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Forum => newLISP in the real world => Topic started by: lyl on April 27, 2019, 04:07:33 PM

Title: difference result by eval in macro and in S-expr
Post by: lyl on April 27, 2019, 04:07:33 PM
I think the following two examples say the same thing, but they give different results, why?



Example No. 1:
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(define-macro (m lst)
  (eval lst))
(m '(+ 4 5)) ;; -> (+ 4 5)

Example No. 2:
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(eval '(+ 4 5)) ;; -> 9
Title: Re: difference result by eval in macro and in S-expr
Post by: rrq on April 27, 2019, 06:52:39 PM
mmm, did you check //http://www.newlisp.org/downloads/newlisp_manual.html#define-macro ?



The key point is that for m, its parameter lst gets bound to the un-evaluated argument '(+ 4 5), whereas in example 2 the eval function's parameter gets the evaluated argument (+ 4 5), i.e, without the single-quote. In other words,
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(m '(+ 4 5)) is actually the same as
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(eval ''(+ 4 5))
If you'd want an "m" function to be the same as an "eval" function, you'd use "define" rather than "define-macro". (Not totally the same, since "m" binds "lst" before calling "eval", but that's a separate concern)



hth
Title: Re: difference result by eval in macro and in S-expr
Post by: lyl on October 03, 2019, 01:18:39 AM
Then, in the following example
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(define(f lst)
  (+ lst 1))

(setq a '(+ 2 3))
(f a)

As arguments of function in lisp are evaled first, I think (f a) is the same as (+ (+ 2 3) 1) which should be 6, but what I get is: "ERR: value expected in function + : (+ 2 3)". Why?
Title: Re: difference result by eval in macro and in S-expr
Post by: cameyo on October 03, 2019, 11:22:15 AM
Maybe you can use letex:
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(define(f lst)
  (letex (x lst) (+ x 1)))

(setq a '(+ 2 3))
;-> (+ 2 3)

(f a)
;-> 6

Try this:
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(define types '("nil" "true" "int" "float" "string" "symbol" "context"
    "primitive" "import" "ffi" "quote" "expression" "lambda" "fexpr" "array"
    "dyn_symbol"))

(define (typeof v)
    (types (& 0xf ((dump v) 1))))

(define(f lst)
  (letex (x lst)
    (println (list? lst))
    (println (symbol? lst))
    (println (list? x))
    (println (symbol? x))
    (println (dump lst))
    (println (dump x))
    (println (typeof lst))
    (println (typeof x))
  (+ x 1)))

(setq a '(+ 2 3))
;-> (+ 2 3)

(f a)
;-> true
;-> nil
;-> nil
;-> nil
;-> (7026864 59 7012912 7030672 7026896)
;-> (7030096 386 7012912 7012912 5)
;-> expression
;-> int
;-> 6
Title: Re: difference result by eval in macro and in S-expr
Post by: lyl on October 03, 2019, 02:59:13 PM
Thank you, cameyo.  I think the argument "lst" of "f" has been evaled during the call of (f a), so why must  it be evaled again by "letex" in function body ?

And could you please give more information about the function "typeof"?
Title: Re: difference result by eval in macro and in S-expr
Post by: cameyo on October 04, 2019, 02:22:12 AM
Hi lyl,

i'm not an expert, but i think "evaluated" is different from "expanded". Maybe a guru will highligth this question.

The typeof function show the type of the argument; the core function is dump (see the manual for information).

ciao
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