Version: 4.2.1
5.1 Quote Literals Used as Expressions
Synopsis: Do not use naked syntactic literals (numbers, strings, etc) as expressions. Instead, put the datum inside of a quote expression.
Macros that consume syntactic literals (eg, literal numbers and literal strings) should avoid the temptation to also use those literals directly as self-evaluating expressions. The meaning of “self-evaluating” data is determined by the binding of #%datum in scope where the literal occurs. That binding of #%datum may not be the standard binding; it might assign other behavior to “self-evaluating” data, or the binding might be absent entirely.
The latter case – an absent #%datum binding, resulting in a
no #%datum transformer bound syntax error – is often
caused by converting the datum into a syntax object with an empty
lexical context, as in the following code:
Another way this problem can manifest is if the lexical context has a
#%datum binding but not in the relevant phase. This is
usually due to a forgotten for-template import in an
auxiliary module (see Code Generators).
5.1.1 Example
Here is a broken variant of the define-record-type macro from the Static Information example:
| (module records scheme |
| (provide define-record-type) |
| ; syntax (define-record-type record-type-id field-count-number) |
| (define-syntax (define-record-type stx) |
| (syntax-case stx () |
| [(define-record-type name field-count) |
| #'(begin (define record-type (gensym)) |
| (define-syntax name |
| (list ((syntax-local-certifier) #'record-type) |
| ; SHOULD BE (quote field-count) |
| field-count)))]))) |
| (module records-client scheme/base |
| (require 'records) |
| (define-record-type pair 2)) |
The 2 in the second module occurs in a context where there is
no transformer-phase binding for #%datum.