Re: Newbie Packaging Question

[Tobias Geerinckx-Rice <me@tobias.gr>]

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Brian,

Brian Woodcox wrote:
> I have a question regarding packaging.
>
> Say I have the following in my sample.scm file:
>
> (define commit "a1ee2ebf087768b5fbffa07f50a294ad9c8ee600")
>
> (define version-date “201903271913”)
>
> and then
>
> (define-public test-me
>   (package
>   (name “test-me”)
>   (version commit)
>   (source …
>                .
>                .
>                . etc...
>
>
> In my modify-phases %standard-phases,  I am trying to use 
> substitute* to patch some files with the version-date define 
> outside my package.
>
> Is this not possible, I always get an unbound variable error.

You've omitted one of the interesting parts (the one where the 
error happens), but I suspect it's something roughly like this:

  (arguments
   '(#:phases
     (modify-phases %standard-phases
      (add-after 'unpack 'uh-oh
       (lambda _
        (substitute* "foo.bar"
         (("198309270000") version-date))
        #t))))) ;          ^^^^^^^^^^^^

(If not: forgive & ignore the basic Scheme lecture below and 
please paste your full package expression :-)

What's happening here is that the value of ARGUMENTS is being 
*quoted* (by the ‘'’ just after ARGUMENTS), meaning that is is not 
immediately evaluated, but passed to the ‘build stage’ as-is.  And 
since VERSION-DATE isn't bound (defined) in the build stage's 
environment, the builder will return the error you're seeing.

Luckily, this is easily fixed:

  (arguments
   `(#:phases
     (modify-phases %standard-phases
      (add-after 'unpack 'uh-oh
       (lambda _
        (substitute* "foo.bar"
         (("198309270000") ,version-date))
        #t))))) ;          ^

Two subtle changes here:

- The ‘'’ (quote) has been changed into a ‘`’ *quasiquote*, which 
  is identical to quote except that it allows *unquoting* inside 
  the quoted expression.

- This unquoting is done by adding ‘,’ to VERSION-DATE.  Any book 
  on Scheme will explain it much better than I can, but here it 
  basically says that you want to send the currently bound *value* 
  of VERSION-DATE to the builder, not the variable name.

(If you've written your package by copying an existing one, you 
might already be using quasiquote without realising it, and all 
that's missing is an unquote.)

Kind regards,

T G-R

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