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Behavior of #!optional

From: Théo Cavignac
Subject: Behavior of #!optional
Date: Fri, 7 Feb 2020 14:24:42 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.2

Hello everyone,

Quick question: is it safe to rely on the fact that positional parameters are available to definition of optional parameters default value ?

In other word, is the following procedure OK or does it rely on implementation detail that may change ?

(define (f a b #!optional (err (lambda () (error "a is not b" (cons a b)))))

 (if (eq? a b) #t (err)))

Thank you !

PS: It's a dumb example but I have a real use case where it would be nice to use that trick.

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