0. #lang racket/base
1.  
2. (require (for-syntax
3.           racket/base
4.           syntax/parse))
5.  
6. (begin-for-syntax
7. ; A ForClause is either
8. ;   - (bind-clause (listof identifier) syntax)
9. ;   - (when-clause syntax)
10. (struct bind-clause (vars seq-expr) #:transparent)
11. (struct when-clause (guard) #:transparent)
12.  
13. (define-splicing-syntax-class for-clause
14.   #:attributes (ast)
15.   (pattern [var:id seq:expr]
16.            #:attr ast (bind-clause (list #'var) #'seq))
17.   (pattern [(var:id ...) seq:expr]
18.            #:attr ast (bind-clause (syntax->list #'(var ...))
19.                                    #'seq))
20.   (pattern (~seq #:when guard:expr)
21.            #:attr ast (when-clause #'guard)))
22. )
23.  
24. (define-syntax (myfor stx)
25.   (syntax-parse stx
26.     ((_ (clause:for-clause ...) body:expr)
27.      #''((clause.ast ...) body))))
28.  
29. (myfor ((a '[1 2 3])) a)

=>

program::-1: clause.ast: bad attribute value for syntax

template

attribute value: (list (bind-clause '(#<syntax:2:0 a>)

#<syntax:2:0 (quote (1 2 3))>))

expected for attribute: list of depth 1 of syntax

sub-value: (bind-clause '(#<syntax:2:0 a>) #<syntax:2:0

(quote (1 2 3))>)

expected for sub-value: syntax

in: clause.ast