluating the expression?
> 
> Note that expressions do not have types at macro-expansion time - that happens
> at a later stage.
> 
> What you can do is write a procedure and associate either a 'validate'
> or a 'compile' property with it.
> 
> There are lots of examples in kawa/lib.  Look for the search string 'validate-apply:'.
> For example in characters.scm:
> 
> (define (char->integer ch::character) ::int
>  validate-apply: "kawa.lib.compile_misc:charToIntegerValidateApply"
>  (as int ch))
> 
> You'll find charToIntegerValidateApply in compile-misc.scm:
> 
> (define-validate charToIntegerValidateApply (exp required proc)
>  ((exp:isSimple 1 1)
>   (let ((e0 (visit-exp (exp:getArg 0) character)))
>     (apply-exp as int
>                (apply-exp gnu.kawa.functions.Convert:cast character e0)))))
> 
> This mean is exp is a simple application (no splices or keywords) with one argument,
> then apply the following transformation.  This is evaluated during the
> InlineCall phase of the compilation.
> 
> When it comes to charToIntegerValidateApply it first "validates" argument 0
> in the context of the "required type" character (which is a 32-bit "primitive"
> character type).  It then wraps the result in a call to the low-lever Convert:cast
> procedure.  Assuming the types are correct, this will be a no-op.
> Then that result is cast to an int.
> 
> If all goes well and the incoming value is a 32-bit unboxed character or a 16-bit unboxed char,
> no code actually needs to be generated - which is the reason
> for using the charToIntegerValidateApply transformer in the first place.  Otherwise,
> you either get a compile-time error of the appropriate run-time conversion.
> 
> You can use (exp:getType) to get the type of the expression.
> 
> There is also the compile-apply property:
> 
> (define (values #!rest (args :: <Object[]>))
>  validate-apply: "kawa.lib.compile_misc:valuesValidateApply"
>  compile-apply: "kawa.lib.compile_misc:valuesCompile"
>  (invoke-static <gnu.mapping.Values> 'make args))
> 
> This causes the valuesCompile method in class kawa.lib.compile_misc
> to be called at code-generation time.  This is more low-level,
> and most of the code-generation are actually written in Java to
> generate custom byte-code depending on the argument expressions
> and their types.
> 
> (If I wanted to get the type of a standalone expression, I'd
> look at the implementation of eval (see kawa/lang/Eval.java), and do the same,
> but stop before code-generation.  Specifically,you would probably want to do
> compilation.process(Compilation.WALKED). Then you'd do something like
> compilation.getModule().body.getType().
> 
> However, that doesn't give you the type on an expression *in a lexical context*.)
> -- 
> 	--Per Bothner
> per@bothner.com   http://per.bothner.com/

--
Jamison Hope
The PTR Group
www.theptrgroup.com