GNU bug report logs - #32847
Wrong macro expansion in eval

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From: Mark H Weaver <mhw <at> netris.org>
To: Stefan Israelsson Tampe <stefan.itampe <at> gmail.com>
Cc: 32847 <at> debbugs.gnu.org
Subject: bug#32847: Wrong macro expansion in eval
Date: Wed, 26 Sep 2018 22:16:06 -0400

Hi Stefan,

Stefan Israelsson Tampe <stefan.itampe <at> gmail.com> writes:

> This for guile 2.4 and master,
>
>> (eval `(let-syntax ((f (lambda (x) ,#'(+ (pk 'a 1) 2)))) f) (current-module)) 
>
> ;;; (#<syntax a> 1)
>
> But without eval:
>> (let-syntax ((f (lambda (x) #'(+ (pk 'a 1) 2)))) f)  
>
> ;;; (a 1)

I think the mistake is in your code above.  In the first case, what you
want is this:

  (eval `(let-syntax ((f (lambda (x) ,'#'(+ (pk 'a 1) 2)))) f)
        (current-module))

Note the addition of a quote (') between the unquote (,) and syntax (#')
above.

The expression that follows unquote (,) should evaluate to an
s-expression.  In this case, you want it to evaluate to the s-expression
#'(+ (pk 'a 1) 2), i.e. (syntax (+ (pk 'a 1) 2)), i.e. a list with two
elements, the first being the symbol 'syntax'.  But that's not what
you're doing above.  Instead, you are returning the syntax object
itself, which is being spliced directly into the code.

Does that make sense?

       Mark

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