8.12
3.1 Datum
| (require cur/stdlib/datum) | package: cur-lib |
This library defines an interface for incrementally extending #%datum,
solving the problem of diamond inheritance for parsing library-defined data
literals.
syntax
(#%datum . syn)
A generic handler for Cur data literals.
Calls current-datum to parse syn.
procedure
(current-datum) → (-> syntax? syntax?)
(current-datum p) → void? p : (-> syntax? procedure? syntax?)
A phase 1 parameter, holding an open-recursive function for parsing data.
Setting current-datum to a new procedure of the right interface
effectively extends the #%datum parser.
Initially set to default-datum.
Examples:
> 0 eval:1:0: invalid-datum-error: Could not find datum parser
in: 0
> (require cur/stdlib/nat)
> (begin-for-syntax (current-datum (lambda (syn f) (syntax-parse syn [e:nat (nat->unary (syntax->datum syn))] ; User must manually call f if parsing fails. [_ (f syn)])))) > 0 (z)
procedure
(default-datum syn f) → syntax?
syn : syntax? f : procedure?
The default datum parser; raises an error, as it doesn’t know how to parse anything.