Question about guard's => syntax for a new Gerbil user

[HMarcien]

Hello everyone,

I'm new to Gerbil and I'm trying to understand the behavior of the guard macro. I'm running into an issue with the => syntax inside a guard clause, which I expected to work based on the R7RS standard and Gerbil's SRFI-34 support.

Here is a minimal piece of code that demonstrates the problem:

(guard (exception
         ((assq 'a exception) => cdr))
  (raise (list (cons 'a 42))))

Based on the R7RS specification, I expect this code to return 42. However, when I run it in the Gerbil REPL, I get the following syntax error:

*** ERROR IN (stdin)@10.30-10.32--- Syntax Error:
Bad syntax; bottom method for apply-macro-expander...
form:   =>

What I find confusing is that this same code works as expected in Gambit.

My question is: am I misunderstanding something about how guard is implemented in the default Gerbil language? Is there a specific library I need to import to get the R7RS-compliant behavior for guard, or could this be a bug?

Thanks in advance for your help!

[vyzo]

Are you using the appropiate prelude for r7rs? guard is not a predefined macro in the gerbil language prelude.

[HMarcien]

Thanks for the quick reply. That clarifies a lot!

To answer your question, no, I was not using a specific R7RS prelude; I was just running my code in the default Gerbil environment.

It's interesting that you mention guard is not predefined in the Gerbil prelude. I had noticed that a guard form without the => clause seemed to work, which is what led to my confusion.

Based on your hint, I tried to import just the R7RS/SRFI-34 version of guard into the default environment, and it seems to solve the problem perfectly. The following code now works as expected and returns 42:

(import :scheme/r7rs)
(import (only (scheme base) guard))

(guard (exception
         ((assq 'a exception) => cdr))
  (raise (list (cons 'a 42))))

I guess this is not what is recommended but rather to use the r7rs prelude. This brings me to a question: by changing the default prelude to r7rs for example, do I lose some features of the Gerbil language (the default prelude)?

Thanks again for your guidance!

[vyzo]

Well, if you use the r7rs prelude the surface syntax and predefined macros are just standard/vanilla scheme. But you can still import everything you need or want to use, even from :gerbil/core, which is the gerbil prelude itself.

Note that you can also use our try macro, it is defined in :std/sugar. It has much saner semantics than guard!

[HMarcien]

Thanks you so much, i will definitely use the try macro.

I'm really enjoying the language so far—Gerbil is incredible!

Thanks again for your guidance.

[vyzo]

my pleasure! Have fun, come hang out with us on gitter/matrix.