To take a simple example:

(define-syntax -->
(syntax-rules ()
((_ . original)
(-->helper () original))))

(define-syntax -->helper
(syntax-rules ()
((_ pairs (x0 y0 . more))
(-->helper ((x0 y0) . pairs) more))
((_ pairs body)
(let pairs . body))))

(defmacro --> (&rest [var-val]*-body)
`(let ,(loop for (var val) on (butlast [var-val]*-body) by (function cddr)
collect (list var val))
,@(last [var-val]*-body)))

The helper macro isn't needed.

Scheme (Gauche and Racket):

The goal is a let-macro that doesn't need parentheses
around the bindings. Only one expression is allowed
after the bindings.


(define-syntax -->
(syntax-rules ()
[ (_ ((k v) ...) expr)
(let ((k v) ...) expr) ]
[ (_ ((k v) ...) a b . more)
(--> ((k v) ... (a b)) . more) ]
[ (_ a b c more ...)
(--> () a b c more ...) ]))

a 2 m 44 z 88 (print (list a m z)))

===>
(2 44 88)

a 2 m 44 z 88 )

===>
; stdin:10:0: -->: bad syntax
; in: (--> ((a 2) (m 44) (z 88)))

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On 2024-07-03, B. Pym <No_spamming@noWhere_7073.org> wrote:

The helper macro isn't needed.

Scheme (Gauche and Racket):

The goal is a let-macro that doesn't need parentheses
around the bindings. Only one expression is allowed
after the bindings.

(define-syntax -->
(syntax-rules ()
[ (_ ((k v) ...) expr)
(let ((k v) ...) expr) ]
[ (_ ((k v) ...) a b . more)
(--> ((k v) ... (a b)) . more) ]
[ (_ a b c more ...)
(--> () a b c more ...) ]))

That's stupid; just make it so the wrapping is not allowed,
rather than optional:

(defmacro flat (. bindings-and-expr)
(tree-case bindings-and-expr
((expr) expr)
((k v . rest) ^(let ((,k ,v)) (flat ,*rest)))
(x (error "flat: bad syntax"))))

(flat 3)

3

(flat x 1 x)

1

(flat x 1 y 2 (+ x y))

3

(flat x 1 y 2 4 (+ x y))

** flat: bad syntax
** during evaluation at expr-1:6 of form (error "flat: bad syntax")

(flat x 1)

** flat: bad syntax
** during evaluation at expr-1:6 of form (error "flat: bad syntax")

(flat)

** flat: bad syntax
** during evaluation at expr-1:6 of form (error "flat: bad syntax")

a 2 m 44 z 88 (print (list a m z)))

(2 44 88)

a 2 m 44 z 88 )

; stdin:10:0: -->: bad syntax
; in: (--> ((a 2) (m 44) (z 88)))

Wut? Your diagnostic is reported against something that the
user didn't write, because it got normalized into
the nested let-style syntax, which is supposd to be
further processed.

What is the point.

--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
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