Am Do., 15. Nov. 2018 um 17:55 Uhr schrieb Marc Nieper-Wißkirchen < marc.nie...@gmail.com>:
> I would like to alias an identifier in Guile. By this, I mean the > following: Given a bound identifier `x', I want to lexically introduce > another identifier `y' with the same binding as `x' so that `x' and `y' > become `free-identifier=?'. > > The following is one use case: I have written a macro `custom-quasiquote`, > which has a similar syntax and does similar things as `quasiquote' does. > Because I would like to use the special reader syntax for `quasiquote', I > would do: > > (letrec-syntax ((quasiquote <custom-quasiquote-transformer>)) > <body>) > > In the <body>, whenever I write ``<template>', my > <custom-quasiquote-transformer> is being applied to <template>. > > There may be parts in the <body> where I would like to use Scheme's > quasiquotation. One try is to surround these parts with: > > (let-syntax ((quasiquote (identifier-syntax scheme-quasiquote))) > <expression>) > > Here, `scheme-quasiquote' is bound to `quasiquote' as exported by > `(guile)'. However, this solution is not correct as the `quasiquote' local > to <expression> is not `free-identifier=?' to `scheme-quasiquote'. Thus > quasiquotations containing quasiquotes (that should become auxiliary > syntax) won't work right. > > What I really need is to make the inner `quasiquote' a true alias of > `scheme-quasiquote' (or rather to restore the binding of `quasiquote' in > <expression> to what the binding outside the outer `letrec-syntax' was). > > Chez Scheme has `alias' for this purpose: > https://cisco.github.io/ChezScheme/csug9.5/syntax.html#./syntax:h10. > > What can I do in Scheme? If this is currently impossible, please consider > this post as a feature request for `alias' or a similar binding construct. > :-) > Typo. "What can I in GUILE?" was the question I intended to ask. :-) I gave it a try to implement `define-alias' in Guile. My first approximation to a solution was to add a clause like ((define-alias-form) (let ((id (wrap value w mod)) (label (id-var-name e w mod))) (extend-ribcage! ribcage id label) (parse (cdr body) (cons id ids) labels var-ids vars vals bindings))) to `expand-body' in `psyntax.scm'. (A similar clause is added to `expand-top-sequence' and `syntax-type' is extended to recognize `define-alias' and return `define-alias-form' for the type.) With identifiers lexically bound in the same module, my change does what it is supposed to do. The following test passes: (pass-if "alias is free-identifier=?" (let ((x #t)) (define-syntax foo (syntax-rules (x) ((foo x) #t) ((foo _) #f))) (let () (define-alias y x) (foo y)))) My code, however, doesn't work when the aliased identifier is global and/or comes from another module: The following test fails: (pass-if-equal "alias is free-identifier=? with globals" '(1 5) (let () (define-alias comma unquote) `(1 (comma (+ 2 3))))) I see that my attempt to bind the identifier `y' to `x's label (if given `(define-alias y x)') is too naive. I have taken a look at `free-id=?' and `resolve-identifier' in `psyntax.scm', but the way modules are handled in `psyntax.scm' is still new to me. Thus I'm glad about any hints. -- Marc
