Re: how to use a variable in a plain-file object?

[raingloom@riseup.net]

On 2024-05-16 17:57, Giovanni Biscuolo wrote:
> Hello,
> 
> sorry for the very guile-absolute-beginner question, but I'd like to use
> a declared variable in a plain-file object, so I can write something
> like in this pseudo-code snippet:
> 
> --8<---------------cut here---------------start------------->8---
> 
> (define variable1 "var1-value")
> (define variable2 "var2-value")
> 
> [...]
> 
> (define %my-file-object
>   ,(plain-file "something.conf" "
> # This is an example configuration file
> 
> attribute1 =" variable1 " 
> 
> attribute2 =" variable2 "
> 
> "))
> 
> --8<---------------cut here---------------end--------------->8---
> 
> and obtain a "something.conf" file like this:
> 
> --8<---------------cut here---------------start------------->8---
> # This is an example configuration file
> 
> attribute1 = var1-value
> 
> attribute2 = var2-value
> 
> --8<---------------cut here---------------end--------------->8---
> 
> how can I do, please?
> 
> Thanks, Gio'

You probably want a G-expression.

Short untested example:

(define name "Giovanni")
(define greeting-gexp
  #~(begin (with-output-to-file #$output (lambda _ (write (string-append
"Hello, " #$name "!"))))))

You should be able to use greeting-gexp in place of %my-file-object.

I highly recommend reading the relevant docs in Guix's info pages.
Online version:
https://guix.gnu.org/manual/en/html_node/G_002dExpressions.html
Related blog post:
https://guix.gnu.org/en/blog/2023/dissecting-guix-part-3-g-expressions/