On 2023-03-20 11:45, Josselin Poiret wrote: > Hi Andrew, > > Andrew Tropin <and...@trop.in> writes: > >> I would expect two last expressions return the same result, but the >> former one doesn't do ungexp: >> >> --8<---------------cut here---------------start------------->8--- >> (define a '(3 4)) >> >> (define b `#(1 2 ,a)) >> >> (eval-with-store >> #~(list '(1 2 #$a))) ;; => ((1 2 (3 4))) >> >> (eval-with-store >> #~(list #(1 2 #$a))) ;; => (#(1 2 (ungexp a))) >> >> (eval-with-store >> #~(list #$b)) ;; => (#(1 2 4)) >> --8<---------------cut here---------------end--------------->8--- >> >> Am I doing/expecting something wrong or there is a bug here? > > It's more related to how the guile reader works, and this is such a > corner case that I don't know whether we should fix. Basically, > anything starting with # is a reader extension, and the next character > identifies which extension it is. #( is the reader extension for > vectors, #~ for gexp and #$ for ungexp. > > To simplify, whenever you use #~, guile will insert (gexp ...) instead, > which is a hygienic macro (not just a procedure!), that will look for > ungexps inside the expression. That traversal is only made on cons > cells though, so it doesn't try to go through any piece of syntax that > is not a cons cell! Since #( doesn't expand to a (vector ...) > cons-cell, the subexpression gets ignored for traversal. > > This is in contrast to another reader extension, #' (for syntax), which > does expand to (syntax ...), and is thus further traversed! > > You can find how both of these reader extensions operate in > <libguile/read.c>. > > I guess the immediate fix is to use (vector ...) rather than #(...). We > could also add code to the gexp traversal to also traverse vectors, but > I am not even sure if they go through the gexp->sexp dance unharmed, and > we also should in principle do that for everything that can get into a > gexp, not just vectors (eg. bytevectors). >
Thank you very much for extensive explanation! I have a few tasks related to the guile reader extensions, so when I get my hands dirty with it, I'll probably share my new thoughts and opinions on this topic. -- Best regards, Andrew Tropin
signature.asc
Description: PGP signature
